Chronic fasting reduces the response of the thyroid to growth hormone and TSH, and alters the growth hormone-related changes in hepatic 5'-monodeiodinase activity in rainbow trout, Oncorhynchus mykiss.

Chronically fasted rainbow trout (Oncorhynchus mykiss) had significantly lower plasma L-thyroxine (T4) and triiodo-L-thyronine (T3), and higher plasma growth hormone (GH) concentrations than fed animals. Fasted and fed trout were administered bovine thyrotropic hormone (bTSH), native ovine GH (oGH), or recombinant human GH (rhGH) alone, or GH in combination with bTSH to further study the effects of food deprivation on the activity of the pituitary-thyroid axis and on the control of hepatic T3 production. Although the fasted rainbow trout retained the ability to respond to bTSH challenge, the resultant elevation in plasma T4 concentration was significantly lower than that of fed animals; there was no plasma T3 response to bTSH challenge in either fed or fasted trout, except for a significant elevation in fed bTSH-injected fish and a significant depression in fed saline-injected fish sampled 2.5 hr after the injection. GH when administered alone had no significant effect on plasma T4 concentrations of either fed or fasted animals, and stimulated an increase in plasma T3 concentration and an increased hepatic T3 content only in the fed fish, despite a significant stimulation by both oGH and rhGH of in vitro hepatic 5'-monodeiodinase activity (MDA) in both fed and fasted groups. bTSH appeared to suppress rhGH- and oGH-stimulated MDA in fasted groups, and rhGH-stimulated MDA in fed trout. The data suggest that chronic fasting induced a down-regulation of the response of thyroid tissue to bTSH challenge, and of the GH-stimulation of T3 production, in vivo, although in vitro hepatic MDA was elevated following GH administration to both fed and fasted rainbow trout.