The effect of prolonged lithium carbonate administration on the thyrotrophin and prolactin response to thyrotrophin-releasing hormone.

Thyrotrophin (TSH) and prolactin (PRL) responses to intravenous thyrotrophin-releasing hormone (TRH) were measured in euthyroid patients suffering from psychiatric disease on long-term lithium carbonate (LC) and phenothiazine therapy. These responses were retested after oral tri-iodothyronine (T3) 120 mug/day had been given for a week. The raised basal TSH and the suppression of the exaggerated responses to TRH by T3 found in some of the patients suggest a mild disturbance of thyroid function in patients on long-term LC therapy, even in the face of clinical euthyroidism and otherwise normal thyroid function. Basal serum PRL levels and the responses to TRH were elevated in 2 of 5 patients, probably associated with phenothiazine administration. In all 5, a moderate blunting of the PRL responses after TRH was produced by T3. The suppressive effect of T3 on TRH-induced PRL responses was unexpected and suggests a modifying role of thyroid hormones on PRL secretion, although further studies are needed to confirm this possibility.