Hepatic bioavailability of serum thyroid hormones in nonthyroidal illness.

Serum was obtained from 11 patients with nonthyroidal illness (NTI) and from 9 control subjects. Patients with NTI demonstrated decreased total T4 and T3 levels; increased rT3, T3 resin uptake, and percent free (dialyzable) T4 levels; and normal TSH and free T4 concentrations in vitro. In addition, the effects of control and patient sera on the first pass extraction of labeled T4 and T3 by rat liver was measured with a tissue sampling-single injection technique. The percent of total serum T4 and T3 that was transported into liver on one pass was 17 +/- 2% and 77 +/- 5%, respectively, in the case of NTI, and these values were no different from control estimates. The concentrations of total serum T4 and T3 available for transport into liver in vivo were 0.69 +/- 0.13 micrograms/100 ml and 21 +/- 2 ng/100 ml, respectively, in NTI, and these values were 46% and 18% of control values, respectively. Therefore, in contrast to in vitro estimates of free T4, in vivo measurements indicate the amount of circulating T4 or T3 that is available for transport into liver cells in NTI is reduced in proportion to the decrease in total plasma hormone.