Immunoassay Interference due to Macro-TSH: A Case Study of a Pediatric Patient with Hashimoto's Thyroiditis

Thyroid-stimulating hormone (TSH) measurement is central to the diagnostic workup of thyroid diseases. TSH activity is inverse log-linearly related to thyroxine (T4) hormone levels. An isolated elevation of TSH with normal T4 is defined as subclinical hypothyroidism, which is treated by hormone replacement therapy. TSH immunoassays are prone to various interferences, including biotin, heterophile antibodies and macro-TSH complexes. These interferences often lead to misdiagnosis of thyroid dysfunction, potentiating unnecessary clinical investigation and management. This case reports a spuriously elevated TSH in a clinically euthyroid 17-year-old female with a past medical history of Hashimoto’s thyroiditis. By performing a series of troubleshooting experiments such as serial measurement, dilution tests and polyethylene glycol precipitation, we demonstrated potential interference due to the macro-TSH complex.