Effects of exogenous monoiodotyrosine on the serum levels of anterior pituitary hormones.

Monoiodotyrosine is a tyrosine hydroxylase inhibitor. Ingestion of one gram monoiodotyrosine caused a 10,000-fold increase of serum monoiodotyrosine from basal levels of 0.69 +/- 0.20 nmol/l to a peak of 10.6 +/- 1.7 mumol/l in women and 7.1 +/- 2.3 mumol/l in men 30 min later, and the t1/2 was 45 min. Monoiodotyrosine stimulated PRL to a peak of 170 +/- 51 micrograms/l in women and 90 +/- 6 micrograms/l in men 30 min after the monoiodotyrosine peak, or 60 min after the ingestion. Other anterior pituitary hormones were unchanged. Dopamine infusion or L-dopa pretreatment attenuated the monoiodotyrosine effect. TRH exaggerated the PRL peak, and chlorpromazine did not increase but prolonged the hyperprolactinemia. These results suggest that dopamine synthesis inhibition may be the mechanism of PRL stimulation.