General and Comparative Endocrinology vol:80 issue:1 pages:33-40
In this study, intravenous injection of several doses of porcine follicle stimulating hormone (pFSH: 0.002, 0.01, 0.05, and 0.5 μg/g body wt), bovine TSH (bTSH: 0.5 μg/g body wt), and ovine growth hormone (oGH: 0.04, 0.02, and 0.4 μg/g body wt) stimulated an increase in plasma thyroxine (T4) and reverse triiodothyronine (rT3) in tilapia. This effect occurred in a dose-dependent manner. pFSH was the most potent in stimulating thyroid function. The dose of 0.002 μg pFSH/g body wt increased plasma levels of T4 over control levels (2.59 ± 0.16 ng/ml) about 2.5-fold within 4 hr, whereas a concentration of 0.5 μg/g body wt caused a great and prolonged increase of T4 and rT3 levels. Control levels (2.59 ± 0.16 ng/ml for T4 and 40.37 ± 8.60 pg/ml for rT3) were increased 19- and 22-fold respectively, over 24 hr. An increase of T4 and rT3 levels occurred also after injection of total hypophyseal extract and a Con A II glycoprotein fraction of a tilapia pituitary homogenate, whereas the protein fraction failed to alter plasma concentrations of T4 and rT3. rT3 levels were also significantly increased at 2 hr, but not at 1 hr, after injection of T4. Basal T3 levels (1.90 ± 0.22 ng/ml) were reduced by half over 24 hr in all experiments. These results suggest the existence, in tilapia, of a 5-D pathway deiodination of T4 which is pituitary independent. Stimulation of T4 release is always followed by an increase in plasma rT3 levels.