Triiodothyronine: a link between the insulin resistance syndrome and bloodpressure?

Objective Overall obesity is associated with elevated serum triiodothyronin e concentrations and insulin resistance. Oral triiodothyronine is known to induce hypertension in laboratory rats, while triiodothyronine also increas es the expression of genes encoding for enzymes involved in the synthesis a nd secretion of insulin by pancreatic beta cells. We investigated the hypot hesis that central obesity and insulin resistance are linked with an increa sed blood pressure and insulin production through elevated free serum triio dothyronine concentrations. Design A cross-sectional study of 47 healthy euthryroid subjects (17 men, 3 0 women; 34 +/- 15 years, mean +/- SD). Methods The waist:hip ratio was used as measure of central obesity, and ins ulin-stimulated glucose disposal during a hyperinsulinaemic euglycaemic cla mp was used as measure of insulin sensitivity. Insulin production was calcu lated from the insulin clearance during the clamp and fasting insulin conce ntrations. Results Free serum triiodothryronine concentrations correlated, independent of age and gender, positively with systolic and diastolic blood pressure, insulin production and fasting insulin. There was only a borderline signifi cant correlation of free serum triiodothyronine with the waist-to-hip ratio , and no correlation with insulin sensitivity as assessed during the clamp. The correlations of free serum triiodothyronine with blood pressure, insul in production and fasting insulin were independent of the waist:hip ratio a nd insulin sensitivity. Conclusion Our hypothesis of free serum triiodothyronine as an intermediate factor in the insulin resistance syndrome is refuted by these data, but we identified free serum triiodothyronine concentrations as a new determinant of blood pressure, insulin production, and fasting insulin in healthy euth yroid subjects. J Hypertens 1999, 17:1725-1730 (C) Lippincott Williams & Wi lkins.