3′-l-Monoiodothyronine: Development of a Radioimmunoassay and Demonstration ofin VivoConversion from 3′,5′-Diiodothyronine*

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Abstract
A sensitive, reproducible RIA for the measurement of 3′-l-monoiodothyronine (3′T1) is described. Mean intraand interassay coefficients of variation were 2.4% and 22.5%, respectively. Cross-reactivity with other iodothyronines was negligible, except for 3,3′-l-diiodothyronine (3.3T2) which started to demonstrate cross-reactivity when 3,3′T2 levels were elevated above 35 ng/dl. Fifty percent displacement occurred when 500 pg 3,3′T2 were added to the 3′T1 assay. Employing this assay, 11 normal subjects and 7 pregnant women had serum 3′T1 levels below the limits of detectability of the assay (1, with the mean (±sd) values being 6.5 ± 3.0 ng/dl. Serum 3′T1 levels were present in all cord sera measured (7.3 ± 2.3 ng/dl; n = 19), and the highest levels of 3′T1 observed were in 38- to 40-week gestation amniotic fluid specimens (15.4 ± 8.4 ng/dl; n = 20). Compared to other iodothyronines, it seems that a relatively low proportion of 3′T1 is bound to circulating proteins, as the mean percentage of dialyzable 3′T1 in 12 normal subjects was 5.7 ± 1.2%. An oral dose of 3′T1 (120 jug) given to 2 euthyroid individuals resulted in peak serum levels of 28 ng/dl 2 h after ingestion. After iv administration of 3′5′T2 to 2 athyreotic patients (1 hypothyroid and the other euthyroid on replacement T4), 3′T1 levels rose from undetectable levels to 20 ng/dl. It was concluded that 3′T1 is routinely detectable in the serum of hyperthyroid but not normaljndividuals, and even higher levels are present in cord sera and amniotic fluid. Moreover, the study demonstrates that in vivo conversion of 3′,5′T2 to 3′T1 may occur.

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