Oral triiodothyronine administration lowers plasma fibronectin levels in humans.

It has been shown that both serum triiodothyronine (T3) and plasma fibronectin values decline with fasting and increase with treatment of diabetic ketoacidosis in a paralles manner. To evaluate the mechanism responsible for these changes, we examined the effects of both oral T3 administration and fasting in six healthy, adult subjects. Initial plasma fibronectin values were normal in four subjects (group 1) and decreased in two subjects (group 2). Initial serum T3 and thyroxine (T4) values were normal in both groups. Despite a substantial rise in serum T3 values with oral T3 administration, plasma fibronectin fell in group 1 subjects. Fasting caused a significant decline in serum T3 levels, but only a small further decline of plasma fibronectin concentrations. Serum T3 levels rose after 1 week of refeeding, whereas plasma fibronectin levels in group 1 did not return toward normal. Plasma fibronectin values did not change in group 2 subjects with oral T3, fasting, or refeeding. In conclusion, there is no simple cause-and-effect relationship between previously observed changes in plasma fibronectin and serum T3 concentrations. High doses of oral T3 will lower plasma fibronectin levels in subjects with initially normal plasma fibronectin values and a normal caloric intake.