Some factors influencing metabolism and distribution of fibrinogen in man and rabbits.

To shed some light on the homeostatic regulation of plasma fibrinogen, metabolic studies were made in healthy females, and in normal, thyroidectomized, and thyroxine-treated rabbits. In females, compared with normal males, plasma fibrinogen concentration, plasma and interstitial fibrinogen decreased consequent to an increased fractional catabolic rate and a normal fibrinogen synthesis rate. The interstitial/plasma fibrinogen ratio remained unchanged. In normal rabbits, with increasing body weight fractional catabolic rate and catabolic rate decreased, while fibrinogen concentration and plasma fibrinogen remained constant owing to a simultaneous decrease in fibrinogen synthesis. In addition, fractional transcapillary transfer rate and transcapillary flux also decreased resulting in a shrinkage of interstitial fibrinogen. Thyroidectomy and thyroxine-injection markedly altered fibrinogen metabolism: thyroid hormone accelerated fibrinogen catabolism but also stimulated synthesis. The net result was an increase in plasma fibrinogen and fibrinogen concentration. The interstitial/plasma fibrinogen ratio decreased in thyroxine-treated, and increased in thyroidectomized animals. This study defines the variations of the fibrinogen system parameters in these physiologic and pathologic conditions, and illustrates some patterns of alterations in fibrinogen metabolism.