MON-373 - Severe Hypothyroidism and Pituitary Enlargement: Diagnostic Dilemma between Tshoma and Pituitary Hyperplasia

We report a case of severe hypothyroidism complicated by pituitary enlargement, posing a diagnostic challenge between thyrotroph hyperplasia and a TSH-secreting pituitary adenoma (TSHoma).

A 29-year-old female with a history of hypothyroidism, noncompliant with levothyroxine for eight months, and a previously incidentally diagnosed pituitary adenoma treated with cabergoline, presented with unintentional 30-pound weight loss, cognitive slowing, fatigue, galactorrhea, alopecia, and irregular menses with heavy bleeding requiring blood transfusion. Outpatient labs revealed a markedly elevated TSH of 1,600 µU/L, prompting emergency department referral. Physical examination was notable for mild bradycardia and periorbital edema.

Admission laboratory studies showed TSH 1,364 µU/L, free T4 0.28 ng/dL, cortisol 5.42 µg/dL, ACTH < 1.5 pg/mL, prolactin 178 ng/mL, and IGF-1 123 ng/dL, consistent with severe primary hypothyroidism, central adrenal insufficiency, and hyperprolactinemia. The patient received stress-dose hydrocortisone followed by intravenous levothyroxine.

Brain MRI demonstrated a homogeneously enhancing pituitary lesion measuring approximately 1.8 cm in craniocaudal dimension, causing superior displacement of the optic chiasm and possible cavernous sinus involvement. Visual field testing was normal, and no acute surgical intervention was recommended. The patient was discharged on oral levothyroxine and hydrocortisone with close endocrinology, ophthalmology, and neurosurgical follow-up.

The pituitary enlargement was attributed to thyrotroph hyperplasia secondary to prolonged untreated hypothyroidism rather than a TSHoma. This case highlights the importance of strict adherence to thyroid hormone replacement and emphasizes the critical role of endocrinologic evaluation, including biochemical assessment and imaging interpretation, to distinguish pituitary hyperplasia from TSHoma, as management strategies differ significantly.

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