Low thyroxine levels in some hyperprolactinemic patients due to dopaminergic suppression of thyrotropin

The serum free T4 index (FT4I) was at or below the lower limit of normal in 8 of 55 unselected patients with hyperprolactinemia. Serum levels of T3 were normal and none of the patients had clinical evidence of hypothyroidism. In patients with low FT4I the serum TSH was within the normal range and TSH was released normally after administration of TRH, indicating normal pituitary TSH reserve. Serum TSH also increased after administration of the dopamine antagonist domperidone. The TSH response to domperidone was significantly greater in the hyperprolactinemic group with low FT4I compared with either normal subjects or hyperprolactinemic patients with normal FT4I, suggesting that depression of FT4I was due to increased dopaminergic activity. Administration of the dopamine antagonist metoclopramide for 4 days led to a supranormal rise in FT4I in 3 of 5 patients with low FT4I. Thus, a minority of hyperprolactinemic patients have a low FT4I which appears due to excessive hypothalamic production of dopamine.