INTERACTION OF THYROID-HORMONE AND HEMOGLOBIN .1. NATURE OF THE INTERACTION AND EFFECT OF HEMOGLOBIN ON THYROID-HORMONE RADIOIMMUNOASSAY

  • 1 January 1980
    • journal article
    • research article
    • Vol. 95  (5) , 714-724
Abstract
Gel filtration of human RBC red blood cell lysate incubated with labeled T4 [thyroxine] or T3 [triiodothyronine] revealed co-elution of a major iodothyronine-binding fraction (R-2) and Hb. Solutions of purified human Hb and T3 also showed co-elution of hormone and Hb. Because hematin and protoporphyrin bind labeled T3, the oxygen-binding site on Hb was excluded as the site of iodothyronine-Hb interaction. Analysis of hormone binding by heme and globin moieties showed T3 binding to be limited to the heme fraction. Addition of excess unlabeled T3 to Hb or heme incubated with labeled T3 indicated 75% to 90% of hormone binding was poorly dissociable. The presence of Hb in RBC lysate or in serum could influence the measurement of T4 and T3 by specific RIA. Subsequent studies of the addition to serum of human Hb revealed a significant reduction in T3 and T4 detectable by RIA in the presence of this protein. The effect was influenced by the concentration of Hb and by duration and temperate of incubations of hemoglobin and serum prior to RIA. Incubated for 5 days at 4.degree. C, 14 sera containing 10 gm/dl Hb showed a mean decrease in T3 concentration of 40% compared to sera incubated in the absence of Hb (160.1 to 93.9 ng/dl, P < 0.001); detectable serum T4 fell by 50% in 13 sera incubated under the same conditions (5.40 .mu.g/dl without Hb to 2.55 .mu.g/dl in the presence of Hb, P < 0.001). Hb concentrations in serum as low as 0.1 and 0.5 gm/dl affected the RIA significantly. Thus a major fraction of thyroid hormone binding in human RBC cytoplasm is accounted for by an interaction with Hb. This interaction in serum or RBC lysates is a significant variable affecting iodothyronine determinations.