Differential Diagnosis:
In this patient, differential diagnoses included prolactinoma, as
elevated prolactin with galactorrhea suggests this common pituitary
tumor, and primary hypothyroidism with pituitary hyperplasia, where
prolonged hypothyroidism can cause pituitary enlargement that may mimic
a macroadenoma and often regresses with thyroid hormone therapy, as
observed here. Medication-induced hyperprolactinemia, from
norethisterone used for menstrual irregularities, could also contribute.
Given her improvement on levothyroxine, pituitary hyperplasia due to
hypothyroidism appears most likely.