No abstract

Oxygen consumption and plasma thyroid hormone concentrations are modified by both low- and high-calorie diets. It has been suggested that the trigger may be changes in weight ("adipostatic" hypothesis involving the difference between the actual weight and the "set point") or changes in amount of carbohydrate in the diet ("carbohydrate" hypothesis). Two experiments were performed in order to test both hypotheses. Fourteen young healthy volunteers were studied: 1) at their spontaneous stable weight; 2) while losing weight rapidly on a calorically restricted diet; 3) and then at their stable new weight when consuming a refeeding diet. The calorie restricted diet resulted in decrease of VO2, and T3, and an increase of rT3; the refeeding diet resulted in values of VO2, T3, and rT3 intermediate between those of the spontaneous diet and those of the restricted diet. Another group of nine subjects were studied at their spontaneous caloric and proteic levels, comparing a diet containing only protein and carbohydrate with a diet containing only protein and fat. During the low carbohydrate diet rT3 increased and T3 decreased but they remained unchanged during the carbohydrate-rich diet. Thus neither the adipostatic hypothesis nor the carbohydrate hypothesis is sufficient alone to explain the observed changes in serum T3 and rT3.

Male Wistar rats, 3 weeks old, were thyroidectomized surgically, kept for 1 month at 25 degrees C and then fasted for 3 days, with or without daily intraperitoneal injection of 3,5,3'-triiodo-L-thyronine (4.6 nmol T3/100 g body weight). Age-matched fed euthyroid rats were used as controls. All the experiments were carried out using isolated epididymal adipocytes. Basal lipolysis was higher during fasting in euthyroid or T3-treated adipocytes than in hypothyroid adipocytes. Adipocytes of fed hypothyroid rats were quite unresponsive to theophylline alone or combined with adrenaline or isoproterenol, whereas lipolysis was stimulated by these drugs in euthyroid or T3-treated adipocytes. Such a stimulated lipolysis was increased partially by fasting in hypothyroid adipocytes and was restored to a euthyroid level in T3-treated adipocytes. Lipolysis was more stimulated by adenosine deaminase in fasted euthyroid adipocytes than in fed ones. Hypothyroid and T3-treated adipocytes were unresponsive to adenosine deaminase except in fasted T3-treated rats. In these adipocytes, lipolysis was activated by the combination of adenosine deaminase plus theophylline. Finally, lipolysis was inhibited strongly in hypothyroidism while it was activated weakly by fasting. Lipolysis was inhibited slightly in fasted hypothyroid rats and thyroid hormone restored lipolysis. The findings are discussed in terms of the dual regulation of lipolysis by fasting and thyroid hormones.

Rats were treated with MnSO4, H2O (1 mg/100 g/day, SC) for a period of 5 weeks. Thyroxine (T4) and triiodothyronine (T3) levels were measured in thyroid by radioimmunoassay. T4, T3 and thyroid-stimulating hormone (TSH) levels were also estimated by radioimmunoassay in serum. Manganese treatment produced no change in thyroid T4 and T3 levels but induced a significant decrease in serum T4, T3 and TSH levels. This decrease can be interpreted as the result of a pituitary alteration which appears to be related to the high accumulation of manganese in the pituitary gland.

No abstract