Endocrine Abstracts

Introduction: Syndrome of inappropriate secretion of thyrotropin (SITSH) is a rare condition characterized by uninhibited serum TSH in the presence of high thyroid hormones, usually caused by TSH secreting adenoma or resistance to thyroid hormones (RTH). Differential diagnosis between the two is often challenging due to their overlapping features.

Case report: We present the case of a 50-year-old postmenopausal woman with a history of -myocardial infarction in 2013, with angioplasty and 4 stent coronary placement. The patient was referred to our clinic for the evaluation of abnormal thyroid function tests (TSH=3.25 μIU/ml RR=0.3-3.6, FT4=30.2 pmol/l RR 9-24, FT3=11.8 pmol/l RR 3.1-6.8). The patient had no family history of thyroid disorders and a personal history showing a consistent pattern of a normal or elevated TSH level accompanied by elevated levels of thyroid hormones, which persisted despite the treatment with thiamazole 10 mg per day between 2017-2020. Clinically, the patient was euthyroid except for tachycardia, which was managed with metoprolol 300 mg per day. Our laboratory results confirmed inappropriate secretion of TSH (TSH=4.07 μIU/ml RR=0.3-3.6, FT4=1.73 ng/dl RR 0.7-1.48, TT4=15.37 μg/dl RR 4.8-11.7, TT3=244 ng/dl RR 76.3-220.8), normal pituitary function tests, negative anti-thyroid autoantibodies and thyroid ultrasound showed an unremarkable multinodular goiter. Due to anxiety the patient refused pituitary MRI, therefore screening CT scan was performed as an alternative diagnostic tool. It revealed an enlarged sella turcica (25/14/19mm) and a 18/11 mm pituitary tumor. Given the ischemic heart disease T3 suppression test was considered to be contraindicated. Furthermore, TRH is not available in our country. Alpha glycoprotein subunit levels were elevated (α-GSU= 1.89IU/l RR for postmenopausal women<1.3). We decided to escalate the thiamazole dose to 20 mg per day and to reassess the patient in 4 weeks adding SHBG, a short-term somatostatin analog test, genetic screening for RTH and psychological counseling in order to obtain a MRI contrast enhanced pituitary scan.

Conclusions: The diagnosis of SITSH can be difficult to establish due to the low prevalence, insufficient awareness or experience of diagnosis or treatment, and unavailability of highly accurate diagnostic tests. This case was challenging due to the patient’s ischemic heart condition, lack of access to certain diagnostic tools and the patient’s refusal to undergo MRI examination. It is also important to remember that TSH-omas are very rare tumors, while pituitary incidentalomas are common. Therefore, a high level of suspicion and a thorough differential diagnosis algorithm are imperative.