Opiate participation in thyroid hormone regulation in domestic fowl

Administration of morphine sulfate (at 5 and 50 mg/kg) to immature domestic fowl lowered the circulating thyroxine (T4) concentration in a dose- and time-dependent manner. The inhibitory effect of morphine on T4 secretion was mediated via opiate receptors, since it was completely blocked in the presence of naloxone (50 mg/kg). The possibility that T4 secretion may be under tonic opioid inhibition was suggested by a stimulatory effect of naloxone on plasma T4 concentrations. Morphine sulfate (50 mg/kg) enhanced plasma triiodothyronine concentrations, an effect that was completely suppressed in the presence of naloxone. These results demonstrate differential effects of opiates on T4 and T3 concentrations and suggest that opiates tonically inhibit T4 release and accelerate its rate of peripheral monodeiodination.