EFFECT OF SMALL DOSES OF IODINE ON THYROID-FUNCTION DURING CALORIC RESTRICTION IN NORMAL SUBJECTS

It is well recognized that starvation and malnutrition are associated with a low-T-3 syndrome in man. A similar condition has been observed after intake of a low carbohydrate hypocaloric diet. However, little i s known about the influence of iodine on these conditions. Therefore, we evaluated the effect of iodine supplementation on thyroid function before and after a short-term intake of a low carbohydrate diet in nor mal subjects residing in an iodine-deficient area. The study was perfo rmed in 16 young euthyroid, nonobese volunteers (11 males, 5 females). The subjects were placed on a low carbohydrate (800 kcal) diet for 4 days. Eight subjects received 500 mu g iodine (oral) daily beginning 4 weeks before diet. The control group (n = 8) received no iodine. Afte r iodine supplementation, iodine excretion increased from 52 to 405 mu g iodine/g of creatinine. Total T-4 showed a slight but significant i ncrease (104.2 nmol/l vs. 115.8 mu g/dl; p < 0.001); fT(4) was unchang ed. The intake of the hypocaloric low carbohydrate diet resulted in a striking decrease in both total and free T-3 and an increase of rT(3) irrespective of iodine supplementation. T-4 and fT(4) were not affecte d in either group. During diet, iodine administration resulted in a de crease of basal TSH from 2.3 to 1.2 mU/l (p < 0.05), Delta TSH from 10 .3 to 4.5 mU/l (p < 0.01) and Delta T-3 (T-3 180 min after TRH) from 0 .7 to 0.3 nmol/l (p < 0.01). In contrast, no significant changes of ba sal TSH and T-3 response after TRH were seen during diet in the contro l group. In conclusion, changes of T-3, fT(3) and rT(3) during caloric restriction occurred irrespective of iodine supplementation. However, significant alterations in basal TSH and T-3 response after TRH could only be demonstrated in subjects receiving iodine supplementation.