In order to assess the role of thyroid hormone on physiologically and pharmacologically induced prolactin (PRL) secretion, serum PRL concentrations were measured in 4 normal women and 4 women with various endocrinopathies before, and 4 to 6 days following, the ingestion of L-thyroxine (T4). A single 1.5 to 3.0 mg dose of oral T4 produced approximately a 2-fold increase in serum T4. Exogenous T4 did not significantly alter the mean concentration, or the pattern of PRL secretion during a 24-h interval in either normal individuals or 3 patients with galactorrhea. The lactating patients had elevated basal PRL levels and a blunted secretory response to intramuscular chlorpromazine; however, neither fasting PRL nor the peak response to chlorpromazine was altered by T4. L-Dopa suppression of serum PRL was not significantly influenced by T4 in these patients. In conclusion, PRL secretion remained unaltered after the administration of thyroxine in doses sufficient to produce approximately a 2-fold increase in serum T4. This challenges the concept that T4 and TRH are important physiologic regulators of PRL secretion.