The effect of long-term metabolic control on free thyroid hormone levels in diabetics during insulin treatment.

Serum concentration of free T3 and, in female patients, FT4, were found to be lower in 20 asymptomatic, moderately-poor or well controlled, diabetics treated with insulin than in a group of non-diabetic subjects. Over a mean 3-month period of the study a significant fall occurred in HbA1 concentration in both groups of diabetics without change in free thyroid hormone levels. The mean capillary blood glucose, fasting free insulin and fasting lipid concentrations, other than high density lipoprotein (HDL) cholesterol, did not change. No correlations were found between the changes in HbA1 and free thyroid hormone concentrations. Improvement in long term metabolic control did not influence free thyroid hormone levels in well controlled and moderately-poor controlled diabetics, taking insulin.