Effect of postweaning thyroid status on endocrine and adipose development in rats

We previously reported that 6 wk after thyroidectomy, rat fat cell diameter and number were 86 and 32% of values in age-matched controls. This study aims to 1) determine the time course of these changes and of parallel changes in the levels of several hormones known to participate in adipose differentiation and 2) try to normalize the altered cellularity with thyroxine (T4) treatment. Rats were thyroidectomized (T) or sham-operated and studied 2, 5, or 6 wk later. At 5 wk some T rats received L-T4. Epididymal fat cells were sized and used to estimate fat cell number. Triiodothyronine (T3), T4, thyroid-stimulating hormone, somatotropin (GH), and insulin plasma levels were assayed. In T rats fat cell hypertrophia was blunted uniformly with time, whereas fat cell hyperplasia was slow for 2 wk and zero thereafter. Insulinemia was less drastically reduced than T4, T3, and GH levels. The fat cell number-to-body weight ratio always stayed close to the ratio present at thyroidectomy. In T rats receiving a T4 substitution dose for 1 wk, fat cell number and body weight increased, but not when the T4 dose was lower even though the plasma thyroid hormone levels were increased. These thyroid hormone effects on adipose tissue are probably secondary to other endocrine changes that occur in hypothyroidism.