The effect of altered thyroid function on serum fructosamine concentrations.

ABSTRACT

The effect of altered thyroid function on serum fructosamine concentrations was investigated in 31 untreated hyperthyroid patients, 18 short-term hypothyroid patients (i.e., 20 days after withdrawal of thyroid hormone suppressive therapy for thyroid cancer), 7 untreated long-term hypothyroid patients, and 25 age-matched normal controls. No differences in serum fructosamine concentrations were observed between hyperthyroid patients and normal controls; conversely, serum fructosamine concentrations were significantly higher both in short-term and in long-term hypothyroid patients than those found in normal controls. Furthermore, long-term hypothyroid patients showed significantly higher serum fructosamine concentrations than short-term hypothyroid patients. L-Thyroxine (L-T4), replacement therapy in two hypothyroid patients, resulted in a marked decrease in serum fructosamine concentrations. In seven hyperthyroid patients, the restoration of euthyroidism with antithyroid drug therapy was associated with no significant changes in serum fructosamine concentrations. The results of the present study indicate that hypothyroidism is associated with a marked increase in serum fructosamine concentrations. This alteration does not appear to be the consequence of gross abnormalities in plasma protein or glucose metabolism. The duration of hypothyroidism seems to be an important factor, even though the mechanism underlying this alteration remains at present unexplained. These results also suggest that caution must be used in the interpretation of elevated serum fructosamine concentrations as an index of the metabolic control of diabetes mellitus in the presence of hypothyroidism.