A radioimmunoassay for measurement of 3,3′-diiodothyronine sulfate: studies in thyroidal and nonthyroidal diseases, pregnancy, and fetal/neonatal life

Abstract

Data suggesting that (1) sulfation of the phenolic hydroxyl of iodothyronines plays an important role in thyroid hormone metabolism and (2) maternal serum 3,3′-diiodothyronone sulfate (3,3′-T 2S) may reflect on the status of fetal thyroid function stimulated us to develop a radioimmunoassay (RIA) for measurement of T 2S. Our T 2S RIA is highly sensitive, practical, and reproducible. T 4S, T 3S, and T 1S crossreacted 3.1%, 0.81%, and 5.3%, respectively; thyroxine (T 4), triiodothyronine (T 3), and reverse (r)T 3, 3,3′-T 2 and 3′-T 1 crossreacted <0.1%. Although rT 3 sulfate (rT 3S) crossreacted 55% in 3,3′-T 2S RIA, its serum levels are very low and have little influence on serum T 2S values reported here. T 2S was measured in ethanol extracts of serum, amniotic fluid, and urine. Recovery of nonradioactive T 2S added to serum was 96%. The dose-response curves of inhibition of binding of 125I-T 2S to anti-T 2S by serial dilutions of ethanol extracts of serum or urine were essentially parallel to the standard curve. The detection threshold of the RIA varied between 0.17 and 0.50 nmol/L (or 10 and 30 ng/dL). The coefficient of variation (CV) averaged 9% within an assay and 13% between assays. The serum concentration of T 2S was [mean ± SE, nmol/L] 0.86 ± 0.59 in 36 normal subjects, 2.2 ± 0.06 in 10 hyperthyroid patients ( P < .05), 0.73 ± 0.10 in 11 hypothyroid patients (not significant [NS]), 6.0 ± 1.5 in 16 patients with systemic nonthyroidal illness ( P < .001), 18 ± 2.5 in 16 newborn cord blood sera ( P < .02), 2.7 ± 0.32 in 25 pregnant women [15 to 40 weeks gestation, P < .001], 0.94 ± 0.10 in 10 hypothyroid women receiving T 4 replacement therapy (NS), and 2.0 ± 0.38 in 11 hypothyroid women treated with T 4 replacement and oral contraceptives ( P < .02); serum T 2S levels in the third trimester of pregnancy were similar to those in the second trimester of pregnancy. T 2S concentration in amniotic fluid was 12.5 ± 2.7 nmol/L (n = 7) at 15 to 20 weeks gestation, and it decreased markedly to 3.3 ± 1.3 nmol/L (n = 3) at 35 to 38 weeks gestation. Urinary excretion of T 2S in random urine samples of 19 normal subjects was 10.9 ± 1.3 nmol/g creatinine. (1) T 2S is a normal component of human serum, urine, and amniotic fluid, and serum T 2S levels change substantially in several physiologic and pathologic conditions; (2) high serum T 2S in pregnancy may signify increased transfer of T 2S from fetal to maternal compartment, estrogen-induced increase in T 2S production, decreased clearance, or a combination of these factors. The data do not support the notion that fetal thyroid function is the only or the predominant factor responsible for high serum T 2S in pregnant women.