| In NTI serum TSH is usually normal or even suppressed despite lowered serum T3 and sometimes also T4. The mechanism of this inappropriately low TSH is not well-known. Everts, Rotterdam, The Netherlands, investigated if TRIAC was taken up by the pituitary and evaluated its biological effects with regard to TSH secretion as it is known that free TRIAC concentrations are markedly elevated in NTI. It was found that TRIAC was taken up in a carrier-mediated energy dependent fashion by rat anterior pituitary cells in culture and also that after TRH, TSH secretion was markedly inhibited by TRIAC. The TSH suppressive effect of TRIAC was higher than that of T3 or T4 (Comment: As low T3 production in NTI is considered as an energy and protein saving mechanism, the teleological significance of the absence of TSH increase is to keep T3 low in these patients. It is therefore important to evaluate the mechanisms that are responsible for continued TSH suppression under these circumstances and TRIAC might be one of the factors causing this effect.) |
