Effect of L-dopa on plasma TSH levels in primary hypothyroidism

Plasma TSH responses after an i.v. injection of 100 mg of L-dopa were evaluated by radioimmunoassay in 4 normal euthyroid subjects and in 8 patients with primary hypothyroidism. In agreement with previous results, no variations to plasma TSH levels were observed in the euthyroid subjects. In contrast, in primary hypothyroidism L-dopa induced a biphasic response in plasma TSH. In fact, we observed a transitory increase witha maximum at 30 min (mean plus or minus SEM equals 54 plus or minus 18%) followed by a decrease reading a minimum level of plasma TSH basal values at 90 min (mean plus or minus SEM equals 15 plus or minus 6%). Thesefindings demonstrate that the plasma TSH response to L-dopa in primary hypothyroidism is time-dependent . Some speculation on the possible mechanism of this action is presented.