Differential effect of homeostatic thymus hormone on plasma thyrotropin and growth hormone in young and old rats

There is increasing evidence that the neuroendocrine system is responsive to hormonal signals generated by the immune system. Thus, interleukin-1, hepatocyte stimulating factor and thymosin have been shown to stimulate adrenocorticotropin, gbēndorphin and luteinizing hormone secretion. We report here that homeostatic thymus hormoen (HTH), a well-characterized thymic preparation, reduces plasma thyrotropin (TSH) and grwoth hormone (GH) in young (3 months) Sprague-Dawley male rats, but fails to do so (TSH) or has a significantly weaker effect (GH) in old (26 months) animals. Young and old conscious, free-moving rats carrying an indwelling atrial cannula received the substances to be tested via the cannulas. Plasma samples were taken every 30 min for 5 h and hormones were measured by RIA. In the young rats, HTH (8 mg/kg body wt) induced a marked reduction in plasma TSH which was significantly greater than the normal circadian decline observed in saline-injected young controls. The old rats displayed high basal levels of TSH which showed no circadian rhythmicity and did not respond to HTH. Plasma thyroxine (T₄) showed a significant age-related reduction but was not affected by HTH. The above dose of HTH significantly reduced plasma GH in young and old rats, but the effect was greater in the young animals. Mean basal levels of plasma GH were significantly lower in old than in young rats. The present results suggest that HTH, whose production by the thymus is known to be stimulated by TSH and GH, is involved in an inhibitory feedback loop regulating plasma TSH and GH in young rats. Our data also suggest an age-related desensitization of the TSH and GH systems to thymic influence in this species.