Pituitary hyperplasia resulting from primary hypothyroidism mimicking macroadenomas

Three patients presented with clinical and imaging features suggestive of a primary functional pituitary adenoma. Surprisingly, thyroid function testi ng revealed co-existing primary hypothyroidism. The pituitary tumours repre sented hyperplasia of anterior pituitary thyrotrophs secondary to hypothyro idism. Complete regression was achieved with thyroxine replacement therapy. Careful attention should be paid to differentiating this condition from pr olactinoma or the rare TSH producing adenoma.