Triiodothyronine (T3)-Binding Immunoglobulins in a Euthyroid Woman: Effects on Measurement of T3(RIA) and on T3Turnover
- 1 April 1976
- journal article
- research article
- Published by The Endocrine Society in Journal of Clinical Endocrinology & Metabolism
- Vol. 42 (4), 642-652
- https://doi.org/10.1210/jcem-42-4-642
Abstract
A 36-year-old woman with nodular goiter, nervousness, and tachycardia was evaluated for T3 toxicosis. Her serum thyroxine level, resin T3 uptake, and thyroidal radioiodine uptake were normal. Her T3, (RIA), by a technique employing charcoal to separate bound and free T3, was reported as indeterminate due to an interfering substance; by a double-antibody method, her T3 (RIA) was 325 ng/dl. Further studies of the patient's serum revealed an abnormal T3-binding protein which migrated in the beta-gamma globulin zone on paper electrophoresis and gel filtration chromatography (Sephadex G-200), and was precipitated from serum by rabbit anti-human Fab antibody. The gamma globulin fraction of the patient's serum, separated by a standard technique, showed strong binding activity toward [125I]T3, with an association constant of 4.1 × 108 l/mole (Scatchard plot). In a similar system, labeled T4 was not bound. To avoid artefacts which this T3-binding protein might produce in assaying unextracted serum, T3 (RIA) was performed on an ethanol extract of serum and found to be 191 ng/dl, a slight elevation. However, the metabolic clearance rate of injected [125I]T3, estimated by non-compartmental analysis of the serum decay curve or by the specific activity of urinary T3, was about 16 l/day, a low value, so that the T3 production rate, 31 μg/day, was normal. The patient's symptoms disappeared with the resolution of domestic problems, and she appeared clinically euthyroid. Serum TSH was 5.0 uU/ml and antithyroglobulin titer, 1:16. A test for antibodies to thyroid microsomes was negative. We postulate that this subject was euthyroid, but had a concentration of T3-binding immunoglobulin which was sufficient to produce modest slowing of T3 turnover, borderline elevation of extractable T3 (RIA), and a major artefact in the T3 (RIA) measurement of unextracted serum. A similar abnormality may account for other instances of. high T3:T4 ratios in serum.Keywords
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