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Triiodothyronine--binding immunoglobulin in a euthyroid patient.
Clinical Biochemistry
|June 1, 1981
Summary
A patient with high thyroxine levels had undetectable triiodothyronine due to a unique T3-binding protein. This immunoglobulin G antibody interfered with thyroid function tests, highlighting the need for careful interpretation.
Area of Science:
- Endocrinology
- Clinical Chemistry
- Immunology
Background:
- Thyroid hormone assays are crucial for diagnosing thyroid dysfunction.
- Radioimmunoassay (RIA) is a common technique for measuring thyroid hormones like triiodothyronine (T3) and thyroxine (T4).
- Elevated thyroid hormone levels can occur after radioactive iodine therapy for hyperthyroidism.
Observation:
- A clinically euthyroid 60-year-old woman presented with elevated thyroxine (188 µg/L) and normal TSH (4.7 mU/L) after radioactive iodine therapy.
- Her T3 levels were initially unmeasurable by RIA, and added T3 could not be recovered, suggesting interference.
- High titers of thyroid microsomal antibodies (1:100,000) were detected.
Findings:
- Ethanol extraction of the patient's serum revealed a significantly elevated T3 level (17 µg/L).
- Agarose gel electrophoresis identified a T3-binding protein in the gamma globulin fraction, which did not bind T4.
- Scatchard analysis indicated this protein, likely immunoglobulin G (IgG), had a high affinity (2 x 10^9 L/mole) and capacity (50 µg/L) for T3.
Implications:
- This case demonstrates a rare instance of an autoantibody interfering with thyroid function tests.
- Circulating thyroid hormone-binding antibodies can lead to misinterpretation of thyroid status.
- Clinical correlation is essential when laboratory results appear incongruent with a patient's condition, especially in the presence of autoantibodies.