Prolactin-thyroid interaction in Fundulus heteroclitus

Injections of adult Fundulus heteroclitus with 0.2 IU of oTSH over 4 days caused an increase in serum T 4, with no effect on serum T 3. Administration of 12 IU of oPRL in conjunction with the same dosage of TSH prevented the TSH-induced rise in T 4, without affecting serum T 3 levels. The failure of TSH to cause T 4 levels to rise in the presence of PRL may result from either inhibition of release or acceleration of metabolic clearance of T 4. We therefore conducted three experiments to examine potential effects of PRL on the kinetics of peripheral clearance of thyroid hormones. 125I T 4 was cleared from serum in a biphasic pattern that was unaltered by PRL. Clearance of labeled T 3 followed a similar pattern that was also not influenced by PRL treatment. Generation of labeled T3 by deiodination of a dose of 125I T 4 was quantified over a 24-h period. Again, PRL-treated fish showed no significant differences. Since PRL is without effects on thyroid hormone clearance patterns or deiodination rate, we conclude that PRL prevents TSH-induced increases in serum T 4 in this species by directly affecting thyroid function.