Influence of dopaminergic inhibition on serum levels of thyrotrophin and prolactin in patients with hypothyroidism before and after prolonged oral administration of TRH.

Forty mg TRH/day given orally for 3 weeks to 8 patients with mild primary hypothyroidism decreased serum TSH from a mean of 4.0 ng/ml +/- 1.2 (SE) to 2.0 ng/ml +/- 0.4 (49%), and their mean incremental TSH response to iv TRH was equally reduced from 8.6 ng/ml +/- 2.5 to 4.0 ng/ml +/- 1.9 (46%). In the same patients serum Prl was 8.2 ng/ml +/- 2.2 before oral TRH treatment and 6.6 ng/ml +/- 1.5 (81%) after treatment, and the mean incremental Prl response to iv TRH was reduced from 43.5 ng/ml +/- 5.0 to 35.9 ng/ml +/- 7.5 (83%). The oral administration of 10 mg of the dopamine antagonist metoclopramide increased mean serum TSH from 0.6 ng/ml +/- 0.1 (SE) to 0.7 ng/ml +/- 0.1 (120%) in euthyroid subjects and from 4.0 ng/ml +/- 1.2 to 5.7 ng/ml +/- 1.6 (145%) in patients with primary hypothyroidism, and mean serum Prl from 8.6 ng/ml +/- 0.8 to 109.5 ng/ml +/- 24.3 (1251%) and from 8.2 ng/ml +/- 2.2 to 119.6 ng/ml +/- 45.5 (1460%), respectively. The incremental TSH responses to iv TRH increased 2.3-fold in euthyroid subjects pre-treated with metoclopramide, while no change was observed in the TSH responsiveness in patients with primary hypothyroidism following metoclopramide pre-treatment. In the euthyroid subjects metoclopramide treatment had no effect on the Prl response to iv TRH.(ABSTRACT TRUNCATED AT 250 WORDS)