Effect of propranolol in combination with TSH and ovine prolactin on plasma thyroid hormone levels and in vitro hepatic monodeiodination of thyroxine in brook charr, Salvelinus fontinalis (Mitchill).

1. Ovine TSH stimulated an increase in plasma T4 levels in brook charr, but plasma T3 levels, hepatic T3 content and hepatic T4-monodeiodination to T3 were unaffected. TSH-induced plasma T4 levels were further enhanced in propranolol-pretreated fish. 2. Fish treated with ovine prolactin + TSH had elevated plasma T3 levels, hepatic T3 content and hepatic T4-monodeiodination rates compared with TSH-treated animals. Propranolol-pretreatment further enhanced hepatic T4-monodeiodination in these fish. 3. Propranolol, a beta-adrenoceptor blocker, appears to enhance peripheral T4 to T3 conversion in physiological situations when plasma thyroid hormone levels and T4-monodeiodination rates are high.