Propylthiouracil-induced hypothyroidism in coho salmon, Oncorhynchus kisutch: Effects on plasma total thyroxine, total triiodothyronine, free thyroxine, and growth hormone

Thyroid hormones transiently increase during parr-smelt transformation in c oho salmon, Oncorhynchus kisutch, and are believed to trigger morphological , physiological, behavioural, and neural changes. The effectiveness of prop ylthiouracil (PTU) to induce hypothyroidism in smelting coho salmon was det ermined by immersing coho salmon, Oncorhynchus kisutch, in 30 mg l(-1) PTU from May 1, two weeks prior to the consistent annual total thyroxine (TT4) peak in mid-May, until the last sampling date. Plasma was obtained at two s ampling dates from control and PTU-treated coho salmon: May 15, during the plasma TT4 peak; and May 26, after the TT4 peak. Radioimmunoassays were use d to measure plasma TT4, total triiodothyronine (TT3), free thyroxine (FT4) , and salmon growth hormone (sGH). The PTU-treatment inhibited the natural smoltification-related increases in plasma TT4, TT3 and GH levels compared with controls, but PTU-treatment did not affect these hormone levels when t hey were low. PTU-treatment increased FT4 and decreased TT3 and sGH levels in the May 26 sample. In the May 15 sample, FT4 levels were unaffected by P TU-treatment, whereas TT4 levels were decreased. These data demonstrate the ability of PTU to induce hypothyroidism in salmonids as shown by the decre ase in TT4 and TT3. These data demonstrate that PTU treatment by immersion can induce hypothyroidism in salmonids as shown by: (1) the inhibition of t he natural increases of TT4 and TT3; (2) the increase in FT4 levels corresp onding to the lowered TT3 levels, suggesting an inhibition of thyroxine 5'- monodeiodinase activity. We also show for the first time that PTU treatment can lower plasma GH levels in salmonids. This lowering of plasma GH level is associated with the decrease in TT3 levels and the increase in FT4 level s. The PTU induced lowering in GH levels may contribute to the observed cha nges in FT4 and TT3, since GH is known to increase thyroxine 5'-monodeiodin ase activity.