Relationship between serum TSH levels and intrarenal hemodynamic parameters in euthyroid subjects.

Objective: Low thyroid function may be associated with a reduced glomerular filtration rate (GFR) calculated on the basis of creatinine metabolism. Thyroid hormone directly affects serum creatinine in muscle and low thyroid function might exert a similar direct effect in the kidney. The goal of the study was to evaluate this possibility by assessment of the inulin-based GFR and to examine the mechanism underlying the reduction of GFR. Patients and methods: Renal and glomerular hemodynamics were assessed by simultaneous measurements of plasma clearance of para-aminohippurate (C_PAH) and inulin (C_in) in 26 patients with serum creatinine<1.00 mg/dl and without thyroid disease. All subjects were normotensive with or without antihypertensive treatment and were kept in a sodium-replete state. Renal and glomerular hemodynamics were calculated using Gomez's formulae. Results: Serum TSH, including within the normal range (0.69-4.30 µIU/ml), was positively correlated with vascular resistance at the afferent arteriole (R_a) (r=0.609, P=0.0010), but not at the efferent arteriole (R_e). Serum TSH was significantly and negatively correlated with renal plasma flow (RPF), renal blood flow (RBF), and GFR (r = -0.456, P = 0.0192; r = -0.438, P = 0.0252; r = -0.505, P=0.0086 respectively). In multiple regression analysis, serum TSH was significantly positively associated with Ra after adjustment for age and mean blood pressure. Conclusions: These findings suggest that low thyroid function, even within the normal range, is associated with reduced RPF, RBF, and GFR, which might be caused by a preferential increase in R_a. [ABSTRACT FROM AUTHOR]