Tonic effect of endogenous TSH on the in vitro thyroid cAMP response to TSH

The present study was undertaken to compare the effects of 3,5,3'-triiodothyronine (T3) alone and T3 plus bovine thyrotrophin (bTSH) given chronically in vivo on the TSH-stimulated cyclic adenosine 3',5'-monophosphate (cAMP) production in a mouse thyroid in vitro. Mice were given T3 (5 micrograms/ml) in drinking water for 4 days. The thyroid cAMP concentrations after an incubation with 10 mU/ml of TSH for 10 min were decreased by 50% in T3-treated mice as compared to the control. In the second experiment, mice were given T3 alone or T3 plus 0.5 mU of bTSH ip daily for 4 days. The combined treatment with T3 and TSH partially restored the reduction of cAMP response to TSH that was induced by T3 alone. In the third experiment, mice were given T3 alone for 7 days, or T3 for 7 days plus TSH for the last 3 days. The reduced cAMP response to TSH induced by T3 alone was again partially restored by the concomitant treatment with TSH. These results indicate 1) that the capacity of the thyroid cAMP to respond to TSH is regulated, at least in part, by a trophic effect of endogenous TSH and 2) that the impaired capacity caused by a loss of tonic effect of endogenous TSH is reversible.