Effect of insulin-induced hypoglycemia on the serum concentrations of thyroxine, triiodothyronine and reverse triiodothyronine.

The effect of insulin-induced hypoglycemia on serum thyroid hormone concentrations was studied in nine healthy individuals. Before, during and after the hypoglycemia blood samples were taken for measurement of the concentrations of glucose, thyroxine (T(4)), triiodothyronine (T(3)), reverse triiodothyronine (rT(3)), catecholamines and pituitary hormones.There was no change in the mean serum T(4) level (+/- the standard error of the mean) of 67 +/- 2 mug/l. However, the T(3) concentrations rose from a mean basal level of 1.86 +/- 0.06 mug/l to a mean peak of 2.51 +/- 0.21 mug/l (P < 0.01) at 45 minutes after the insulin injection, and the rT(3) concentrations fell from a mean basal level of 0.184 +/- 0.008 mug/l to a mean nadir of 0.171 +/- 0.022 mug/l (not a significant change). The mean peak epinephrine level was 545 +/- 103 ng/l and it occurred between 30 and 45 minutes after the insulin injection; the mean peak norepinephrine level was 584 +/- 114 ng/l and it occurred between 30 and 90 minutes after the injection. The growth hormone levels reached a mean peak of 26.1 +/- 4.8 mug/l and the plasma cortisol levels rose to 215 +/- 9 mug/l. The mean basal prolactin level was 8.5 +/- 0.9 mug/l; in five subjects there was a rise to a mean peak of 50.6 +/- 14.6 mug/l, whereas in the remaining four no significant increase occurred. No correlation was found between the changes in the serum T(3) concentration and any of the other factors studied.It was concluded that acute hypoglycemia is associated with a rapid increase in the serum T(3) concentration.