A 67-year-old woman presented with a one-week history of generalized muscle weakness. On physical examination, she had a left facial palsy, dysarthria, grade 1 strength in lower extremities and grade 3 in upper extremities, diminished deep tendon reflexes, and peripheral sensation in a glove and stocking pattern. Anal sphincter tone and perianal sensation were normal. She was being treated with atenolol for hypertension. She was clinically euthyroid. The provisional clinical diagnosis was Guillain- Barre syndrome. Her laboratories profile showed hyponatremia so that thyroid function test was investigated. TSH 7.090 mIU/L (0.27 to 4.20), FT3 13.33 pg/ml (2.00 to 4.40), FT4 1.37 ng/dl (0.93 to 1.70) tested by one-step electrochemiluminescent immunoassay (Cobas® platform). Negative results of anti-thyroid peroxidase antibody (16.25 IU/ml) and anti-thyroglobulin antibody (<10 IU/ml). Serum morning cortisol concentration was normal. The sample was repeated at the same lab to exclude laboratory error. The results confirmed mildly elevated TSH with a high FT3 concentration but a FT4 level in the reference range. Thus, the authors assessed FT3, FT4, and TSH in a different laboratory (Architect® platform). This revealed FT3, FT4, and TSH within reference ranges. Confirming the assay interference, the sample was sent to Roche for further analysis as well. The results showed that she had antibodies to anti-idiotype of T3 and FT3 assay.

Keywords: Free T3, Assay interference, Human anti-mouse antibodies, Anti-idiotype antibodies

Received 27 Jun 2019 | Revised 19 Aug 2019 | Accepted 22 Aug 2019