Hypopituitarism, characterized by reduced secretion of pituitary hormones, profoundly impacts systemic metabolic homeostasis and quality of life. Its etiology ranges from congenital anomalies in pituitary development to acquired conditions involving inflammation and autoimmune processes. Despite advances in understanding its pathogenesis, diagnostic challenges persist, particularly in cases with complex extra-pituitary manifestations or novel genetic variations. Congenital hypopituitarism often stems from disruptions in transcription factors and signaling pathways critical for pituitary organogenesis. Emerging studies employing next-generation sequencing and developmental biology techniques have revealed new genetic loci and mechanisms implicated in combined pituitary hormone deficiency. However, the pathogenesis of most congenital cases remains elusive, underscoring the need for functional and phenotypic analyses of novel variants. Acquired hypopituitarism, frequently associated with pituitary tumors or systemic diseases, has also been increasingly linked to autoimmune mechanisms. Notably, the concept of paraneoplastic autoimmune hypophysitis has emerged, highlighting malignancy-driven immune responses as a novel etiological framework. Investigations into immune checkpoint inhibitor-related hypophysitis and anti-PIT-1 hypophysitis exemplify the intricate interplay between tumor immunity and endocrine dysfunction, suggesting shared mechanisms involving ectopic antigen expression and autoimmunity. This review synthesizes recent insights into the genetic, developmental, and immunological underpinnings of hypopituitarism. By exploring both congenital and acquired etiologies, we aim to bridge gaps in the current understanding of this complex disorder and provide a foundation for improved diagnostic and therapeutic strategies. Future perspectives emphasize the integration of advanced genetic tools, deeper exploration of tumor-immunity interactions, and a heightened focus on extra-pituitary phenotypes to refine clinical practice and enhance patient outcomes.

Cushing’s disease is a rare endocrine disorder that presents many systemic complications, and its initial phase management can be difficult in atypical and severe cases or at institutes with limited experience. It is a disease in which several complications may have already progressed at the time of diagnosis, and complications may become more severe during the initial management phase, potentially becoming life-threatening. In addition, many patients are young, and depending on this phase management, their quality of life will significantly decline later on. Therefore, this review summarizes the evidence accumulated to date and outlines strategies for disease management, focusing on the initial stages from detection, diagnosis, and referral of patients to surgery.

Ghrelin produced in the stomach promotes food intake and GH secretion, and acts as an anabolic peptide during starvation. Ghrelin binds to the growth hormone secretagogue receptor, a G protein-coupled receptor (GPCR), whose high-resolution complex structures have been determined in the apo state and when bound to an antagonist. Anamorelin, a low-molecular-weight ghrelin agonist, has been launched in Japan for the treatment of cancer cachexia, and its therapeutic potential has attracted attention due to the various biological activities of ghrelin. In 2019, liver-expressed antimicrobial peptide (LEAP2), initially discovered as an antimicrobial peptide produced in the liver, was identified to be upregulated in the stomach of diet-induced obese mice after vertical sleeve gastrectomy. LEAP2 binds to the GHSR and antagonizes ghrelin’s activities. The serum concentrations of human LEAP2 are positively correlated with body mass index, body fat accumulation, and fasting serum concentrations of glucose and triglyceride. Serum LEAP2 elevated and ghrelin reduced in obesity. Ghrelin and LEAP2 regulate body weight, food intake, and GH and blood glucose concentrations, and other physiological phenomena through their interactions with the same receptor, GHSR.

Androgen is widely acknowledged to regulate skeletal muscle mass. However, the specific mechanism driving muscle atrophy resulting from androgen deficiency remains elusive. Systemic androgen receptor knockout (ARKO) mice exhibit reduction in both muscle strength and muscle mass while skeletal muscle fiber specific ARKO mice have decreased muscle strength without affecting skeletal muscle mass in the limbs. Therefore, androgens may indirectly regulate skeletal muscle mass through effects on non-myofibers. Considering this, our investigation focused on blood fluid factors that might play a role in the regulation of skeletal muscle mass under the influence of androgens. Using a male mouse model of sham, orchidectomy and DHT replacement, mass spectrometry for serum samples of each group identified epidermal growth factor receptor (EGFR) as a candidate protein involving the regulation of skeletal muscle mass affected by androgens. Egfr expression in both liver and epididymal white adipose tissue correlated with androgen levels. Furthermore, Egfr expression in these tissues was predominantly elevated in male compared to female mice. Interestingly, male mice exhibited significantly elevated serum EGFR concentrations compared to their female counterparts, suggesting a connection with androgen levels. Treatment of EGFR to C2C12 cells promoted phosphorylation of AKT and its downstream S6K, and enhanced the protein synthesis in vitro. Furthermore, the administration of EGFR to female mice revealed a potential role in promoting an increase in skeletal muscle mass. These findings collectively enhance our understanding of the complex interplay among androgens, EGFR, and the regulation of skeletal muscle mass.

This study aimed to focus on the role of radiologists in the diagnosis and management of adrenal lesions, particularly primary aldosteronism (PA) and secondary hypertension. As hypertension affects more than one-third of the population in Japan, identifying secondary causes such as PA and adrenal lesions is crucial. Establishing a radiological differential diagnosis of adrenal lesions using advanced imaging techniques, such as computed tomography and magnetic resonance imaging, is crucial. Knowledge of the imaging findings of various benign and malignant adrenal lesions, such as adrenocortical adenomas, cortisol-producing lesions, pheochromocytomas, adrenocortical carcinoma, malignant lymphoma, and metastatic tumors, is necessary. Adrenal venous sampling (AVS) plays a crucial role in accurately localizing aldosterone hypersecretion in PA, especially when imaging fails to provide a clear diagnosis. This paper details the technical aspects of AVS, emphasizing catheterization techniques, anatomical considerations, and the importance of preprocedural imaging for successful sampling. Furthermore, we explore segmental adrenal venous sampling (SAVS), a more refined technique that samples specific adrenal tributary veins, offering enhanced diagnostic accuracy, particularly for microadenomas or challenging cases that may be missed with conventional AVS. The methodology for performing SAVS, along with the interpretation criteria for successful sampling and lateralization, is also outlined. Furthermore, radiologists have initiated treatments for unilateral PA, such as radiofrequency ablation, and play an integral role in the management of adrenal lesions. Collaborative approaches across clinical departments are required to enhance patient management in medical care involving the adrenal gland.

Almost a century has passed since Plummer reported the efficacy of short-term preoperative inorganic iodine therapy for Graves’ disease in the 1920s. Since there were concerns about the escape phenomenon and exacerbation with inorganic iodine, antithyroid drugs became the mainstay of pharmacotherapy for Graves’ disease following their development in the 1940s. With regard to long-term inorganic iodine monotherapy, Trousseau reported a case in the 1860s, and several subsequent reports suggested its efficacy. Around 1930, Thompson et al. published a number of papers and concluded that long-term inorganic iodine monotherapy was useful if limited to mild cases under careful follow-up. From Japan, in 1970, Nagataki et al. reported that, of 12 patients treated with inorganic iodine, three remained eumetabolic for more than two years. Since 2014, some reports have also been published from Japan. A summary of these recent reports is given below. The starting dose of potassium iodide is around 50 mg/day, and candidate responders have mild disease, with FT4 <2.76 ng/dL (35.5 pmol/L), a small goiter, and are female and elderly. Response rates are relatively high, at 60–80%, and the remission rate is about 40%. In cases of insufficient response, changing therapy should be considered. Inorganic iodine can be used as a possible alternative if the patient experiences adverse events with antithyroid drugs and/or prefers conservative treatments, with an understanding of their efficacy and limitations. These recent reports have been published from Japan, where iodine is sufficient, and the dose of inorganic iodine is empirical and requires further study.

Cushing’s disease is associated with increased morbidity and mortality. Osilodrostat, a potent oral 11β-hydroxylase inhibitor, provided rapid, sustained mean urinary free cortisol (mUFC) normalization in Cushing’s disease patients in two Phase III studies (LINC 3, NCT02180217; LINC 4, NCT02697734). Here, we evaluate the efficacy and safety of osilodrostat in Cushing’s disease in patients of Asian origin compared with patients of non-Asian origin. Pooled data from LINC 3 and LINC 4 were analyzed. Outcomes were evaluated separately for Asian and non-Asian patients. For the analysis, 210 patients were included; 56 (27%) were of Asian origin. Median (minimum–maximum) osilodrostat dose was 3.8 (1–25) and 7.3 (1–47) mg/day in Asian and non-Asian patients, respectively. mUFC control was achieved at weeks 48 and 72 in 64.3% and 68.1% of Asian and 68.2% and 75.8% of non-Asian patients. Improvements in cardiovascular and metabolic-related parameters, physical manifestations of hypercortisolism, and quality of life were similar in both groups. Most common adverse events (AEs) were adrenal insufficiency (44.6%) in Asian and nausea (45.5%) in non-Asian patients. AEs related to hypocortisolism and pituitary tumor enlargement occurred in more Asian (58.9% and 21.4%) than non-Asian patients (40.3% and 9.1%). Of Asian and non-Asian patients, 23.2% and 13.6%, respectively, discontinued because of AEs. Asian patients with Cushing’s disease generally required numerically lower osilodrostat doses than non-Asian patients to achieve beneficial effects. Hypocortisolism-related AEs were reported in more Asian than non-Asian patients. Together, these findings suggest that Asian patients are more sensitive to osilodrostat than non-Asian patients.

Elevated Fulminant Index (FI), [plasma glucose (PG)/glycosylated hemoglobin A1c (HbA1c)], was reportedly a sensitive index to differentiate fulminant type 1 diabetes (FT1D) from non-fulminant T1D (nFT1D). Aim of this study was to describe a better, but simpler index of FT1D. 49 and 52 patients with FT1D and nFT1D, respectively, were registered, and the discriminating ability of the rounded, normalized ratio, [PG (mmol/L) – 5.0]/[HbA1c (%) – 5.0], and the original ratio, [PG (mmol/L)]/[HbA1c (%)], was compared. Normalizing the ratio significantly raised its accuracy: area under the curve for receiver operating curve, AUROC (95%CI), 0.927 (0.858–0.964) and 0.851 (0.763–0.910), respectively, with and without the normalization (p < 0.01). Rounding of the figure into [PG (mmol/L) – 5.0]/[HbA1c (%) – 5.0] did not significantly sacrifice the discriminating ability of the index. Namely, the optimal cut point of rounded and normalized GAR, 10.0, showed 89.8% sensitivity. In conclusion, rounded, normalized (rn) GAR ≥10 (the rounded optimal cut-off) could be used for the snap diagnosis of FT1D.

A 38-year-old Japanese woman with a history of abnormal thyroid function of non-autoimmune origin, pituitary endocrine tumor, and untreated diabetes mellitus was referred to our outpatient clinic when she became pregnant with twins. Physical findings consistent with Cushing’s syndrome (CS) were absent at the time of presentation. Although baseline plasma adrenocorticotropic hormone, serum cortisol, and 24-hour urinary free cortisol excretion levels were above the upper limits of normal non-pregnant reference ranges, we could not exclude a physiological increase associated with pregnancy. No medical or surgical intervention for hypercortisolism was performed during pregnancy. Spontaneous vaginal delivery resulted in the normal delivery of live twins. A diagnosis of Cushing’s disease (CD) was established when papery skin developed postpartum. Transsphenoidal surgery was performed and the hypercortisolism partially resolved post-operatively. The patient’s abnormal thyroid function also resolved. Pregnancy in women with endogenous CS is rare, with less than 300 cases reported. Most reported cases of CS during pregnancy are of adrenal origin. Only two cases of twin pregnancies with CD have been reported. Therefore, we reported the third case of CD in a twin pregnancy and reviewed the diagnostic and therapeutic challenges associated with CD during pregnancy.

Although growth hormone (GH) and prolactin (PRL) are usually recognized as pituitary hormones, their expression is not restricted to the adenohypophysis and can also be found in extra-pituitary tissues including placenta. Furthermore, GH, PRL, and their receptors structurally belong to the cytokine family of proteins, and indeed they have remarkable pleiotropic effects. In this review, we analyzed the biological roles of GH/PRL from an evolutionary perspective. We have recognized that the biological significance of GH/PRL can be summarized as follows: cytokines (metabokines) that regulate the shift of nutrients and even of whole bodies to live in the most appropriate environment(s) for conducting growth and reproduction. In this sense, the common keyword of the two metabokines is “shift” for environmental adaptation. Considering that these metabokines flexibly changed their biological roles, GH/PRL may have played important roles during vertebrate evolution.

The endometrium during the sexual cycle undergoes detachment, tissue remodeling, and differentiation during the menstrual cycle. Localized and transient destruction and regeneration of endometrial tissue are also essential for pregnancy. It is possible to attribute many causes of failure in infertility treatment to the implantation stage. To improve the success rate of plateau fertility treatment, it is important to understand the regeneration mechanism of the endometrium, a unique regenerative tissue in the human body. In association with cell proliferation, tissue remodeling requires the relocation of proliferative cells, and the steady-state epithelial cells need to be motile for the relocation. Transient add-on motile activity in epithelial cells is mediated by epithelial to mesenchymal transition (EMT) and reversible mesenchymal to epithelial transition (MET). The destruction and regeneration of endometrial tissue over a period of days to weeks requires a system with a rapid and characteristic mechanism similar to that of wound healing. Here, I review the relationship between the well-known phenomenon of EMT in wound healing and endometrial tissue remodeling during the sexual cycle and pregnancy establishment, which are automatically triggered by menstruation and embryonal invasion.

Achondroplasia (ACH) is a representative skeletal disorder characterized by rhizomelic shortened limbs and short stature. ACH is classified as belonging to the fibroblast growth factor receptor 3 (FGFR3) group. The downstream signal transduction of FGFR3 consists of STAT1 and RAS/RAF/MEK/ERK pathways. The mutant FGFR3 found in ACH is continuously phosphorylated and activates downstream signals, resulting in abnormal proliferation and differentiation of chondrocytes in the growth plate and cranial base synchondrosis. A patient registry has been developed and has contributed to revealing the natural history of ACH patients. Concerning the short stature, the adult height of ACH patients ranges between 126.7–135.2 cm for men and 119.9–125.5 cm for women in many countries. Along with severe short stature, foramen magnum stenosis and spinal canal stenosis are major complications: the former leads to sleep apnea, breathing disorders, myelopathy, hydrocephalus, and sudden death, and the latter causes pain in the extremities, numbness, muscle weakness, movement disorders, intermittent claudication, and bladder-rectal disorders. Growth hormone treatment is available for ACH only in Japan. However, the effect of the treatment on adult height is not satisfactory. Recently, the neutral endopeptidase-resistant CNP analogue vosoritide has been approved as a new drug for ACH. Additionally in development are a tyrosine kinase inhibitor, a soluble FGFR3, an antibody against FGFR3, meclizine, and the FGF2-aptamer. New drugs will bring a brighter future for patients with ACH.

The pituitary gland is endocrine tissue composed of two distinct parts with different origins: the adenohypophysis (adenohypophyseal placode origin) and the neurohypophysis (neuroectoderm origin). Differentiation of endocrine cells in the pituitary gland leads to hormone synthesis, secretion into the capillary network, and transportation to target organs. In 1988, the discovery of the pituitary transcription factor PIT1 sparked research on endocrine cell differentiation. In the twenty-first century, the discovery that SOX2-positive stem/progenitor cells give rise to all types of pituitary endocrine cells advanced research on differentiation processes using diverse marker molecules. Lineage tracing using specific marker genes from early embryos revealed that during construction of the anterior pituitary from the adenohypophyseal placodal cells the developing anterior pituitary incorporates diverse cell types originating from the neural crest-derived and ectodermal-derived cells. Consequently, the postnatal anterior pituitary becomes a mosaic of terminally differentiated cells of different origin and with different life histories. It has also been revealed that most of the postnatal stem/progenitor cells form at least solid clusters in the parenchyma. Moreover, the classification and role of S100β-positive cells had been ambiguous, but now they are identified as a major component of postnatal stem/progenitor cells. This paper provides an updated overview of pituitary development.

Androgens play a vital role not only in promoting the development of male sexual characteristics but also in exerting diverse physiological effects, including the regulation of skeletal muscle growth and function. Given that the effects of androgens are mediated through androgen receptor (AR) binding, an understanding of AR functionality is crucial for comprehending the mechanisms of androgen action on skeletal muscles. Drawing from insights gained using conditional knockout mouse models facilitated by Cre/loxP technology, we review the cell-specific functions of AR in skeletal muscles. We focus on three specific cell populations expressing AR within skeletal muscles: skeletal muscle cells, responsible for muscle contraction; satellite cells, which are essential stem cells contributing to the growth and regeneration of skeletal muscles; and mesenchymal progenitors, situated in interstitial areas and playing a crucial role in muscle homeostasis. Furthermore, the indirect effects of androgens on skeletal muscle through extra-muscle tissue are essential, especially for the regulation of skeletal muscle mass. The regulation of genes by AR varies across different cell types and contexts, including homeostasis, regeneration and hypertrophy of skeletal muscles. The varied mechanisms orchestrated by AR collectively influence the physiology of skeletal muscles.

Bone secrets the hormone, fibroblast growth factor 23 (FGF23), as an endocrine organ to regulate blood phosphate level. Phosphate is an essential mineral for the human body, and around 85% of phosphate is present in bone as a constituent of hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂. Because hypophosphatemia induces rickets/osteomalacia, and hyperphosphatemia results in ectopic calcification, blood phosphate (inorganic form) level must be regulated in a narrow range (2.5 mg/dL to 4.5 me/dL in adults). However, as yet it is unknown how bone senses changes in blood phosphate level, and how bone regulates the production of FGF23. Our previous data indicated that high extracellular phosphate phosphorylates FGF receptor 1 (FGFR1) in an unliganded manner, and its downstream intracellular signaling pathway regulates the expression of GALNT3. Furthermore, the post-translational modification of FGF23 protein via a gene product of GALNT3 is the main regulatory mechanism of enhanced FGF23 production due to high dietary phosphate. Therefore, our research group proposes that FGFR1 works as a phosphate-sensing receptor at least in the regulation of FGF23 production and blood phosphate level, and phosphate behaves as a first messenger. Phosphate is involved in various effects, such as stimulation of parathyroid hormone (PTH) synthesis, vascular calcification, and renal dysfunction. Several of these responses to phosphate are considered as phosphate toxicity. However, it is not clear whether FGFR1 is involved in these responses to phosphate. The elucidation of phosphate-sensing mechanisms may lead to the identification of treatment strategies for patients with abnormal phosphate metabolism.

Collision tumors involving the metastasis of malignant neoplasms to pituitary neuroendocrine tumors (PitNETs) are extremely rare. We herein report a case involving a patient with lung adenocarcinoma metastasis within a PitNET who exhibited relatively rapid progression of neurological symptoms. A 75-year-old man who underwent tumor resection 36 and 18 years prior to presentation for bladder and colon cancer, respectively, without recurrence presented with bitemporal hemianopsia, ptosis, and diplopia of the right eye. Subsequent magnetic resonance imaging (MRI) revealed a tumor 3.2 cm in diameter that extended from the anterior pituitary gland to the suprasellar region. Gadolinium-enhanced MRI of the tumor showed heterogeneous contrast enhancement. Considering the relatively rapid progression of neurological symptoms, semi-emergency endoscopic endonasal transsphenoidal surgery was performed. Histopathological examination revealed a group of thyroid transcription factor-1- and napsin A-positive papillary proliferating cells intermingled with α-subunit- and steroidogenic factor-1-positive PitNET cells. Thus, the patient was diagnosed with lung adenocarcinoma metastasis within a gonadotroph PitNET. Genetic testing revealed the presence of an EGFR (Ex-19del) mutation, after which chemotherapy was initiated. Additional stereotactic radiotherapy was performed for the residual tumor in the sella turcica. With continued chemotherapy, good control of both the primary and metastatic tumors was noted after 24 months after surgery. Cases of malignant neoplasm metastasis within a PitNET are difficult to diagnose. In the case of a sella turcica tumor with relatively rapid progression of neurological symptoms, early surgical intervention is recommended given the possibility of a highly proliferative tumor and the need to obtain pathologic specimens.

Accumulating evidence suggests that cellular heterogeneity in organs and cell-cell and tissue-tissue interactions are crucial for maintaining physical homeostasis and disease progression. Endocrine organs also exhibit cellular heterogeneity and comprise multiple cell types. For instance, the pituitary gland comprises five types of pituitary hormone-producing cells as well as non-hormone-producing supporting cells, such as fibroblasts, endothelial cells, and folliculostellate cells. However, the functional roles of the interactions between hormone-producing and non-producing cells in the pituitary gland remain incompletely understood. Over the past decade, emerging technologies such as single-cell and spatial transcriptomics have provided excellent tools for studying cellular heterogeneity and their interactions; however, the application of these technologies in endocrine research remains limited. This review provides an overview of these technologies and discusses their strengths and limitations. Additionally, we also summarize the potential future applications of single-cell and spatial transcriptomics in the study of endocrine organs and their disorders.

Immune checkpoint inhibitors (ICIs) can cause immune-related adverse events (irAEs) in several organs including endocrine glands. Among endocrine irAEs, thyroid and pituitary irAEs are frequently observed, followed by primary adrenal insufficiency, insulin-dependent diabetes mellitus, and hypoparathyroidism. These conditions could lead to life-threatening consequences, such as adrenal crisis and diabetic ketoacidosis. On the other hand, several types of irAEs including thyroid and pituitary irAEs are reported to be associated with better overall survival. Therefore, it is important to understand and manage endocrine irAEs, which differ depending on the ICI regimen used. In this review, we describe the clinical features, potential biomarkers, management strategies, and possible mechanisms of thyroid and pituitary irAEs.

In adrenal fasciculata cells stimulated by ACTH, Ca²⁺ and cAMP play indispensable roles as second messengers in cortisol production. However, whether their second messengers cooperatively or independently participate in steroid production remains unclear. We focused on the roles of Ca²⁺ and cAMP in cortisol production in bovine adrenal fasciculata cells stimulated by ACTH for a relatively short period (1 h). Incubation of the cells with 100 pM ACTH in Ca²⁺-containing (normal) medium for 1 h increased cortisol production without affecting cAMP content. In contrast, treatment of the cells with the peptide at a higher concentration (1 nM) significantly augmented both cortisol production and cAMP content. However, ACTH did not increase either of them in the Ca²⁺-free medium. ACTH rapidly increased the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) in the normal medium, but did not influence [Ca²⁺]_i in the Ca²⁺-free medium, indicating that ACTH caused Ca²⁺ influx into the cells. ACTH-induced Ca²⁺ influx and cortisol production were suppressed by a voltage-sensitive L-type Ca²⁺ channel blocker but not by a T-type, N-type, or P-type Ca²⁺ channel blocker. In contrast, dibutyryl cAMP, a cell-permeable cAMP analog, greatly enhanced cortisol production in the normal or Ca²⁺-free medium and slowly caused Ca²⁺ influx into the cells. These results strongly suggest that Ca²⁺, as a second messenger, is more critical than cAMP for cortisol production. However, both second messengers jointly participate in the production in adrenal fasciculata cells stimulated by ACTH.

Immune checkpoint inhibitors (ICIs) are used for various malignancies, although they frequently cause immune-related adverse events involving the thyroid gland (thyroid irAEs). We conducted a retrospective cohort study to elucidate thyroid function outcomes. Fifty of 639 patients who received PD-1 blockade therapy met criteria and were divided into the following groups: thyrotoxicosis with subsequent hypothyroidism (Toxic-Hypo, n = 21); thyrotoxicosis without subsequent hypothyroidism (Toxic, n = 9); and hypothyroidism without prior thyrotoxicosis (Hypo, n = 20). The Toxic-Hypo group developed thyroid irAEs earlier than the Toxic group (26 vs. 91 days; p < 0.001), and had higher serum free T4 levels (3.210 vs. 1.880 ng/dL; p = 0.011). In addition, positive anti-thyroglobulin antibodies (TgAbs) at thyroid irAE onset were more common in the Toxic-Hypo group (93.3%) than in the Toxic group (0.0%; p = 0.005) and Hypo group (44.4%; p = 0.007). The Toxic-Hypo group developed severe hypothyroidism and required larger levothyroxine (LT4) doses than the Hypo group (75 vs. 25 μg/day; p = 0.007). We predicted that patients with positive TgAbs who developed severe thyrotoxicosis within 4 weeks after the first ICI administration would develop subsequent hypothyroidism. We treated 4 such patients with prompt LT4 replacement, characterized by LT4 initiation after thyrotoxicosis improvement and quick dose titration. Their euthyroid state was successfully maintained, in contrast with patients receiving conventional replacement. In conclusion, rapid-onset severe thyrotoxicosis in patients with TgAbs correlated with a high likelihood of subsequent hypothyroidism. Accordingly, prompt LT4 replacement is suggested to prevent a severely hypothyroid state.