Abstract
1. 1. Administration of morphine sulfate (at 5 and 50 mg/kg) to immature domestic fowl lowered the circulating thyroxine (T 4) concentration in a dose- and time-dependent manner. 2. 2. The inhibitory effect of morphine on T 4 secretion was mediated via opiate receptors, since it was completely blocked in the presence of naloxone (50 mg/kg). 3. 3. The possibility that T 4 secretion may be under tonic opioid inhibition was suggested by a stimulatory effect of naloxone on plasma T 4 concentrations. 4. 4. Morphine sulfate (50 mg/kg) enhanced plasma triiodothyronine concentrations, an effect that was completely suppressed in the presence of naloxone. 5. 5. These results demonstrate differential effects of opiates on T 4 and T 3 concentrations and suggest that opiates tonically inhibit T 4 release and accelerate its rate of peripheral monodeiodination.
Published Version
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