Short-term effect of morphine on the thyroid gland in male rats

Experiments were designed to demonstrate that morphine may exert a direct short-term effect on the hormone release of the thyroid gland. Groups of male rats were injected with single doses of 1, 5 and 10 mg/kg morphine, or with 2 mg/kg naloxone and in addition with morphine 30 min after naloxone and/or with naloxone 30 min after morphine pre-treatment. The rats were killed by decapitation 15, 30 and 60 min after the injection and serum was collected and stored for subsequent TSH, T4 and T3 radioassays. All doses of morphine resulted in an increase of serum T4 and T3 concentrations 15 and 30 min after the injection, with a tendency to return to control levels by the 60 min samples. Serum TSH concentrations were to suppressed by administration of 5 and 10 mg/kg but not by 1 mg/kg morphine. Naloxone treatment did not increase the T4 and T3 concentrations; however, serum TSH was elevated in the 15 min sample. Naloxone pre-treatment inhibited the morphine induced release of T4 and T3 into the serum, but naloxone administration after morphine pre-treatment failed to prevent the increase of T4 and T3 secretion. These data suggest that morphine may exert a short-term stimulatory effect on the thyroid gland with a concomitant inhibitory action on the hypothalamo-pituitary TSH system.