Thyroid function in a diabetic population.

Several alterations in thyroid function are found in diabetes mellitus (see Table I). The most profound changes occur in patients with insulin-dependent diabetes. Plasma T4 is normal whereas plasma T3 is diminished, and the plasma level of rT3 is elevated in diabetic ketoacidosis or in patients with severely uncontrolled diabetes. These changes arise from alterations in the monodeiodination pathways of T4. Both hypo- and hyperthyroidism occur with increased frequency in diabetes. There is an increased prevalence of thyroid autoantibodies in insulin-dependent diabetics. Animals studies suggest a defect in the hypothalamic regulation of the thyroid-pituitary feedback system and an impaired response of the thyroid gland to TSH. Clinical studies are not yet available to confirm the occurrence of these regulatory disturbances in human diabetic patients. It is not clear whether the deiodination and regulatory changes in thyroid hormone economy that are associated with diabetes result in hypothyroidism at the cellular level.