Evidence check: FT4 in the context of allergy

The relationship between FT4 (free thyroxine) and allergies is not well-established. Some observational studies suggest thyroid hormones can modulate immune function—for instance, hypothyroidism (low FT4) may shift the Th1/Th2 balance, potentially influencing allergic responses. However, effect sizes are modest, evidence is mixed, and no causal link has been proven in clinical trials. FT4 is a clinical marker for thyroid status, not a validated allergy biomarker. MyBody-X may include FT4 in hormone tests, but using it for allergy assessment is speculative. Caveat: Do not use FT4 to diagnose or treat allergies without medical supervision. Evidence: human-moderate (observational).