Thyroid function in patients with proteinuria

Background: Patients with proteinuria may suffer from substantial losses of functional proteins such as hormones and hormone-binding proteins. A limited number of studies have reported urinary losses of thyroid hormones and thyroxin-binding globulin. Overt hypothyroidism attributable to these urinary losses has been described. However, the impact of proteinuria on thyroid function parameters has not been studied in a large patient cohort. Methods: We evaluated thyroid function parameters in patients with proteinurea who are negative to thyroxine peroxidase antibodies (TPOAbs). Values of free thyroxin and thyroid-stimulating hormone (TSH) were compared with data from age- and gender-matched controls derived from the Nijmegen Biomedical Study, a population-based survey conducted in our hospital. Results: We evaluated 159 patients. There were 111 males and 48 females. Median (IQR) age was 52 (40 to 62) years, serum creatinine concentration 99 (82 to 134) μmol/l, serum albumin concentration 29 (22 to 35) g/l, and proteinuria 6.6 (3.1 to 10.9) g/10 mmol creatinine. Median TSH was significantly higher in the patients than the controls (1.81 mU/l vs 1.34 mU/l, p<0.0001). In the patients, TSH was negatively correlated with serum albumin (r=-0.21; p<0.01). Subclinical hypothyroidism was six times more frequent in the patients (11.3 vs 1.8%, p<0.001); however, overt hypothyroidism was observed in only one patient. Conclusion: Patients with proteinuria have higher TSH levels, consistent with urinary loss of thyroid hormones. However, these urinary losses do not result in overt, clinically relevant, hypothyroidism. The role of subclinical hypothyroidism in these patients needs further evaluation.