A 32-year-old woman with persistent postpartum galactorrhea, hyperprolactinemia and a sellar configuration compatible with pituitary adenoma underwent transsphenoidal surgery. Mild hyperthyroidism had been suspected clinically preoperatively and thyroxine plus cortisol therapy was given. Morphological features of pituitary thyrotroph hyperplasia were present as shown by light microscopy, immunoperoxidase staining and electron microscopy. The thyrotroph hyperplasia was secondary to primary hypothyroidism as demonstrated by the TSH response to TRH stimulation and subsequent suppression with thyroid hormone treatment. Although prolonged severe hypothyroidism has been reported to cause pituitary enlargement, this is the first case of thyrotroph hyperplasia with detailed morphologic study of the surgically-removed adenohypophysial tissue. The presentation of this patient calls attention to the possibility of mistaking pituitary enlargement due to primary hypothyroidism for a prolactinoma.