Effect of aging on cold tolerance and thyroid activity in CBA/Ca inbred mice

Relationships between cold tolerance, serum levels of thyroxine (T₄), thyronine (T₃), and thyrotropine (TSH), and thyroid morphometry have been investigated in male CBA/Ca inbred mice at various ages through their life span. From the data obtained it appeared that there was an age-related decrease in cold tolerance up to 18 months of age which was followed by an increase, the age effect being most apparent in relation to cold resistance and cold tolerance during the recovery period following cold exposure. The age-related changes in cold tolerance appeared to be associated with changes in the serum concentrations of T₃, T₄, and TSH. In contrast to the T₃ serum levels which showed a decrease at 36 months, the thyroxine contents showed a perceptible decrease from the age of 12–18 months onwards. A similar pattern was observed for the TSH levels, with a peak at 21 months, followed by a decline at 30 months. A relationship with age between serum T₄ level and thyroid weight was indicated together with structural changes in the thyroid gland, particularly during senescence, for example the size and number of thyroid epithelial cells had become enlarged by 30 months of age.