Secondary pituitary enlargement due to primary hypothyroidism is not a common manifestation. The loss of thyroxin feedback inhibition in primary hypothyroidism causes overproduction of thyrotropin-releasing-hormone (TRH), which results in secondary pituitary enlargement. TRH has a weak stimulatory effect on the lactotroph cells of the pituitary, so a mild to moderate increase in prolactin (PRL) levels is expected. We report the case of a 67-year-old female who presented with a large pituitary mass and a very high level of TSH in association with a significant rise in PRL level. In this case, diagnosing a sellar mass was challenging; it was difficult to distinguish between pituitary prolactinoma and primary hypothyroidism with secondary pituitary hyperplasia. Thyroid hormone replacement proved that this patient's hyperprolactinemia was due to hyperplasia of the pituitary gland. As such, making the correct diagnosis and initiating thyroid hormone therapy can prevent unnecessary treatment with dopamine agonists.