[High blood triiodothyronine and the euthyroid state]

Area of Science:

• Endocrinology
• Immunology
• Clinical Chemistry

Background:

• Thyroid hormones, including triiodothyronine (T3) and thyroxine (T4), are crucial for metabolic regulation.
• Abnormalities in thyroid hormone levels can indicate various endocrine disorders.
• Accurate measurement of thyroid hormones is essential for diagnosis and management.

Observation:

• A euthyroid (normal thyroid function) woman presented with unexpectedly high circulating levels of T3.
• Serum analysis using agarose gel electrophoresis revealed abnormal T3 binding within the gamma-globulin region.
• This abnormal binding was found to interfere with standard T3 radioimmunoassay procedures.

Findings:

• Immunological characterization identified the interfering substance as a polyclonal antibody, specifically IgG-kappa and IgG-lambda.
• This autoantibody demonstrated specific binding to T3 but not to T4.
• Scatchard analysis of the isolated gamma-globulin fraction showed a high-affinity binding site for T3 (Ka = 0.4 x 10^9 L/M) with a maximal binding capacity of 5.2 x 10^-9 M.

Implications:

• This case highlights a rare cause of falsely elevated T3 levels due to autoantibodies.
• The presence of T3-specific autoantibodies can complicate thyroid function testing and interpretation.
• Understanding such antibody-mediated interference is crucial for accurate endocrine diagnostics.