Hepatic Bioavailability of Serum Thyroid Hormones in Nonthyroidal Illness*

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Abstract
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 an patient sera on the first pass extraction of labeled TSerum was obtained from 1 patients with nonthyroidal illness (NTI) and from 9 control subjects. Patients with NTI demonstrated decreased total T4 and T3 levels; increased rT,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% an 77 ± 5%, respectively, in the case of NTI, and these values were n different from control estimates. The concentrations of total serum T4 and T3available for transport into liver in vivo were 0.69 ± 0.13 jug/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 i available for transport into liver cells in NTI is reduced in proportion to thedecrease in total plasma hormone. (JClin Endocrinol Metab53 913,1981)