Objective: Low thyroid function may be associated with reduced glomerular filtration rate (GFR). We therefore studied the association of thyroid function with estimated GFR (eGFR) in a population-based study.
Design: A cross-sectional, population-based study of 29 480 individuals above 40 years of age, without previously known thyroid disease.
Methods: We calculated geometric mean eGFR and odds ratio (OR) of chronic kidney disease (CKD; eGFR <60 ml/min per 1.73 m(2)) according to categories of thyroid function, using people with TSH in the lower third of the reference range (0.50-1.4 mU/l) as the comparison group.
Results: TSH within the reference range (0.50-3.5 mU/l) was negatively associated with eGFR (P for trend <0.001). Compared with people with TSH in the lower third of the reference range (83.0 ml/min per 1.73 m(2)), eGFR was lower in people with TSH in the middle (81.6 ml/min per 1.73 m(2)) and highest third (80.3 ml/min per 1.73 m(2)) of the reference range, and in people with subclinical (79.3 ml/min per 1.73 m(2), P<0.001) or overt hypothyroidism (76.5 ml/min per 1.73 m(2), P<0.001). The prevalence of CKD was higher in people with TSH in the middle (OR 1.20, 95% confidence interval (CI) 1.07-1.35) or highest third (OR 1.31, 95% CI 1.13-1.52) of the reference range, compared with people in the reference group. Also, CKD was more common in people with subclinical (OR 1.63, 95% CI 1.38-1.93) or overt (OR 1.98, 95% CI 1.22-3.20) hypothyroidism.
Conclusions: These findings suggest that low thyroid function, also within the clinically normal range, is associated with reduced GFR.