The effect of fasting and refeeding on thyroid follicule structure and thyroid hormone levels in young and old rats.

In order to assess morphological and functional plasticity of the thyroid gland in aging the effects of fasting and refeeding on the thyroid morphology and thyroid hormone serum levels were compared with morphometry and RIA in young and old rats. Young (4 months) and old (24 months) male Wistar rats were fasted for 40 h and sampled, or fasted and fed thereafter for 4 or 24 h. In control (fed) old animals the thyroid follicules were larger, the follicular epithelium was smaller and colloid resorption was smaller than in young rats. 'Thyroid activation index' (epithelial volume/colloid volume ratio) was almost twice lower in the thyroids of control old rats. As the result of fasting, height, surface area and volume of epithelial follicular cells decreased in the thyroids of fasted young rats but not in old ones. On the contrary, in thyroids of fasted old rats the dimensions of epithelial cells did not change and thyroid colloid resorption was increased. After 24 h of refeeding, thyroid morphology in both young and old rats did not differ significantly as compared with control animals. Upon fasting, serum levels of thyroxine (T4) and triiodothyronine (T3) decreased by 28 and 38% in young and by 35 and 46% in old rats, respectively. However, T4 and T3 serum concentrations did not differ significantly between age groups in both fed and fasted states. During refeeding the increase in serum thyroxine concentration was delayed in old rats as compared with young ones. The results of morphological, morphometric and hormonal investigations indicate the existence of age-related changes in the structure and function of thyroid follicular cells.