Observations concerning the binding of thyroid hormones in sera of normal subjects of varying ages.

Abstract

Indexes of thyroid hormone binding have been assessed in sera obtained from 99 normal subjects whose ages ranged between 2 and 87 years. Electrophoretic analyses revealed a highly significant age-related change in the proportion of "endogenous" thyroxine (T4) bound by thyraxine-binding globulin (TBG). This decreased from childhood to adulthood, reached a nadir in the 4th or 5th decade, and then increased with advancing age. The proportion of endogenous T4 bound by the thyraxine-binding prealbumin (TBPA) displayed a change that was reciprocal to that bound by TBG. These changes were associated with changes in the T4-binding capacities of TBG and TBPA, as assessed electro-phoretically. Specimens obtained from children (1st decade) revealed a higher T4-binding capacity of TBG and a lower T4-binding capacity for TBPA than did specimens from individuals in the 4th decade. The latter specimens, in turn, revealed a slightly lower T4-binding capacity of TBG and a higher T4-binding capacity of TBPA than did specimens from subjects in the 7th and 8th decades. The age-related change in the distribution of T4 between TBG and TBPA was not reflected in changes in several indexes of the over-all intensity of thyroid hormone binding. Values for the protein-bound iodine, the proportion and absolute concentration of free T4, and the in vitro resin uptake of triiodothyronine displayed no significant correlation with age. It is concluded that the progressive slowing of the peripheral turnover of T4 that occurs from childhood through senescence does not result from alterations in the binding of thyroid hormone in the plasma, but is probably due to factors associated with the cellular uptake or metabolism of the hormone per se.