Low-affinity IgG-mediated macro-TSH: diagnostic limitations of peak-based gel filtration chromatography-a case report with review of literature.

Macro-thyroid-stimulating hormone (macro-TSH) is a well-recognized cause of spuriously elevated TSH results. However, its characterization still relies largely on gel filtration chromatography (GFC), which may fail to detect macro-TSH associated with unstable or low-affinity immune complexes. In particular, when second-line screening tests, such as polyethylene glycol (PEG) precipitation, strongly suggest macro-TSH but GFC shows no distinct high-molecular-weight TSH fraction, macro-TSH may be incorrectly ruled out as the source of interference. A 43-year-old woman treated with levothyroxine for subclinical hypothyroidism showed persistently elevated TSH despite high-normal free thyroxine levels and no clinical signs of hypothyroidism. PEG precipitation and Protein G treatment resulted in TSH recovery rates of 8% and 23%, respectively, indicating macro-TSH mediated by immunoglobulin G, while heterophile and assay-specific interferences were considered unlikely. GFC under neutral conditions revealed a subtly broadened TSH distribution without a distinct high-molecular-weight peak. Quantitative peak analysis showed increased full width at half maximum and area under the curve, with values approximately 1.5- to 2-fold higher than those in controls in both the initial and repeat runs, confirming significant peak broadening. Under acidic conditions, the patient's TSH elution profile completely overlapped with those of the controls, supporting the presence of macro-TSH due to low-affinity TSH-IgG complexes. This case highlights the diagnostic challenge posed by macro-TSH, particularly in cases involving low-affinity antibody interactions. It underscores the importance of assessing the overall elution profile rather than relying solely on peak position. Recognizing such atypical presentations can prevent unnecessary thyroid hormone treatment and improve diagnostic accuracy.