title: Triiodothyronine (T3)-binding immunoglobulins in a euthyroid woman: effects on measurement of T3 (RIA) and on T3 turnover.
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 misgrated 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 X 10(8) 1/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 or urinary T3, was about 16 1/day, a low value, so that the T3 production rate, 31 mug/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.
mesh_terms: D000328:Adult; D001665:Binding Sites; D004590:Electrophoresis, Paper; D005260:Female; D006044:Goiter, Nodular / Q000097:blood; D006801:Humans; D007122:Immunoelectrophoresis; D007136:Immunoglobulins / Q000378:metabolism*; D011485:Protein Binding; D011863:Radioimmunoassay; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood* / Q000276:immunology / Q000652:urine
chemical_list: D007136:Immunoglobulins; D014284:Triiodothyronine; D013974:Thyroxine