SAT-612 An Anti-Idiotype Antibody to T3 Measurements: A Misleading Cause of Inappropriate TSH Secretion

Background: Measurements of thyrotropin (TSH), total and free thyroxine (TT4,FT4), triiodothyronine (TT3,FT3) are widely used diagnostic methods for thyroid function evaluation. However, some serum samples demonstrate a nonspecific binding with assay reagents that can interfere with the measurement of these hormones. Several steps can be taken to identify results that may reflect the presence of interfering factors including: 1) discrepancies between current values and previous results using the same methods; 2) clues from the clinical setting; 3) the pattern of discrepant and non-discrepant thyroid profile results (1,2) Clinical case: A 67-year-old woman admitted with generalized muscle weakness for one week. On physical examination she had left facial palsy (LMN), dysarthria, motor power grade 1 in her lower extremities and grade 3 in her upper extremities, her deep tendon reflex was diminished and her peripheral sensation was decreased as glove and stocking pattern. She was also experiencing normal anal sphincter tone and perianal sensation. She was diagnosed with hypertension and she was prescribed atenolol for treatment. She was clinically euthyroid. Provisional clinical diagnosis was Guillain-Barre syndrome. Because her laboratory profile showed hyponatremia, the following thyroid function tests were ordered: TSH 7.090 mIU/L (0.27-4.20 normal range [NR]); FT3 13.33 pg/ml (2.00-4.40 NR); FT4 1.37 ng/dl (0.93-1.70 NR) tested by one-step electrochemiluminescent immunoassay (ECLIA,Roche platform,Cobas(®));anti-thyroid peroxidase antibody (16.25 IU/ml, normal); anti-thyroglobulin antibody (<10 IU/ml); serum morning cortisol concentration, normal. The thyroid profile was repeated on new samples from the same patient 3 and 7 days later. The results confirmed mildly elevated TSH with high FT3 concentration but normal FT4. Then, the thyroid profile also was processed in a different laboratory using a different technique (Abbott(®) platform). Using this platform, the FT3, FT4 and TSH were all within reference ranges. To confirm that the sample contained an interfering, an aliquot also was sent to Roche for further analysis. The results showed that the patient had antibodies to anti-idiotype of both the T3 and FT3 assays. Conclusion To avoid misdiagnosis or inappropriate treatment decisions, when there are discrepancies between the clinical setting and the thyroid function tests, the clinician should always consider that there is a substance interfering with the assay. References: (1)Tate J, ward G. Interferences in immunoassay. The Clinical Biochemist Reviews. 2004;25(2):105-120(2) Ismail Y, Ismail AA. Erroneous laboratory results: what clinicians need to know. Clinical medicine (London,England).2007;7(4):357-361.