EXPERIMENTAL HYPOTHYROIDISM INCREASES PLASMINOGEN-ACTIVATOR INHIBITORACTIVITY IN RAT PLASMA

The aim of the present study was to investigate whether hypothyroidism would affect the components of the rat plasma fibrinolytic system. Hy pothyroidism was induced by feeding rats a thiouracil-containing dict for 2 weeks. During this period, half of the animals received subcutan eous injections Of L-thyroxine (20 mug/kg daily) to restore thyroid ho rmone levels. Groups of euthyroid rats were similarly treated With L-t hyroxine. Plasma tissue-type PA (t-PA) activity was not affected by th yroid status. Plasma urokinase-type PA (u-PA) activity was significant ly decreased by the thiouracil-containing diet; however, this effect w as not abolished by L-thyroxine. Plasminogen, alpha2-antiplasmin and f ibrinogen were not affected by hypothyroidism. In the hypothyroid grou p, plasma PAI activity was significantly increased; no such increase w as found in animals simultaneously receiving L-thyroxine. Plasma PAI a ctivity correlated with T3 (r = -0.586; P < 0.005) and cholesterol (r = + 0.737; P < 0.001); in multiple regression analysis, however, the c orrelation between PAI activity and T3 was not significant. The increa se in plasma PAI activity during hypothyroidism may thus not be a dire ct effect of the thyroid hormone, but an indirect one through some oth er, possibly cholesterol related, component.