Impaired renal concentrating ability in hypothyroid man.

The renal concentrating ability was studied in ten patients with hypothyroidism and in 15 euthyroid controls. Solute-free water reabsorption was reduced in the patients with myxedema (4.2 +/- 0.3 ml/min: controls 5.8 +/- 0.6 ml/min; p less than 0.01). This defect was apparent at high rates of solute excretion, and was associated with enhanced excretion of sodium (p less than 0.01) despite a decreased filtered load (p less than 0.005). The myxedema patients had a modest reduction in maximal urine osmolality (p less than 0.04), which was entirely attributable to the lower values observed in younger patients. The results may be explained best by decreased sodium chloride reabsorption in the ascending limb of Henle's loop and/or diminished permeability of the distal nephron in myxedema.