Changes in glucose metabolism based on 75-g oral glucose tolerance tests before and after surgery for adrenal Cushing’s syndrome

Abstract

Adrenal Cushing’s syndrome (CS) is caused by cortisol-producing adrenal adenoma and is frequently accompanied by glucose metabolism disorders, which are characterized by increased insulin resistance and insufficient β-cell compensation. However, considering the rarity of CS, few studies have assessed whether the glucose metabolism disorders could be ameliorated by surgical treatment. In this case series, we evaluated glucose metabolism before and after surgery in 11 patients (10 women and 1 man) who underwent unilateral adrenalectomy for overt adrenal CS between 2005 and 2016. Patients with pre-diagnosed diabetes mellitus (DM) were excluded. Pre- and post-operative 75-g oral glucose tolerance tests were performed. Cortisol secretion decreased significantly after surgery (median 24-h urinary free cortisol: 582.0 μg/day [interquartile range: 321.0–743.0 μg/day] to 31.3 μg/day [23.6–40.6 μg/day], p = 0.001). The results of the OGTT generally improved after surgery (normal glucose tolerance/impaired glucose tolerance/DM: 2/8/1 to 8/3/0), with significant decreases in the immunoreactive insulin and glucose levels. We also found a decrease in the median homeostatic model assessment of insulin resistance (2.4 [1.4–2.8] to 1.0 [0.6–1.1], p = 0.002), and increases in the median Matsuda index (3.0 [2.3–4.5] to 8.2 [6.3–11.4], p < 0.001), median insulinogenic index (0.70 [0.22–1.51] to 1.22 [0.78–1.64], p = 0.08), and median disposition index (609.1 [237.8–1,095.2] to 1,286.0 [1,034.6–1,857.6], p = 0.002). These findings indicate that adrenalectomy for adrenal CS without overt DM may help ameliorate glucose metabolism disorders, and improve both insulin resistance and insulin secretion.

CUSHING’S SYNDROME (CS) is caused by cortisol overproduction and is frequently accompanied by impaired glucose tolerance (IGT) or diabetes mellitus (DM), with the reported prevalence of DM among patients with CS ranging from 20% to 50% [1]. Glucose metabolism disorders in CS are thought to occur via two mechanisms: increased insulin resistance and insufficient β-cell compensation. In this context, glucocorticoid excess causes insulin resistance by increasing hepatic gluconeogenesis and lipogenesis, reducing glucose uptake and glycogen synthesis in skeletal muscle, and inducing abdominal obesity [2, 3]. In addition, glucocorticoid excess causes β-cell dysfunction by increasing β-cell apoptosis and decreasing insulin exocytosis [4]. Several previous studies have cross-sectionally analyzed the characteristics of glucose metabolism disorders in CS [5, 6], although the rarity of CS has resulted in few studies evaluating patients’ glucose metabolism before and after surgical treatment of CS [7, 8]. Furthermore, the previous studies have not evaluated the detailed changes in insulin resistance or secretion.

There are many subtypes of CS, including pituitary Cushing’s disease, ectopic pituitary adrenocorticotropic hormone (ACTH) syndrome, ACTH-independent macronodular adrenal hyperplasia, adrenal carcinoma, and cortisol-producing adrenal adenoma (CPA) [9, 10]. Almost all patients with unilateral cortisol-producing adrenal adenoma are expected to achieve complete remission from cortisol overproduction after adrenalectomy [11]. However, neurosurgery has a lower remission rate for patients with pituitary adenoma [12, 13]. Interestingly, a previous study showed that IGT was significantly reduced after surgical treatment for adrenal adenoma, but not in patients with pituitary Cushing’s disease [8]. Thus, we speculated that we could isolate the postoperative change in glucose metabolism, without the possibility of residual disease affecting the results, by selecting only patients with adrenal CS. This report described our findings regarding glucose metabolism after adrenalectomy in a series of 11 patients with adrenal CS but without overt DM, based on data from 75-g oral glucose tolerance tests (OGTTs) that were performed before and after adrenalectomy.

Materials and Methods

Study population

We retrospectively evaluated the clinical records of 11 Japanese patients with adrenal CS who visited our outpatient clinic between 2005 and 2016. The retrospective study protocol was approved by the research ethics committee of Yokohama Rosai Hospital (no. 30-30).

All patients fulfilled the CS diagnostic criteria based on the Endocrine Society guidelines [14], had a unilateral adenoma that was detected using computed tomography (CT) or magnetic resonance imaging (MRI), and completed 75-g OGTTs that were performed before the adrenalectomy and 1–3 years after the adrenalectomy. The exclusion criteria were fulfilling the DM diagnostic criteria based on the Japan Diabetes Society guidelines [15] before the hospitalization, subclinical CS (e.g., serum cortisol levels of ≥1.8 μg/dL after an overnight 1-mg DST without symptoms of CS) [16], Cushing’s disease, bilateral adenoma, adrenal carcinoma, and renal insufficiency with an estimated glomerular filtration rate (eGFR) of <30 mL/min/1.73 m².

Oral glucose tolerance test results with immunoreactive insulin and glycated hemoglobin levels

After the baseline blood sampling, a standard 75-g OGTT was performed [17], and serum samples for measuring glucose and immunoreactive insulin (IRI) were obtained at 0 min, 30 min, 60 min, 90 min, and 120 min after the glucose loading. Serum IRI levels were measured using an enzyme immunoassay (LS “Eiken” Insulin; Eiken Chemical Co., Ltd., Tokyo, Japan) between January 2005 and June 2006, using a chemiluminescent immunoassay (CLEIA) (Lumipulse Presto Insulin; Fujirebio, Inc., Tokyo, Japan) between June 2006 and March 2012, and using an electro-chemiluminescent immunoassay (ECLIA) (ECLusys Insulin; Roche Diagnostics, Basel, Switzerland) between April 2012 and December 2016. The results of the OGTT were defined as normal glucose tolerance (NGT), impaired fasting glucose (IFG), IGT, or DM according to the Japan Diabetes Society guidelines [15]. Plasma glycated hemoglobin (HbA1c) levels were reported in National Glycohemoglobin Standardization Program units and were measured using high-performance liquid chromatography (ADAMS A1c; Arkray, Inc., Tokyo, Japan).

Measurements and calculation of the indices

This study used the following indices [18]: insulinogenic index = ΔIRI_0-30/Δplasma glucose (PG)_0-30 [19]; homeostasis model assessment of insulin resistance (HOMA-IR) = (fasting plasma glucose [FPG] × fasting IRI [FIRI])/405 [20]; HOMA of β-cell function (HOMA-β) = FIRI × 360/(FPG – 63) [20]; Matsuda index = 10,000/(sqrt [FPG × FIRI × mean PG × mean IRI]) [21], and disposition index = Stumvoll-1/HOMA-IR, which has been shown to have the best-fit hyperbolic line for insulin sensitivity and insulin secretion in the Japanese population [22]. Stumvoll-1 = 1,283 + 1.829 × IRI₃₀ – 138.7 × PG₃₀ + 3.772 × FIRI [23]. The units were mg/dL for PG and μU/mL for IRI in the insulinogenic index, HOMA-IR, and ISI_matsuda. In Stumvoll-1, the units were pmol/mL for PG and mmol/L for IRI. The area under the curve (AUC) was calculated using a trapezoidal method.

Measurements of cortisol and ACTH levels

The serum cortisol levels were measured between January 2005 and March 2009 using a radioimmunoassay (RIA) (Cortisol kit “TFB”; TFB Inc., Tokyo, Japan), which had a lowest detection limit of 1.0 μg/dL. Serum cortisol levels were measured between April 2009 and December 2016 using a CLEIA (Access Cortisol; Beckman Coulter Inc., CA, USA), which had a lowest detection limit of 0.4 μg/dL. Urinary cortisol levels were measured using a RIA (Cortisol kit “FR”; Fujirebio, Inc., Tokyo Japan). Postoperative urine cortisol levels were measured without stopping the hydrocortisone supplementation. The plasma ACTH levels were measured between January 2005 and March 2010 using an immunoradiometric assay (ACTH IRMA kit “Mitsubishi”; Mitsubishi Kagaku Iatron, Inc. or Mitsubishi Kagaku Medience, Co, Tokyo, Japan), which had a lowest detection limit of 5.0 pg/mL. Plasma ACTH levels were measured between April 2010 and December 2016 using an ECLIA (ECLusys ACTH kit; Roche Diagnostics, Basel, Switzerland), which had a lowest detection limit of 1.0 pg/mL.

ACTH stimulation test

The ACTH stimulation test was performed using a 250-μg intravenous injection of ACTH (Cortrosyn; Daiichi Pharmaceutical Co., Tokyo, Japan) at 8:00 AM. Blood samples were subsequently drawn at 0 min, 30 min, and 60 min after the injection to measure cortisol levels. Postoperative tests were performed with stopping the hydrocortisone supplementation for >24 h.

Statistical analysis

Data were reported as median [interquartile range] or number, as appropriate. The Wilcoxon signed-rank test was used to compare parameters from before and after surgery. The Mann-Whitney U test was used to compare clinical variables between the groups with NGT or IGT after surgery or between the groups with cortisol levels of <18 μg/dL or ≥18 μg/dL after the 250-μg ACTH stimulation test. The relative proportions of categorical variables were assessed using Fisher’s exact test. Spearman’s rank correlation coefficients were calculated to assess the relationships between continuous variables. If a measured value for cortisol or ACTH was below the detection limit, we used one-half of the detection limit as the value for that measurement in the statistical analyses. Differences were considered statistically significant at p < 0.05, and all analyses were performed using JMP^® software (version 12; SAS Institute Inc., Cary, NC, USA).

Results

Patient characteristics before and after the surgery

The patients included 10 women and 1 man (Table 1), with a disease duration range of 0.5–5.5 years (median: 2 years [1–3 years]). All patients were treated using unilateral adrenalectomy, and 9 patients needed steroid supplementation at the postoperative 75-g OGTT. All patients had no signs of adrenal insufficiency at the time of the postoperative OGTT. The patients preoperative and postoperative clinical characteristics are shown in Table 2. Significant postoperative decreases were observed for serum cortisol levels at 8:00 AM (18.1 μg/dL [17.6–21.7 μg/dL] to 7.9 μg/dL [1.9–9.5 μg/dL], p = 0.003), serum cortisol levels after an overnight 1-mg DST (20.0 μg/dL [17.2–25.1 μg/dL] to 0.5 μg/dL [0.4–0.8 μg/dL], p = 0.001), and 24-h urinary cortisol levels (582.0 μg/day [321.0–743.0 μg/day] to 31.3 μg/day [23.6–40.6 μg/day], p = 0.001), with a significant postoperative increase in ACTH levels at 8:00 AM (1.7 pg/mL [0.5–2.5 pg/mL] to 30.1 pg/mL [17.3–52.1 pg/mL], p = 0.002). All patients had their serum cortisol levels after an overnight 1-mg DST decrease to <1.8 μg/dL and their ACTH levels increase to a detectable level. Seven patients had postoperative maximum cortisol levels of <18 μg/dL after the 250-μg ACTH stimulation test, which has been determined as the cut-off for adrenal insufficiency (Table 1) [24]. In particular, all patients who received ≥10 mg of hydrocortisone supplementation had low cortisol responses to the ACTH stimulation. There was a trend towards a reduction in the postoperative HbA1c levels (5.8% [5.5–6.2%] to 5.4% [5.2–5.5%], p = 0.06). Significant postoperative decreases were observed for body mass index (BMI), dehydroepiandrosterone sulfate level, liver enzyme levels, serum triglyceride level, and serum high-density lipoprotein-cholesterol level, with a significant postoperative increase in serum potassium level.

Table 1 Profiles of the 11 patients

Patients			Disease duration (years)	Adrenal tumor	Results from the OGTT		Steroid supplementation*	Postoperative maximum cortisol level after 250-μg ACTH stimulation test (μg/dL)
Case no.	Sex	Age (years)			Preoperative	Postoperative
1	Female	37	2.0	lt, 25 mm	IGT	NGT	hydrocortisone 15 mg	1.3
2	Female	40	5.5	lt, 32 mm	IGT	NGT	hydrocortisone 17.5 mg	7.7
3	Female	23	2.0	rt, 36 mm	IGT	NGT	hydrocortisone 5 mg	19.1
4	Female	45	3.0	lt, 31 mm	DM	NGT	hydrocortisone 10 mg	4.0
5	Female	38	5.0	rt, 30 mm	DM	IGT	hydrocortisone 5 mg	15.3
6	Female	45	1.0	lt, 33 mm	IGT	NGT	hydrocortisone 10 mg	3.7
7	Male	45	0.5	lt, 33 mm	IGT	NGT	none	19.7
8	Female	46	3.0	lt, 25 mm	IGT	IGT	none	16.6
9	Female	51	2.0	lt, 30 mm	IGT	NGT	hydrocortisone 5 mg	18.0
10	Female	35	1.0	rt, 22 mm	IGT	NGT	hydrocortisone 3 mg	14.0
11	Female	45	3.0	rt, 28 mm	NGT	IGT	hydrocortisone 5 mg	18.7

* Steroid supplementation at the postoperative OGTT. OGTT, oral glucose tolerance test; ACTH, adrenocorticotropic hormone; IGT, impaired glucose tolerance; NGT, normal glucose tolerance; DM, diabetes mellitus; lt, left; rt, right.

Table 2 Clinical characteristics before and after adrenalectomy

	Preoperative	Postoperative	p-value
Sex (male/female)	1/10
Age (years)	45 [37–45]
Estimated duration of CS (years)	2 [1–3]
BMI (kg/m2)	22.6 [20.6–26.2]	21.4 [19.6–24.2]	0.02
Systolic blood pressure (mmHg)	138.5 [116.8–148.0]	115.0 [102.0–130.0]	0.03
Diastolic blood pressure (mmHg)	85.0 [74.3–90.0]	74.0 [70.0–80.0]	0.03
Mean blood pressure (mmHg)*	101.0 [90.5–108.75]	87.0 [80.0–97.0]	0.02
ACTH at 8:00 AM (pg/mL)	1.7 [0.5–2.5]	30.1 [17.3–52.1]	0.002
Cortisol at 8:00 AM (μg/dL)	18.1 [17.6–21.7]	7.9 [1.9–9.5]	0.003
Cortisol at midnight (μg/dL)	20.4 [17.2–22.0]	1.1 [0.6–2.4]	0.001
Cortisol after 1-mg DST (μg/dL)	20.0 [17.2–25.1]	0.5 [0.4–0.8]	0.001
24-h urinary cortisol (μg/day)	582.0 [321.0–743.0]	31.3 [23.6–40.6]	0.001
Maximum cortisol level after 250-μg ACTH stimulation test (μg/dL)	34.2 [30.1–40.8]	15.3 [4–19.7]	0.002
DHEA-S (μg/dL)	18 [8.9–39.8]	4 [2.5–11]	0.008
Plasma aldosterone (pg/mL)	79 [59–85]	69 [48–95]	0.96
Potassium (mEq/L)	3.7 [3.4–3.9]	3.9 [3.8–4.0]	0.008
HbA1c (%)	5.8 [5.5–6.2]	5.4 [5.2–5.5]	0.06
AST (IU/L)	18.0 [16.0–23.0]	18.0 [13.0–20.0]	0.05
ALT (IU/L)	29.0 [20.0–34.0]	13.0 [9.0–17.0]	0.001
γ-GTP (IU/L)	38.0 [20.0–101.0]	13.0 [12.0–22.0]	0.001
Triglycerides (mg/dL)	106.0 [65.0–142.0]	93.0 [46.0–99.0]	0.01
HDL-cholesterol (mg/dL)	75.7 [61.0–92.0]	54.0 [45.0–66.0]	0.02
LDL-cholesterol (mg/dL)	115.0 [88.1–161.0]	95.5 [81.3–113.8]	0.13
eGFR (mL/min/1.73 m2)	93.2 [86.7–109.9]	84.8 [76.5–107.2]	0.21
Cases treated using antihypertensive drugs	6	3	0.39
Cases treated using antihyperlipidemic drugs	1	0	1.0

* Mean blood pressure was defined as diastolic pressure plus one third of the pulse pressure. Data are reported as median [interquartile range] or number. CS, Cushing’s syndrome; BMI, body mass index; ACTH, adrenocorticotropic hormone; DST, dexamethasone suppression test; DHEA-S, dehydroepiandrosterone sulfate; HbA1c, glycated hemoglobin; AST, aspartate amino transferase; ALT, alanine amino transferase; γ-GTP, g-glutamyltranspeptidase; HDL, high-density lipoprotein; LDL, low-density lipoprotein; eGFR, estimated glomerular filtration rate.

The results of the 75-g OGTTs before and after surgery

The results of the 75-g OGTTs from before and after surgery revealed a substantial postoperative improvement (NGT/IFG/IGT/IFG + IGT/DM: 2/0/8/0/1 to 8/0/3/0/0). Significant postoperative decreases were also observed for plasma glucose levels during the OGTT and the AUC-PG (312 mg/dL × h [290.3–391.3 mg/dL × h] to 241 mg/dL × h [201.3–258 mg/dL × h], p = 0.001) (Table 3 and Fig. 1). Significant postoperative decreases were observed for most IRI levels (except at 30 min) and the AUC-IRI (147.13 μU/mL × h [114.7–194.9 μU/mL × h] to 74.8 μU/mL × h [61.9–91.9 μU/mL × h], p = 0.005) (Table 3 and Fig. 1).

Table 3 Plasma glucose and IRI levels during the preoperative and postoperative 75-g OGTTs

	Preoperative	Postoperative	p-value
PG 0 min (mg/dL)	90 [81–99]	78 [74–84]	0.04
PG 30 min (mg/dL)	158 [143–181]	122 [108–136]	0.05
PG 60 min (mg/dL)	170 [163–220]	125 [86–142]	0.001
PG 90 min (mg/dL)	183 [167–222]	121 [100–136]	0.002
PG 120 min (mg/dL)	178 [153–195]	111 [104–144]	0.01
IRI 0 min (μU/mL)	11.3 [6.7–12.1]	4.72 [3.2–5.5]	0.002
IRI 30 min (μU/mL)	57.7 [25.2–111.0]	44.5 [26.4–78.2]	0.46
IRI 60 min (μU/mL)	63.6 [46.0–91.0]	35.1 [15.9–42.1]	0.005
IRI 90 min (μU/mL)	77.2 [73.9–113.0]	42.2 [21.3–44.7]	0.003
IRI 120 min (μU/mL)	113.0 [86.6–133.7]	40.7 [25.8–52.1]	0.005

Data are reported as median [interquartile range]. OGTT, oral glucose tolerance test; PG, plasma glucose; IRI, immunoreactive insulin.

Fig. 1

Indices from the 75-g OGTT before and after the adrenalectomy in patients with adrenal Cushing’s syndrome.

Data from the 75-g OGTT for the 11 cases were analyzed to show the AUC-PG (A), AUC-IRI (B), insulinogenic index (C), disposition index (D), HOMA-IR (E), and Matsuda index (F).

OGTT, oral glucose tolerance test; PG, plasma glucose; IRI, immunoreactive insulin; AUC, area under the curve; HOMA-IR, homeostasis model assessment of insulin resistance.

Insulin secretion and insulin resistance indices before and after surgery

The results of the 75-g OGTT were used to calculate the insulin secretion and insulin resistance (or sensitivity) indices (Fig. 1). There was a trend towards a postoperative increase in the insulinogenic index (0.70 [0.22–1.51] to 1.22 [0.78–1.64], p = 0.08), as well as a significant postoperative increase in the disposition index, which is a product of insulin secretion and insulin sensitivity (609.1 [237.8–1,095.2] to 1,286.0 [1,034.6–1,857.6], p = 0.002). A significant postoperative decrease was observed for HOMA-IR, which is an index of hepatic insulin resistance (2.4 [1.4–2.8] to 1.0 [0.6–1.1], p = 0.002). A significant postoperative increase was observed for the Matsuda index, which is an index of whole-body insulin sensitivity (3.0 [2.3–4.5] to 8.2 [6.3–11.4], p = 0.001). Almost all patients exhibited improvements in the disposition index, HOMA-IR, and Matsuda index.

Comparing the postoperative clinical characteristics of the NGT and IGT groups

To clarify which factors were associated with IGT after surgery, we compared the postoperative clinical characteristics of the NGT group (8 patients) and the IGT group (3 patients). No significant differences (all p > 0.05) were observed in preoperative age, disease duration, pre- and post-operative BMI, postoperative amount of hydrocortisone supplementation, postoperative maximum cortisol level after the 250-μg ACTH stimulation test, pre- and post-operative urinary cortisol, or any indices from the preoperative 75-g OGTT (Table 4). We also failed to detect significant correlations between the preoperative indices for insulin secretion and insulin resistance with the levels of urinary cortisol, dehydroepiandrosterone sulfate, plasma aldosterone, and serum potassium (data not shown).

Table 4 Comparing the NGT and IGT groups after surgery

	NGT (n = 8)	IGT (n = 3)
Preoperative age (years)	42.5 [35.5–45]	45 [38–46]
Disease duration (years)	2 [1–2.75]	3 [3–5]
Preoperative BMI (kg/m2)	23.0 [19.6–27.5]	21.8 [20.6–26.2]
Postoperative BMI (kg/m2)	21.9 [20.1–24.4]	19.7 [17.7–22.4]
Postoperative hydrocrotisone supplementation (mg)	7.5 [3.5–13.75]	5 [0–5]
Postoperative maximum cortisol level after 250-μg ACTH stimulation test (μg/dL)	10.9 [3.8–18.8]	16.6 [15.3–18.7]
Preoperative urinary cortisol (μg/day)	561.5 [245.5–894.5]	636 [156–636]
Postoperative urinary cortisol (μg/day)	30.8 [21.4–41.6]	35.1 [19.1–40.0]
Preoperative insulinogenic index	0.83 [0.53–1.39]	0.22 [0.18–2.29]
Preoperative disposition index	621.7 [303.2–1,240.4]	237.7 [130.2–1,095.1]
Preoperative HOMA-IR	2.1 [1.3–2.7]	2.5 [1.4–4.4]
Preoperative Matsuda index	3.5 [2.5–4.4]	2.0 [2.0–6.6]

Data are reported as median [interquartile range]. NGT, normal glucose tolerance; IGT, impaired glucose tolerance; BMI, body mass index; ACTH, adrenocorticotropic hormone; HOMA-IR, homeostasis model assessment of insulin resistance.

Discussion

This case series revealed that adrenalectomy could ameliorate glucose metabolism disorders in patients with adrenal CS but without overt DM, based on postoperative improvements in both insulin resistance and insulin secretion.

Nine of the 11 patients exhibited postoperative improvements in their OGTT results, with 8 patients postoperatively achieving 120-min plasma glucose levels of <140 mg/dL, which is the cut-off level for IGT [15]. Faggiano et al. have also compared pre-operative and 1-year post-operative fasting and 2-h OGTT results from 25 patients with pituitary Cushing’s disease, although they reported that the median 2-h OGTT blood glucose levels did not significantly change (9.4 mmol/L [169 mg/dL] to 8.50 mmol/L [153 mg/dL]) [7]. Interestingly, Giordano et al. have reported that the prevalence of IGT was significantly reduced after surgery for patients with adrenal adenoma, but not for patients with pituitary Cushing’s disease [8], which may indicate that the heterogeneity in the glucose metabolism changes is related to residual Cushing’s disease or other pituitary hormonal deficiencies [8]. Therefore, we only evaluated patients with adrenal CS, rather than all cases of Cushing’s disease, to better focus on the surgery-related changes in glucose metabolism without the possibility of residual disease [11].

The present study also revealed improvements in the HOMA-IR and the Matsuda index for most patients. The HOMA-IR reflects hepatic insulin resistance in the fasting state and the Matsuda index reflects whole-body (including liver, muscle, and fat) insulin sensitivity in the fasting and post-glucose loading states. The improvement in hepatic insulin resistance was probably associated with amelioration of cortisol overproduction, as glucocorticoid excess increases hepatic gluconeogenesis and lipogenesis [3]. We also observed significant decreases in liver enzyme levels and serum triglycerides levels, which suggest that the liver’s lipid contents decreased after surgery. The improvement in the Matsuda index suggests there was less insulin resistance in the skeletal muscle and adipose tissue. Although we did not have access to data regarding skeletal muscle and visceral fat masses, we detected a significant postoperative decrease in median waist circumference for 5 patients with available data (80 cm [75.3–91.5 cm] to 71 cm [66–80 cm], p = 0.002). A postoperative decrease in blood pressure might also affect the change in insulin resistance through decreased blood flow into the muscles [18, 25, 26]. However, we did not find a significant correlation between the change in the mean blood pressure and the change in the Matsuda index (ρ = –0.115, p = 0.41), which suggests that further investigation is needed in a larger sample.

There was also a postoperative improvement in the disposition index, which reflects β-cell function based on the change in insulin resistance that is calculated using measures of insulin sensitivity and the first phase insulin secretion. The increase in the disposition index suggested an amelioration of β-cell dysfunction after surgery, and we also detected a trend towards an improved insulinogenic index, which reflects the acute insulin secretion after glucose loading. Glucocorticoid excess causes β-cell dysfunction by increasing β-cell apoptosis and decreasing insulin exocytosis [4]. Thus, if the negative effects of glucocorticoid excess persisted after surgery, β-cell dysfunction would not improve, although our results suggest that β-cell dysfunction caused by glucocorticoid excess can be ameliorated. We suspected that IGT would not improve in patients with a long duration of CS, but the duration of CS was not associated with persistent postoperative IGT, although further analysis of a larger group of patients is likely needed to more accurately analyze this relationship. Moreover, the present study excluded patients with pre-diagnosed DM and it remains unclear whether the postoperative amelioration of β-cell dysfunction takes place in patients with overt DM.

Nine of our patients required hydrocortisone supplementation at the postoperative OGTT. Thus, as hydrocortisone usually exhibits 100% cross-reactivity in serum or urine cortisol assays, the 24-h urine cortisol levels did not reflect the pure adrenocortical function in our patients. We also evaluated the postoperative adrenal function using a 250-μg ACTH stimulation test while pausing the hydrocortisone supplementation. As shown in Table 1, 7 patients had a low cortisol response to the ACTH stimulation test, based on a maximum cortisol level of <18 μg/dL [24]. Given that most of these patients received ≥10 mg of supplemental hydrocortisone, more than enough steroid supplementation might have decreased their endogenous cortisol response to the ACTH. While insufficient cortisol production may increase insulin sensitivity [27], we did not observe significant differences in the insulin resistance or secretion indices between patients with low or normal cortisol responses to the ACTH stimulation test (all p > 0.05, Supplementary Table 1). We also did not find any significant postoperative differences between the NGT and IGT groups in terms of the postoperative maximum cortisol level after the ACTH stimulation test, urinary cortisol level, or the amount of steroid supplementation (Table 4).

This study has several limitations. First, the small sample size is associated with risks of bias. Second, we assessed insulin secretion and insulin resistance using OGTT indices, although the insulin clamp method is the gold standard for analyzing insulin resistance. In addition, other tests, such as the glucagon loading test, should be performed to more accurately evaluate β-cell function. Third, the interval between the preoperative and postoperative tests was variable (1–3 years), and a prospective analysis should be performed to evaluate patients under standardized conditions.

In conclusion, our data revealed that glucose metabolism disorders were ameliorated after adrenalectomy for patients with adrenal CS but without overt DM. Improvements were also observed in both insulin resistance and insulin secretion. These findings provide new insights into the pathophysiology of glucose metabolism disorders in patients with CS.

Acknowledgements

This work was partly supported by the Grant for Research on Intractable Diseases provided by the Japanese Ministry of Health, Labour and Welfare.

Disclosures

None of the authors have any potential conflicts of interest associated with this research.

References

• 1   Arnaldi  G,  Angeli  A,  Atkinson  AB,  Bertagna  X,  Cavagnini  F, et al. (2003) Diagnosis and complications of Cushing’s syndrome: a consensus statement. J Clin Endocrinol Metab 88: 5593–5602.
• 2  Pivonello R, De Leo M, Vitale P, Cozzolino A, Simeoli C, et al. (2010) Pathophysiology of diabetes mellitus in Cushing’s syndrome. Neuroendocrinology 92(Suppl. 1): 77–81.
• 3   Scaroni  C,  Zilio  M,  Foti  M,  Boscaro  M (2017) Glucose metabolism abnormalities in Cushing syndrome: from molecular basis to clinical management. Endocr Rev 38: 189–219.
• 4   Mazziotti  G,  Gazzaruso  C,  Giustina  A (2011) Diabetes in Cushing syndrome: basic and clinical aspects. Trends Endocrinol Metab 22: 499–506.
• 5   Otsuki  M,  Kitamura  T,  Tamada  D,  Tabuchi  Y,  Mukai  K, et al. (2016) Incompatibility between fasting and postprandial plasma glucose in patients with Cushing’s syndrome. Endocr J 63: 1017–1023.
• 6   Giordano  C,  Guarnotta  V,  Pivonello  R,  Amato  MC,  Simeoli  C, et al. (2014) Is diabetes in Cushing’s syndrome only a consequence of hypercortisolism. Eur J Endocrinol 170: 311–319.
• 7   Faggiano  A,  Pivonello  R,  Spiezia  S,  De Martino  MC,  Filippella  M, et al. (2003) Cardiovascular risk factors and common carotid artery caliber and stiffness in patients with Cushing’s disease during active disease and 1 year after disease remission. J Clin Endocrinol Metab 88: 2527–2533.
• 8   Giordano  R,  Picu  A,  Marinazzo  E,  D’Angelo  V,  Berardelli  R, et al. (2011) Metabolic and cardiovascular outcomes in patients with Cushing’s syndrome of different aetiologies during active disease and 1 year after remission. Clin Endocrinol (Oxf) 75: 354–360.
• 9   Sharma  ST,  Nieman  LK,  Feelders  RA (2015) Cushing’s syndrome: epidemiology and developments in disease management. Clin Epidemiol 7: 281–293.
• 10   Loriaux  DL (2017) Diagnosis and differential diagnosis of Cushing’s syndrome. N Engl J Med 376: 1451–1459.
• 11   Brunt  LM,  Moley  JF,  Doherty  GM,  Lairmore  TC,  DeBenedetti  MK, et al. (2001) Outcomes analysis in patients undergoing laparoscopic adrenalectomy for hormonally active adrenal tumors. Surgery 130: 629–635.
• 12   Woo  YS,  Isidori  AM,  Wat  WZ,  Kaltsas  GA,  Afshar  F, et al. (2005) Clinical and biochemical characteristics of adrenocorticotropin-secreting macroadenomas. J Clin Endocrinol Metab 90: 4963–4969.
• 13   Mancini  T,  Porcelli  T,  Giustina  A (2010) Treatment of Cushing disease: overview and recent findings. Ther Clin Risk Manag 6: 505–516.
• 14   Nieman  LK,  Biller  BMK,  Findling  JW,  Newell-Price  J,  Savage  MO, et al. (2008) The diagnosis of Cushing’s syndrome: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 93: 1526–1540.
• 15   Seino  Y,  Nanjo  K,  Tajim  N,  Kadowaki  T,  Kashiwagi  A, et al. (2010) Report of the committee on the classification and diagnostic criteria of diabetes mellitus. J Diabetes Investig 1: 212–228.
• 16   Akehi  Y,  Kawate  H,  Murase  K,  Nagaishi  R,  Nomiyama  T, et al. (2013) Proposed diagnostic criteria for subclinical Cushing’s syndrome associated with adrenal incidentaloma. Endocr J 60: 903–912.
• 17   Soyama  A,  Nishikawa  T,  Ishizuka  T,  Ito  H,  Saito  J, et al. (2005) Clinical usefulness of the thickness of preperitoneal and subcutaneous fat layer in the abdomen estimated by ultrasonography for diagnosing abdominal obesity in each type of impaired glucose tolerance in man. Endocr J 52: 229–236.
• 18   Tsurutani  Y,  Sugisawa  C,  Ishida  A,  Inoue  K,  Saito  J, et al. (2017) Aldosterone excess may inhibit insulin secretion: a comparative study on glucose metabolism pre- and post-adrenalectomy in patients with primary aldosteronism. Endocr J 64: 339–346.
• 19   Phillips  DI,  Clark  PM,  Hales  CN,  Osmond  C (1994) Understanding oral glucose tolerance: comparison of glucose or insulin measurements during the oral glucose tolerance test with specific measurements of insulin resistance and insulin secretion. Diabet Med 11: 286–292.
• 20   Matthews  DR,  Hosker  JP,  Rudenski  AS,  Naylor  BA,  Treacher  DF, et al. (1985) Homeostasis model assessment: insulin resistance and β-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 28: 412–419.
• 21   Matsuda  M,  DeFronzo  RA (1999) Insulin sensitivity indices obtained from oral glucose tolerance testing: comparison with the euglycemic insulin clamp. Diabetes Care 22: 1462–1470.
• 22   Sato  Y,  Oka  R,  Nakasone  Y,  Katakura  M,  Yamauchi  K, et al. (2015) Impact of one-hour postchallenge glucose on the relationship between insulin sensitivity and secretion. Endocr J 62: 573–583.
• 23   Stumvoll  M,  Mitrakou  A,  Pimenta  W,  Jenssen  T,  Yki-Järvinen  H, et al. (2000) Use of the oral glucose tolerance test to assess insulin release and insulin sensitivity. Diabetes Care 23: 295–301.
• 24   Yanase  T,  Tajima  T,  Katabami  T,  Iwasaki  Y,  Tanahashi  Y, et al. (2016) Diagnosis and treatment of adrenal insufficiency including adrenal crisis: a Japan Endocrine Society clinical practice guideline. Endocr J 63: 765–784.
• 25   Laakso  M,  Edelman  SV,  Brechtel  G,  Baron  AD (1990) Decreased effect of insulin to stimulate skeletal muscle blood flow in obese man. A novel mechanism for insulin resistance. J Clin Invest 85: 1844–1852.
• 26   Laakso  M,  Edelman  SV,  Brechtel  G,  Baron  AD (1992) Impaired insulin-mediated skeletal muscle blood flow in patients with NIDDM. Diabetes 41: 1076–1083.
• 27   Christiansen  JJ,  Djurhuus  CB,  Gravholt  CH,  Iversen  P,  Christiansen  JS, et al. (2007) Effects of cortisol on carbohydrate, lipid, and protein metabolism: studies of acute cortisol withdrawal in adrenocortical failure. J Clin Endocrinol Metab 92: 3553–3559.