Variation in thyroid response to thyroid-stimulating hormone in juvenile coho salmon (Oncorhynchus kisutch)

Changes in the circulating levels of thyroid hormones measured by radioimmunoassay in yearling coho salmon in response to injection of bovine thyroid-stimulating hormone (TSH) were evaluated from January through August. Doses of TSH ranging from 0.2 to 6.4 micrograms/g body wt were ineffective in stimulating T4 in January and at one time in April. Responses to TSH injection at this dose range were observed beginning in February and continuing through August. The minimal effective dose of TSH was either 0.2 or 0.4 microgram/g from May through August. The dose-response relationships of bovine and coho salmon TSH were compared and found to be similar. The time course of response of juvenile salmon to either bovine TSH or partially purified salmon TSH was found to be similar in January, May, and August, although the magnitude of the response differed at these different times. It is concluded that the increase in thyroid response to TSH may play a part in the onset of the increase in circulating T4 at the time of smoltification, but such changes in sensitivity do not explain the decrease in circulating T4 at the completion of smoltification.