Somatostatin increases plasma T 3 concentrations in Tilapia nilotica in the presence of increased plasma T 4 levels

Abstract

An injection of ovine growth hormone, porcine follicle stimulating hormone, and bovine thyrotropin stimulating hormone increased in Tilapia nilotica plasma concentrations of thyroxine (T 4) and reverse triiodothyronine (rT 3) after 4 and 8 hr, whereas plasma concentrations of T 3 were unaffected. An injection of somatostatin (SRIF) alone did not influence thyroid hormone levels. If, however, SRIF was injected together with these hormones, which raised plasma T 4, or together with T 4 itself, an increase in plasma concentrations of T 3 could be observed, whereas the increase in rT 3 was less pronounced. It is concluded that SRIF may change the normal 5-deiodinase (5-D) activity and increased rT 3 during hyperthyroxinemia into a 5′-D activity and a rise in T 3, respectively, in T. nilotica.