Plasma and urine osmolality in untreated thyroprivic hypothyroidism: effect of eltroxin therapy.

Plasma and urine osmolality were determined in 56 thyroprivic hypothyroid patients before and at 2 and 6 months of eltroxin treatment. Normal range of plasma osmolality (Posm) was defined at 95% confidence limits of values obtained from 25 euthyroid healthy controls. Thirty patients (53%) had lower Posm. The mean Posm in 56 hypothyroid patients increased from 283.4 +/- 2.06 SEM pretreatment, to 295.3 +/- 1.5 at 2 months (p < 0.001) and 296.5 +/- 1.07 at 6 months. The improvements in Posm from those with lower values (30 patients) were more appreciable: 272.3 +/- 1.03 pretreatment, 290.3 +/- 1.9 at 2 months (p < 0.001) and, 296.16 +/- 1.1 at 6 months (p < 0.001). Urine osmolality also increased from a low normal mean of 477.04 +/- 29.64 to 582.48 +/- 28.67 at 2 months (P < 0.05) and 651.46 +/- 28.35 at 6 months (P < 0.001) of eltroxin treatment. Plasma and urine osmolality correlated positively with the clinical severity of hypothyroid state (plasma T3,T4,&TSH) (P < 0.001). These observations suggested a positive influence of thyroid hormones on plasma and urine osmolality and that plasma and urine osmolality increased with achievement of euthyroid state.