Short-term treatment with testosterone increases plasma 3,5,3'-triiodo-L-thyronine and hepatic L-thyroxine 5'-monodeiodinase levels in arctic charr, Salvelinus alpinus.

Testosterone (T), methyl testosterone (MT), and testosterone propionate (TP) (0.5 mg/100 g body wt in 40 microliter peanut oil) or peanut oil alone were injected (ip) on Days 0 and 3 into immature arctic charr (Salvelinus alpinus). Charr were sampled on Days 7 and 12 and plasma testosterone, L-thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3), and hepatic microsomal T4 5'-monodeiodinase (T4 5'D) measured. Plasma androgen levels were elevated by all androgen treatments to levels similar to those observed in spawning salmonids. Plasma T3 was significantly increased by all forms of testosterone on Days 7 and 12. T4 levels remained unchanged or significantly decreased on Day 7, with no significant differences on Day 12. T4 5'D activity was increased on both Days 7 and 12 in the experimental groups due to increases in the Vmax (1.3 to 5.9 X control groups) with negligible changes in the Km. In conclusion, T, MT, and TP all stimulate thyroidal status by increasing plasma T3 levels in part as a result of increased hepatic T4 5'D activity.