Hypothyroidism is associated with a number of functional renal disorders primarily affecting water and salt handling. In this study, we aim to investigate functional renal disorders in overt and subclinical hypothyroidism. We utilised urinary beta-2 microglobulin (beta2M) as a sensitive marker of tubular dysfunction. Urinary beta2M excretion and tests for renal functions were repeated before and after T4 replacement therapy. Forty-four patients (28 with overt disease, 16 with subclinical disease) and 31 healthy control subjects were involved in the study. There was a significant increase in urinary beta2M in both overt and subclinical hypothyroidism (p < 0.05 in both). TSH levels were correlated with beta2M excretion. Effects of hypothyroidism on renal functions were readily reversible by 3 weeks of thyroid hormone replacement therapy. Urinary beta2M was a sensitive marker of renal tubular dysfunction associated with overt and subclinical hypothyroidism. (C) 2004 Black-well Publishing Ltd.