Overview
Macro-TSH is an uncommon laboratory phenomenon in which thyroid-stimulating hormone (TSH) circulates in the blood as a large molecular complex, usually bound to one or more immunoglobulins. Because the complex is much larger than native TSH, it is cleared more slowly from the bloodstream and can cause persistently elevated measured TSH levels despite normal free thyroxine (fT4). This can mimic subclinical hypothyroidism and lead to unnecessary treatment with levothyroxine (LT4).
This study examined the prevalence and clinical characteristics of macro-TSH in patients evaluated after TSH harmonization, a laboratory standardization process that improves comparability between TSH assays. The results are clinically important because they show that macro-TSH remains rare, but when present it can meaningfully complicate diagnosis and treatment decisions.
