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ABSTRACT

The effect of age on the responsiveness of rat thymocytes to 3,5,3′-tri-iodothyronine (T₃) was studied. It has been demonstrated previously that the plasma membrane-mediated effect of T₃ to increase sugar uptake by rat thymocytes is influenced by age and sex. In both sexes, T₃ given in vitro stimulated sugar uptake in cells from animals of 15 days of age, had no effect at 21 days and was again effective at 26 days. In the male, thymocytes from animals of 40 days of age and older were refractory to T₃. However, in the female, T₃, although less effective than in cells from 26-day-old animals, remained stimulatory in cells from 40- and 60-day-old rats. T₃ had no effect in cells from animals of 90 days of age and older. In in-vivo studies in which female rats of 26, 60 and 90 days of age were first injected with T₃ and 1 h later with [³H]2-deoxyglucose, the responsiveness of thymocytes to T₃ also declined progressively with advancing age; T₃ was most effective in cells from 26-day-old animals, less stimulatory in 60-day-old and essentially without effect in cells from 90-day-old animals. From these observations we have concluded that in both male and female rats the responsiveness of thymocytes to T₃ declines progressively with age, and that this decline occurs at an earlier age in cells obtained from males.

J. Endocr. (1986) 110, 511–515

Studies of the influence of age and sex on the concentrations of total thyroxine (T₄) and 3,5,3′-tri-iodothyronine (T₃) in serum and on the free T₄ and free T₃ indices, were conducted in Sprague–Dawley rats of the CD strain varying in age between 10 days and 12 months. Both sex- and age-related differences were found. In all age-groups studied, serum T₄ concentrations were higher in the male than in the female, whereas serum T₃ concentrations were higher in the female. In both sexes, concentrations of T₄ and T₃ in serum reached a peak early in life, between the first and second month of age, and declined thereafter. In addition, in both sexes the intensity of thyroid hormone binding, as judged from values of the in-vitro uptake of T₃, did not change with age, suggesting that free T₄ and T₃ concentrations in the serum display the same sex differences and age-related changes as do the concentrations of total T₄ and T₃. It remains to be determined whether these sex-and age-related alterations in serum thyroid hormone concentration are expressed in differences in the activity of various thyroid hormone-dependent processes.