Measurement of 3'-monoiodothyronine in human serum.

An heterologous radioimmunoassay for measurement of 3'-monoiodothyronine (3'T1) has been developed using pure 3'T1 as standard, (125I)DL3'T1 and an anti 3,3'L-diiodothyronine antiserum. The assay utilizes Sephadex G25F minicolumns to separate 3'T1 from other endogenous iodothyronines. 8-anilino-1-naphthalene sulphonic acid was used to inhibit binding of 3'T1 to serum binding proteins. Sensitivity was approximately 3.2 pmol/l. The mean serum 3'T1 concentration was 7.4 pmol/l in normal subjects, 30.8 pmol/l in thyrotoxic patients, 4.1 pmol/l in hypothyroid patients, 23.2 pmol/l in patients with severe non-thyroidal illness and 109.8 pmol/l in cord blood. Increased levels of 3'T1 were found in two normal volunteers who were injected with 3,3'T2, demonstrating that 3'T1 is derived from 3,3'T2 in extrathyroidal tissues. These studies suggest that 3'T1 is a minor iodothyronine metabolite in the human. It is unlikely to have significant biological relevance.