Abstract
The present study aimed to establish new reference intervals (RIs) for serum free triiodothyronine (fT3), free thyroxine (fT4), and thyroid stimulating hormone (TSH) levels in Japanese children and adolescents aged 4 to 19 years. A total of 2,036 (1,611 girls, 425 boys) participants were included over a 17-year period; they all tested negative for antithyroid antibodies (TgAb, TPOAb) and were found to have no abnormalities on ultrasonography. RIs were determined by nonparametric methods. The results showed that serum fT3 was significantly higher in the 4–15-year-olds than in the 19-year-olds. The serum fT4 was significantly higher in the 4–10-year-olds than in the 19-year-olds. The serum TSH was significantly higher in the 4–12-year-olds than in the 19-year-olds. All of them gradually decreased with age to approximate the adult levels. The upper limit of TSH was lower in those aged 13 to 19 years than in adults. The differences were examined by sex. The serum fT3 was significantly higher in boys than in girls between the ages of 11 and 19 years. The serum fT4 was significantly higher in boys than in girls between the ages of 16 and 19 years. There did not seem to be any sex difference in those under 10 years of age. In conclusion, serum fT3, fT4, and TSH levels in children and adolescents differ from those in adults. It is important to evaluate thyroid function using the new RIs that are appropriate for chronological age.
THE REFERENCE INTERVALS (RIS) of free triiodothyronine (fT3), free thyroxine (fT4), and thyroid stimulating hormone (TSH) levels have been reported to be different in children and adolescents than in adults [1-19]. Assessing children and adolescents’ thyroid status using adult RIs may lead to incorrect assessments. Therefore, it is crucial to establish the RIs of children and adolescents. The serum hormone levels of fT3, fT4, and TSH have been shown to vary by age, sex, weight, ethnicity, assay method, and iodine intake [1-25]. It has been recommended that RIs be established for each country. The serum concentrations of fT3, fT4, and TSH in Japanese children were first reported in the 1980s [26, 27] and subsequently in 1996 by the Pediatric Reference Interval Study Group in a large-scale study [28]. FT3 and fT4 levels were determined by radioimmunoassay (RIA) kits, and TSH levels were determined by RIA or immunoradiometric assay (IRMA). Recently, the ECLIA assay kit has been mainly used in Japan. We reported the RIs of 342 healthy children between 4 and 15 years of age in 2013 measured by the ECLIA assay for the first time in Japan. The results showed that the RIs of fT3, fT4, and TSH in children varied by age group [6]. Recently, it has been difficult to recruit healthy children as subjects. Nine years have elapsed since then, and the number of subjects has increased. We conducted a new study to establish RIs in Japanese children, including adolescents aged 16–19 years.
Subjects and Methods
Subjects
All 15,435 children and adolescents aged 4 to 19 years who were initially examined at Ito Hospital between January 2003 and September 2020 were reviewed. All subjects underwent ultrasonography and blood tests including thyroid antibodies, and 2,036 (1,611 girls, 425 boys) fulfilling the following criteria were enrolled in this study: no thyroid nodules, including micronodules, and normal thyroid patterns, free of abnormalities such as thyroid enlargement, atrophy, or abnormal echogenicity, as defined by the Japan Society of Ultrasonics in Medicine, on ultrasound examination of the thyroid gland [29]; and serologically negative for antithyroid antibodies (TPOAb and TgAb). Subjects with congenital anomalies of the thyroid gland, with conditions or concomitant medications likely to affect thyroid function, eating disorders, and pituitary disease were excluded from participation. The C28-A3 document from the Clinical and Laboratory Standards Institute (CLSI) recommends establishing a reference interval by collecting samples from a sufficient number of qualified reference individuals to yield a minimum of 120 samples for analysis, by nonparametric means, for each partition [30]. As subjects aged under 14 years were relatively few, the subjects were divided into the following groups by age: 4–6 years, 7–8 years, 9–10 years, 11–12 years, 13–14 years, and the rest. The study was approved by the Ethics Committee of Ito Hospital, Tokyo, Japan, and was conducted with the informed consent of the subjects and their families, with meticulous care taken to protect the confidentiality of individual subjects, in conformity with the Declaration of Helsinki.
Methods
Serum fT3, fT4, and TSH concentrations were measured by ECLIA assay (ECLusys fT3, and ECLusys fT4, ECLusys TSH, respectively; Roche Diagnostics, Basel, Switzerland). Their RIs for adults are: fT3 2.2–4.3 pg/mL, fT4 0.8–1.6 ng/dL, and TSH 0.2–4.5 μU/mL. The traceability of the reagents for fT3, fT4, and TSH during the study period was good, and there were no significant changes in the test results. TgAb and TPOAb were measured using RIA assay kits (TgAb Cosmic II and TPOAb Cosmic II; Cosmic Co., Tokyo, Japan) from January 2003 to May 2006. RIs for these two parameters were set as follows: TgAb <2.6 U/mL and TPOAb <6.7 U/mL. Subsequently, they were determined by ECLIA using an ECLusys Anti-Tg and Anti-TPO (Roche Diagnostics) since June 2007. RIs were based on TPO Ab ≤28 IU/mL and TgAb ≤40 IU/mL.
Statistical analysis
Statistical evaluation of the data was performed using JMP, version 14.2.0 (SAS Institute Inc., Cary, NC). The fT3 and fT4 values were normally distributed. The reference limits are expressed the 2.5th to 97.5th percentiles and median as 50th percentile, respectively. Medians and 2.5th to 97.5th percentiles for each variable were determined as the RI. Analysis of thyroid hormone levels between groups was performed using Mann-Whitney U test. Steel’s multiple comparison test was used to determine the significance of between-group differences compared to the 19-year-old age group. The level of significance was set at p < 0.05.
Results
The characteristics of the subjects enrolled in the study are shown in Table 1. Sixty-six percent of the subjects were female. The mean age at the time of entry was 15.0 ± 3.9 years. Twenty-seven percent of subjects had a family history of autoimmune thyroid disease, Graves’ disease and/or chronic thyroiditis. The number of participants, grouped by age, is shown in Table 2.
Table 1
Baseline characteristics
| n |
2,036 |
| Age (y) |
15.0 ± 3.9 |
| Girls |
1,611 (79%) |
| Boys |
425 (21%) |
| Family history |
Thyroid autoimmune disease (Graves’ disease and/or chronic thyroiditis) |
549 (27%) |
|
Thyroid nodules |
119 (5.8%) |
|
Others* |
114 (5.5%) |
|
None |
1,254 (62%) |
Plus-minus values are means ± SD and n (%).
* Thyroid function abnormalities of unknown origin, cretinism, multiple endocrine neoplasia, etc.
Table 2
Numbers by age group
| Age (y) |
n |
Girls |
Boys |
| 4–6 |
104 |
51 |
53 |
| 7–8 |
92 |
55 |
37 |
| 9–10 |
112 |
67 |
45 |
| 11–12 |
174 |
114 |
60 |
| 13–14 |
204 |
141 |
63 |
| 15 |
153 |
133 |
20 |
| 16 |
240 |
199 |
41 |
| 17 |
261 |
227 |
34 |
| 18 |
334 |
301 |
33 |
| 19 |
362 |
323 |
39 |
| Total |
2,036 |
1,611 |
425 |
The fT3, fT4, and TSH concentrations were compared between those with and without a family history of autoimmune thyroid disease (Table 3). There were no significant differences between those with and without a family history of autoimmune thyroid disease except for fT4 for 18-year-olds. Therefore, subjects with a family history were included for the establishment of the RIs.
Table 3
Comparison of thyroid hormone and thyroid stimulating hormone levels with and without a family history
| fT3 (pg/mL) |
fT4 (ng/dL) |
TSH (μU/mL) |
| Age Group (y) |
Family history (–) |
n |
Family history (+) |
n |
p-Value |
Family history (–) |
n |
Family history (+) |
n |
p-Value |
Family history (–) |
n |
Family history (+) |
n |
p-Value |
| 4–6 |
2.96–5.04 (4.10) |
63 |
3.00–5.18 (4.10) |
41 |
0.51 |
1.12–1.68 (1.33) |
63 |
1.05–1.70 (1.34) |
41 |
0.76 |
0.62–4.83 (2.36) |
63 |
0.80–4.50 (2.74) |
41 |
0.78 |
| 7–8 |
2.98–5.10 (4.15) |
50 |
3.01–5.08 (4.20) |
42 |
0.72 |
1.06–1.81 (1.33) |
50 |
1.00–1.67 (1.32) |
42 |
0.70 |
0.67–4.85 (2.00) |
50 |
0.63–5.20 (2.18) |
42 |
0.81 |
| 9–10 |
3.10–5.13 (4.10) |
67 |
3.30–5.07 (4.00) |
45 |
0.93 |
0.96–1.63 (1.32) |
67 |
0.97–1.59 (1.30) |
45 |
1.00 |
0.78–4.68 (2.10) |
67 |
0.64–5.81 (1.98) |
45 |
0.95 |
| 11–12 |
3.10–4.87 (4.00) |
93 |
2.81–4.90 (3.80) |
81 |
0.19 |
0.94–1.57 (1.23) |
93 |
0.99–1.58 (1.24) |
81 |
0.62 |
0.55–4.50 (1.69) |
93 |
0.67–3.93 (1.72) |
81 |
0.91 |
| 13–14 |
2.54–4.70 (3.50) |
147 |
2.84–5.06 (3.65) |
56 |
0.09 |
0.95–1.63 (1.26) |
147 |
0.96–1.67 (1.21) |
57 |
0.83 |
0.47–3.60 (1.39) |
147 |
0.24–4.06 (1.34) |
57 |
0.92 |
| 15 |
2.20–4.10 (3.20) |
104 |
2.10–4.38 (3.10) |
49 |
0.30 |
0.95–1.53 (1.28) |
104 |
0.91–1.91 (1.27) |
49 |
0.46 |
0.46–3.74 (1.27) |
104 |
0.31–6.20 (1.54) |
49 |
0.07 |
| 16 |
2.20–4.05 (3.10) |
179 |
246–4.19 (3.10) |
61 |
0.84 |
0.94–1.60 (1.25) |
179 |
0.96–1.78 (1.23) |
61 |
0.49 |
0.25–4.18 (1.31) |
179 |
0.50–5.38 (1.27) |
61 |
0.74 |
| 17 |
2.30–4.10 (3.20) |
199 |
2.62–4.24 (3.10) |
62 |
0.69 |
0.98–1.59 (1.24) |
199 |
0.92–1.65 (1.27) |
62 |
0.31 |
0.25–4.43 (1.23) |
199 |
0.45–3.53 (1.21) |
62 |
0.88 |
| 18 |
2.10–4.10 (3.10) |
280 |
2.15–4.19 (3.15) |
54 |
0.32 |
0.92–1.58 (1.26) |
280 |
0.96–1.61 (1.31) |
54 |
0.03 |
0.32–3.61 (1.33) |
280 |
0.03–4.38 (1.20) |
54 |
0.46 |
| 19 |
2.00–4.00 (3.10) |
304 |
1.95–4.07 (3.10) |
57 |
0.40 |
0.94–1.66 (1.27) |
305 |
0.89–1.69 (1.25) |
57 |
0.41 |
0.33–3.67 (1.27) |
305 |
0.09–4.34 (1.38) |
57 |
0.49 |
Data listed are 2.5th percentile–97.5th percentile (median).
The RIs of the age groups are presented in Table 4. All fT3, fT4, and TSH concentrations differed depending on the age group.
Table 4
Reference intervals by age group
| Age Group (y) |
n |
fT3 (pg/mL) |
fT4 (ng/dL) |
TSH (μU/mL) |
| 4–6 |
104 |
3.00–5.04 (4.10) |
1.11–1.68 (1.33) |
0.68–4.77 (2.56) |
| 7–8 |
92 |
3.03–5.10 (4.20) |
1.05–1.69 (1.33) |
0.68–5.01 (2.07) |
| 9–10 |
112 |
3.27–5.10 (4.05) |
0.96–1.59 (1.31) |
0.80–4.69 (2.08) |
| 11–12 |
174 |
3.04–4.90 (3.90) |
0.99–1.57 (1.24) |
0.60–4.19 (1.71) |
| 13–14 |
204 |
2.80–4.79 (3.50) |
0.95–1.63 (1.24) |
0.42–3.75 (1.38) |
| 15 |
153 |
2.20–4.30 (3.20) |
0.95–1.66 (1.28) |
0.40–3.49 (1.28) |
| 16 |
240 |
2.30–4.10 (3.10) |
0.94–1.60 (1.25) |
0.29–4.02 (1.30) |
| 17 |
261 |
2.36–4.15 (3.20) |
0.97–1.59 (1.25) |
0.29–3.98 (1.22) |
| 18 |
334 |
2.14–4.10 (3.10) |
0.93–1.58 (1.27) |
0.30–3.62 (1.29) |
| 19 |
362 |
2.00–4.00 (3.10) |
0.91–1.68 (1.27) |
0.33–3.70 (1.30) |
| Adults |
|
2.20–4.30 |
0.80–1.60 |
0.20–4.50 |
Data listed are 2.5th percentile–97.5th percentile (median). For fT3, one of the 13–14-year-olds and one of the 19-year-olds were not measured.
For fT3, the RI in the 4–14-year-olds was significantly higher than in the 19-year-olds as the control group. The upper limits of the RIs showed a decreasing trend with age, but in the 4–14-year-olds, in whom the value of the upper limit was higher than the upper limit of adults. The highest values of upper limits were seen in the 7–8-year-olds and the 9–10-year-olds. The lower limit of the RI was also higher in the 4–14-year-olds than the lower limits of adults. The lower limit of the post 15-year-old age group was the same as for adults (Fig. 1A, Table 4).
For fT4, the RI in the 4–10-year-olds was significantly higher than in the 19-year-olds as the control group. The upper limits of the RIs in the 4–8-year-olds were higher than the upper limit of adults. The highest values of upper limits were seen in the 7–8-year-olds. The lower limit of the RI in all groups remained higher than the lower limit of adults (Fig. 1B, Table 4).
For TSH, the RI in the 4–12-year-olds was significantly higher than in the 19-year-olds as the control group. The upper limit of the RI in the 4–10-year-olds was higher than the upper limit of adults, and that of the post-11-year-olds was rather lower than that of adults. The lower limits of the RIs tended to decrease with age, but the lower limits of the RIs in all groups remained higher than the lower limit of adults (Fig. 1C, Table 4).
Second, the RIs of fT3, fT4, and TSH were analyzed by sex (Fig. 2).
For fT3, the RI of girls in the 4–14-year-olds was significantly higher than in the 19-year-olds as the control group. The RI of boys in the 4–15-year-olds was significantly higher than in the 19-year-olds.The upper limits of girls remained higher in the 4–12-year-olds. However, the values of post-13-year-olds were lower than those of the adults. In contrast, the upper limits of boys remained higher from 4 years to 18 years. The lower limit of girls remained higher in the 4–17-year-olds, and that of 18–19-year-olds was lower than that of adults. The lower limit of boys remained higher from 4 years to 19 years (Fig. 2A).
For fT4, the RI of girls in the 4–8-year-olds was significantly higher than in the 19-year-olds. The RIs of boys in the 7–8-year-olds, the 11–12-year-olds, and the 13–14-year-olds were significantly lower than in the 19-year-olds. The peak upper limit of girls was seen in the 7–8-year-olds, and in the post-11-year-olds, the upper limit of girls was slightly lower than that of adults. The upper limits of boys remained higher from 4 years to 19 years. The peak lower limit of girls was the highest in the 4–6-year-olds (Fig. 2B).
For TSH, the RIs of boys and girls in the 4–12-year-olds were significantly higher than in the 19-year-olds. The lower limit of girls of the 16-year-olds was the lowest; that of boys of the 18-year-olds was the lowest (Fig. 2C).
The comparisons of hormone levels between girls and boys in different age groups are shown in Table 5. The fT3 was higher in boys than girls after the 11–12-year-olds. The fT4 was also higher in boys than girls in the 9–10-year-old and the 16–19-year-olds. TSH was significantly higher in boys than in girls in the 13–15-year-olds. There were no significant differences in TSH in the other groups.
Table 5
Comparison of thyroid hormone and thyroid stimulating hormone levels of girls and boys by age group
| fT3 (pg/mL) |
fT4 (ng/dL) |
TSH (μU/mL) |
| Age Group (y) |
Girls |
n |
Boys |
n |
p-Value |
Girls |
n |
Boys |
n |
p-Value |
Girls |
n |
Boys |
n |
p-Value |
| 4–6 |
3.15–5.14 (4.10) |
51 |
2.94–4.99 (4.20) |
53 |
0.83 |
1.12–1.70 (1.35) |
51 |
1.07–1.68 (1.32) |
53 |
0.40 |
0.61–4.83 (2.28) |
51 |
1.07–4.77 (2.79) |
53 |
0.17 |
| 7–8 |
3.04–5.10 (4.20) |
55 |
2.90–4.80 (4.10) |
37 |
0.46 |
1.02–1.79 (1.35) |
55 |
1.04–1.67 (1.29) |
37 |
0.06 |
0.62–5.13 (1.88) |
55 |
0.90–4.86 (2.13) |
37 |
0.29 |
| 9–10 |
3.50–5.13 (4.10) |
67 |
3.10–5.07 (3.90) |
45 |
0.05 |
0.96–1.62 (1.28) |
67 |
0.98–1.61 (1.36) |
45 |
0.008 |
0.75–5.50 (2.08) |
67 |
0.71–4.54 (2.09) |
45 |
0.69 |
| 11–12 |
2.89–4.90 (3.80) |
114 |
3.35–5.00 (4.20) |
60 |
<0.0001 |
0.98–1.49 (1.24) |
114 |
0.95–1.64 (1.24) |
60 |
0.14 |
0.54–4.35 (1.64) |
114 |
0.68–3.94 (1.97) |
60 |
0.08 |
| 13–14 |
2.51–4.20 (3.35) |
140 |
3.16–5.04 (4.10) |
63 |
<0.0001 |
0.95–1.55 (1.23) |
141 |
0.94–1.72 (1.27) |
63 |
0.08 |
0.39–3.69 (1.28) |
141 |
0.49–4.15 (1.67) |
63 |
0.0002 |
| 15 |
2.20–3.90 (3.10) |
133 |
3.00–4.80 (3.75) |
20 |
<0.0001 |
0.93–1.57 (1.27) |
133 |
1.04–1.95 (1.34) |
20 |
0.06 |
0.39–3.02 (1.27) |
133 |
0.76–7.84 (1.79) |
20 |
0.013 |
| 16 |
2.30–3.80 (3.10) |
199 |
2.71–4.30 (3.70) |
41 |
<0.0001 |
0.93–1.51 (1.22) |
199 |
1.10–1.86 (1.41) |
41 |
<0.0001 |
0.26–3.91 (1.28) |
199 |
0.35–7.75 (1.35) |
41 |
0.35 |
| 17 |
2.30–3.80 (3.10) |
227 |
2.70–4.80 (3.60) |
34 |
<0.0001 |
0.95–1.57 (1.23) |
227 |
1.00–1.79 (1.37) |
34 |
<0.0001 |
0.28–3.94 (1.21) |
227 |
0.41–4.83 (1.31) |
34 |
0.68 |
| 18 |
2.10–3.90 (3.10) |
301 |
2.90–4.80 (3.70) |
33 |
<0.0001 |
0.93–1.58 (1.26) |
301 |
1.03–1.75 (1.44) |
33 |
<0.0001 |
0.30–3.47 (1.26) |
301 |
0.25–3.85 (1.46) |
33 |
0.10 |
| 19 |
2.00–3.80 (3.00) |
322 |
2.90–4.30 (3.50) |
39 |
<0.0001 |
0.91–1.59 (1.25) |
323 |
1.04–1.80 (1.40) |
39 |
<0.0001 |
0.33–3.81 (1.28) |
323 |
0.27–3.51 (1.52) |
39 |
0.13 |
Data listed are 2.5th percentile–97.5th percentile (median)
Discussion
In this study, the RIs of fT3, fT4, and TSH levels were established in 4–19-year-olds using the data accumulated over 17 years. This report provides guidance for appropriate diagnosis and medical care.
We reported the RIs of 4–15-year-old children on the basis of testing 342 children in 2013. The RIs in children differed from those in adults by age [6]. Due to ethical issues, it is very difficult to recruit young, healthy participants. Thus, we expanded our study to include 16–19-year-olds and resurveyed the RIs in children and adolescents. In defining healthy subjects, we directly took their medical and family histories and performed ultrasonography on all of them. Whereas most studies did not measure autoantibodies or only measured TPOAb [2, 3, 5, 7, 9-14, 16-19, 25], both TPOAb and TgAb were measured, and those who tested positive were excluded. Besides, our hospital has reserved QC records of the laboratory. And our hospital obtained ISO 15189 accreditation from the Japan Board for Conformity Assessment (the Public Interest Incorporated Association) in 2013. Although the CVs of the immunoassay test is prone to fluctuation, the CVs of TSH, fT3, and fT4 were almost less than 5% at our hospital over the 17-year period. IFCC (International Federation of Clinical Chemistry and Laboratory Medicine) organized C-STFT (Committee for Standardization of Thyroid Function Tests) and has been leading the harmonization of TSH measurements worldwide. Recently, Japan also decided to measure each kit harmonized in TSH. The policy of the Standardization Committee of the Japanese Society of Laboratory Medicine regarding the harmonization of TSH values is to multiply the actual measured values by a correction factor to make the measured values conform to the IFCC standard-compliant test values (Phase IV). The list of a correction factor of each kit with harmonization is showed on the website of the Japan Endocrine Society. Our Laboratory instruments in this paper were Roche, with a correction of 0.98–1.00, and our results were considered usable as is. Thus, it was possible to establish RIs in healthy children more strictly than in other studies.
The results showed that the serum fT3 levels were significantly higher in the 4–14-year-olds than in the 19-year-olds and gradually decreased with age to approximate the adult level. The serum fT4 levels were significantly higher in the 4–10-year-olds than in the 19-year-olds and decreased with age. The serum TSH was significantly higher in the 4–12-year-olds than in the 19-year-olds and decreased with age. The upper limit of TSH varied widely and was lower than that of the RI for adults in those aged 11 to 19 years.
We have previously investigated the RIs in the Japanese population aged 20–80 years [31]. In combination with this study, it appears that TSH in the Japanese population is at its highest level in early childhood, decreases in adolescence, rises in adulthood, and then increases at over 70 years of age. In this report, there are notable mismatches of 19-year-olds RIs (for fT3, fT4, and TSH) from those of adult RIs. According to our previous report [31], TSH was (n = 477) 0.39–4.29 (median 1.30) for 20- to 29-year-olds and fT4 was (n = 477) 0.91–1.58 (median 1.25) for 20- to 29-year-olds, which were close to the RIs for 19-year-olds. The present study aimed to focus on the difference in RIs between children and adults RIs for not misdiagnosing childhood and adolescents thyroid diseases. Therefore we used the comparison with the RIs of adults.
The reports of fT3 levels in children have been relatively few, and the present results were consistent with the other reports, in that the level was high in early childhood and tended to decrease with age [2, 5, 8, 11, 18, 19]. FT4 and TSH were also high in childhood and decreased with age, similar to reports from other countries [2, 4, 5, 7-10, 12, 16-20]. The serum fT3 concentration reported by Sonia et al. was higher than in the present study [8], but it was measured by mass spectrometry, and 96% of the subjects were Caucasian. In other reports, the fT3 levels were almost the same as in the present study [2, 5, 11, 18, 19]. The serum fT4 concentration did not seem to be much different in each report. As for TSH, it seems that the serum TSH level of the Japanese population is lower than of Chinese and Korean populations, which are also Asian populations [9, 19]. These differences between reports were considered to be due to ethnicity, assay methods, and iodine intakes. NHANES III showed that the TSH level was higher in whites and Mexican Americans than in blacks [1]. It has been reported that TSH levels and iodine intake are correlated among the same ethnic population [12, 20]. Therefore, it was considered essential to establish the RIs of children for each region or country.
In the present study, differences by sex were examined. It was found that fT3 was significantly higher in boys than in girls between the ages of 11 and 19 years, fT4 was significantly higher in boys than in girls in the 9–10-year-old and the 16–19-year-olds, and TSH did not show much difference. There did not seem to be any sex difference in those under 9 years of age.
Though there have been some reports of no sex differences in thyroid hormone levels [3, 11], that may be due to the small number of subjects and age groupings. The present results were consistent with other reports: a longitudinal study in Australia [14], a cohort study in the Danish/North-European population [13], and a longitudinal study in Germany [18], which found elevated fT3 concentrations that were higher in adolescent boys than in girls. For fT4, the German and Australian reports showed that boys’ levels were higher than girls’ in adolescence [14, 18], whereas the Danish study showed no difference [13]. TSH was also higher in adolescent boys than girls [14, 18], whereas the present study and the Danish data did not find many differences [13]. It seems that the differences by sex of fT4 and TSH may be less than of fT3.
The physiological reasons for the differences between boys and girls are still uncertain. Some of them may be due to growth hormone or sex hormone effects. Autopsy studies of Japanese children have shown that thyroid volume changes rapidly during puberty [27]. The changes were observed several years earlier in girls than in boys, at around 15 years of age. Furthermore, the peak IGF-1 level is also known to be a few years earlier in girls than in boys. Yamauchi et al. reported that injection of recombinant human GH increased serum fT3 levels and decreased serum fT4 levels in severe adult GH deficiency and consecutive acromegaly patients who underwent transsphenoidal surgery [32]. This effect is thought to be mediated by deiodinase 2 (D2) expression, since GH significantly increased D2 expression at the mRNA level, as well as D2 protein and its activity in vitro.
On the other hand, Surup et al. [18] analyzed thyroid hormones in five stages of sexual maturity (Tanner stages) regardless of age. For fT3, whereas the median values in girls decreased across all pubertal stages, those for males increased significantly until Tanner 3, followed by a significant decrease into post-puberty. When classified by Tanner stage, fT4 and TSH seemed to move with the same trends in boys and girls. These results may suggest that sex hormones may be responsible for part of the difference in fT3 by sex. In a study of sex steroids and thyroid hormones, transgender persons who took oral estradiol had increased TBG levels, whereas testosterone injection decreased TBG levels. In addition, administration of anti-androgens lowered the T3/T4 ratio, and administration of testosterone increased the T3/T4 ratio [33]. In a study of thyroid hormone levels over the lifespan and of differences between the sexes, both fT3 and fT4 levels were higher in men, but the difference disappeared or reversed in older age. These results may be related to decreasing sex hormone levels due to aging [21].
It is well-known that obesity is associated with a higher level of TSH. Leptin is produced by adipose tissue, so that leptin secretion increases exponentially with increasing fat mass. Leptin is the regulator of the hypothalamic-pituitary-thyroid axis by regulation of TRH gene expression in the paraventricular nucleus. In addition, leptin itself also affects thyroid deiodinase activities with T4 to T3 conversion [34, 35]. Surup et al. have shown a correlation between obesity and thyroid function in children in Germany. The BMI standard deviation score (SDS) was positively associated with the TSH SDS and the fT3 SDS and negatively associated with the fT4 SDS [18].
In the present study, it was not possible to assess the relationship between weight and thyroid function, since weight was not measured in all subjects. Analysis of the children whose weight was measured showed that the median BMI-SDS of 565 children aged 4–15 years was –0.34, and only seven children (1.2%) had a BMI of +2.0 or higher, whereas the median BMI of 676 children aged 16 years or older was 20.06 kg/m2, and only 27 children (0.4%) had a BMI of 25 kg/m2 or higher (data not shown).
In general, the number of obese children in the Japanese population was small, and it is considered difficult to evaluate the correlation between body weight and thyroid function.
The limitation of this study is that the number of participants under 10 years of age and of boys was relatively small in total. Moreover, our hospital specializes in thyroid diseases, and children are more likely to have a family history of thyroid disease than children who visit a regular facility. In addition, iodine intake was unknown.
In conclusion, the RIs of fT3, fT4, and TSH levels in children and adolescents aged 4 to 19 years, including new healthy subjects aged 16 to 19 years and using data accumulated over 17 years, were determined. The data are valuable because it is very difficult to collect data from healthy children.
Disclosure
None of the authors has any potential conflicts of interest associated with this research.
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