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A Radioimmunoassay for 3,3',5'-Triiodo-L-Thyronine in Unextracted Serum: Method and Clinical Results

Division of Endocrinology, University of Geneva, Sioitzerland

Reprint requests should be sent to: Dr. A. Burger, Laboratoire d'Investigation Clinique, Hopital Cantonal, CH-1211 Geneve 4, Switzerland.

Serum 3,3',5'-triiodothyronine (rT₃) was measured with a radioimmunoassay in unextracted serum. The assay was specific and reproducible. The coefficients of variation for 3 different sera known for high, normal, and low rT₃ concentrations between assays were 4, 6, and 9% and within assays 4, 9, and 7%, respectively. In euthyroid subjects 20 to 60 years, old, rT₃ was 450 ± 200 pg/ml (mean ± 2 SD, n = 83). Serum rT₃ was found to be increased in hyperthyroidism (range: 762-2581 pg/ml; n = 11) but also in acute and chronic illness (up to 2400 pg/ml; n = 24) and in anorexia nervosa (536–1058 pg/ml; n = 7). In the latter two situations there was mostly an inverse change in serum 3,5,3'-triiodothyronine (T₃) which was in the low normal range or decreased. These findings suggest a metabolic control of thyroxine deiodination. A low serum rT₃ was found in 9 of 12 hypothyroid patients and in the serum of 1 chronically ill patient. Long-term treatment (1–7 years) with lithium carbonate slightly reduced serum rT₃, although the changes were inside the normal range. Kidney function was not found to be necessary for its production, as anephric patients had normal rT₃ values. In addition, hemodialysis increased serum rT₃, which is probably due to the heparin therapy.

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