MON-453 - Unmasking the Unexpected: Thyrotropin-Secreting Pituitary Adenoma Presenting after Subclinical Hypothyroidism

Introduction

TSH‑secreting pituitary adenomas (TSHomas) are rare causes of central hyperthyroidism, accounting for 0.5–2% of all pituitary adenomas and with a prevalence of 1–2 cases per million. Diagnosis is often delayed due to atypical biochemical profiles and overlapping features with more common thyroid disorders.

Case Report

A 34‑year‑old man with a complex medical history including narcolepsy, hereditary hemochromatosis, celiac disease, and previously treated subclinical hypothyroidism progressively developed symptoms of thyrotoxicosis over several months. Symptoms included anxiety, mood lability, insomnia, diarrhea, diffuse alopecia, and tremors. Physical examination revealed no goiter, ophthalmopathy, or thyroid bruit; resting heart rate was in the low 70s, punctuated by intermittent tachycardia. TSH levels were persistently elevated (5-6 mcUnits/mL) during the prior months. Initial evaluation excluded autoimmune thyroid disease, with normal thyroid peroxidase (TPO), thyroglobulin (TG), and thyroid-stimulating immunoglobulin (TSI) antibody levels. Thyroid ultrasound demonstrated normal echotexture without nodularity or increased vascularity, making Graves disease and multinodular goiter an unlikely diagnosis. Repeat thyroid function testing, including free T4 by equilibrium dialysis, confirmed elevated thyroid hormones in the presence of non-suppressed TSH. TSH receptor gene sequencing was negative, excluding TSH resistance syndrome. Even though the serum α-glycoprotein subunit (α-GSU) and α-GSU/TSH molar ratio were within reference limits, making a pituitary adenoma less likely, the team decided to pursue pituitary MRI imaging which revealed a 1.7‑cm sellar macroadenoma. The patient underwent transsphenoidal surgical resection, and histopathology confirmed a thyrotropin-secreting pituitary adenoma (TSHoma). Postoperatively, the patient achieved biochemical euthyroidism with complete resolution of symptoms.
Discussion
TSHomas are rare but important causes of central hyperthyroidism. This case highlights the diagnostic challenge posed by prior thyroid dysfunction and comorbidities, which can obscure interpretation of thyroid function tests. Clinicians should include TSHoma in the differential diagnosis whenever thyroid hormones are elevated with non-suppressed TSH. Early recognition and surgical intervention are essential to restore euthyroidism and prevent complications from hormonal excess and pituitary mass effects.

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