Reversible Pituitary Hyperplasia Mimicking a Macroadenoma in Severe Autoimmune Primary Hypothyroidism

Pituitary hyperplasia is a reversible but underrecognized consequence of severe primary hypothyroidism that may closely mimic a pituitary macroadenoma; radiological overlap with hypophysitis can further complicate the differential diagnosis.A 26-year-old man underwent brain magnetic resonance imaging (MRI) for headache and balance disturbance, which revealed an approximately 18 × 12 mm sellar enlargement with suprasellar extension.The initial radiological interpretation favored a macroadenoma.Endocrine evaluation demonstrated profound autoimmune primary hypothyroidism, with markedly elevated thyroid-stimulating hormone (TSH), reduced free thyroxine (FT4), positive thyroid autoantibodies, and mild hyperprolactinemia.Levothyroxine replacement was initiated; no dopamine agonist, glucocorticoid treatment, or pituitary-directed intervention was administered.Follow-up MRI on Day 175 showed normal pituitary morphology despite residual biochemical abnormalities, and long-term imaging confirmed sustained resolution.Hypophysitis was considered retrospectively because of the radiological overlap.However, preservation of the posterior pituitary bright spot, absence of reported cavernous sinus involvement, lack of documented arginine vasopressin deficiency, and complete resolution during levothyroxine replacement favored reactive thyrotroph hyperplasia.A complete baseline etiological work-up for secondary hypophysitis and histological confirmation were not available, preventing absolute exclusion of an inflammatory pituitary disorder.This case emphasizes the need for integrated endocrine and imaging assessment of sellar enlargement before pituitary-directed treatment.