Is FT4 useful for allergy?

FT4 (free thyroxine) is a thyroid hormone and not a standard biomarker for allergies. Allergies are primarily diagnosed via IgE antibodies, skin prick tests, or clinical history. There is only an indirect link: thyroid dysfunction (e.g., hyperthyroidism) can cause allergy-like symptoms such as urticaria or exacerbate existing allergies. In such cases, measuring FT4 may help rule out thyroid involvement. However, the evidence for FT4 as an allergy marker is weak, based mainly on clinical practice and observational studies. A normal FT4 does not exclude allergies. For allergy evaluation, specific allergy tests are required. FT4 is not useful for routine allergy screening or diagnosis.