Reversal of increased whole blood and plasma viscosity after treatment of hypothyroidism in man.

Blood and plasma viscosity was measured in 13 patients with hypothyroidism before and during replacement therapy with l-thyroxine. Blood viscosity was measured at natural hematocrit and after adjustment to 40%. The values were compared to those of 12 healthy subjects. Both blood viscosity at hematocrit 40% and plasma viscosity were increased in the hypothyroid state and decreased to the levels of the reference group after treatment. The therapy-induced changes in blood and plasma viscosity were intercorrelated at low but not at high shear rates. The changes in viscosity were not correlated to the reductions in lipoprotein concentrations resulting from therapy. The institution of l-thyroxine replacement therapy was also followed by reductions in erythrocyte sedimentation rate and diastolic blood pressure, both with significant correlations to the decrease in blood viscosity. It is concluded that in hypothyroidism there are changes in both plasma and erythrocytes that increase blood viscosity, with normalization upon treatment with l-thyroxine to euthyroidism.