AbstractThis study examines the effect of food deprivation, increased ambient salinity and prolactin administration on the thyroidal response to ovine TSH, andin vitrohepatic monodeiodination of T4 to T3 in coho salmon,Oncorhynchus kisutch.Fed fish and fish food‐deprived for 18 days showed similar significant increases in plasma T4 9 and 24 h after a single injection of TSH. Plasma T3 levels were also elevated in both fed and food‐deprived fish 9 h after the TSH injection but plasma T3 levels in the food‐deprived fish were markedly lower than in the fed salmon. The increase in T4 and T3 evident in freshwater‐acclimated fish after TSH administration was not found in salmon acclimated to 65% sea water. Prolactin, given alone (either as a single injection or a series of three daily injections) had no effect on plasma T4 or T3 levels. When given together with TSH prolactin prolonged the T4 and T3 elevating effect of TSH. Food‐deprived salmon had lowerin vitrohepatic T4 to T3 conversion rates than fed groups but T4 to T3 conversion did not appear to be affected by increased ambient salinity, or by prolactin and/or TSH admin