An Anti-Idiotype Antibody to T3: A Misleading Cause of
Inappropriate TSH Secretion -
Case Report:
Treesaranuwattana T, MD¹, Napartivaumnuay N, MD¹, Niramitmahapanya S, MD¹, Sarinnapakorn V, MD¹,
Deerochanawong C, MD¹
Affiliation : ¹ Department of Medicine, Rajavithi Hospital, College of Medicine, Rangsit University, Bangkok, Thailand
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
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