Changes in extrathyroidal conversion of thyroxine (T4) to 3,3',5-triiodothyronine (T3) in vitro during development and aging of the rat

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.

Perinatal age and sex variations of the triiodothyronine nuclear receptors in the chick pectoralis major muscle

The ontogenesis of the nuclear triiodothyronine receptors was determined in the pectoralis muscle of male and female chicken at 18 days in ovo and 0, 3, 6, 14 and 35 days ex ovo. Our results show the presence of putative T3 nuclear receptors with equilibrium dissociation constant values (Kd approximately 5.50 X 10(-10) M) in good agreement with these reported in other tissues. The T3 receptor numbers decrease from 18 days incubation to 6-day-old animals, then increase until 35 days of age. Compared to the already reported levels of thyroid hormone in plasma and tissues, the results seem to correspond to a down-regulation of the muscle T3 receptors. The nuclear binding capacity of T3 was higher in females than in males, which could be related to the known effects of various sex steroid hormones on the T4 to T3 tissue conversion.

Thyroid function in the elderly

Thyroid dysfunction is common in older individuals, yet the diagnosis is often complicated by atypical clinical presentations and difficulty in interpretation of laboratory tests. An understanding of the alterations in thyroid function occurring normally as a consequence of the aging process is necessary for correct laboratory diagnosis of thyroid dysfunction in the elderly. There are subtle alterations in hypothalamic and pituitary function but normal feedback control of TSH secretion persists. In the thyroid itself, morphologic changes develop with age, but have little impact on thyroid hormone economy. Thyroidal secretion of thyroxine decreases, but parallels the decrease in thyroxine degradation rate, resulting in unaltered plasma thyroxine levels. Decreased peripheral conversion of thyroxine to triiodothyronine causes a fall in triiodothyronine concentrations. Nonthyroidal illnesses in the elderly may perturb the laboratory assessment of thyroid function by producing isolated high or low thyroxine levels in euthyroid individuals.