Chronophysiological modifications of the triiodothyronine's peripheral metabolism(T3)

An interesting aspect relative to the thyroid function consists in determining the variations produced by the age. Though this relation has been referred to T4 related with age, there are no differences about T3 kinetics. For this purpose, different parameters were studied in 68 euthyroid controls of both sexs, in good nutritional state and normal activity, whose ages oscillate between 6 and 83 years old. A kinetic study method of single i.v. T3-I125 injection was used, employing 0,5 μCi/Kg of body weight with maximum activity of 10 μCi to 10 years old; 15 μCi to 15 years old and 30 μCi above this age. Determinations of T4, T3 and TSH serum by radioimmunoassay were carried out. From these studies, the following conclusions were obtained: T4 and TSH are within the normal values, while a decrease of T3 related with age was observed. Parallely to said decrease, a prolongation of plasmatic half time and a decrease of the daily value production, which is expressed statistically by a double exponential function are appreciated. The first one, which is of fast slope, corresponds to subjects below 10 years old and the second one, which is slower, starts from that age to the seventh decade of life. (M.E.L.)