was ist ft3 — what are common causes?

FT3 (free triiodothyronine) is the active thyroid hormone circulating in blood, regulating metabolism. It is mainly produced by the thyroid gland but also converted from T4 (thyroxine) in peripheral tissues. Common causes of elevated FT3 include hyperthyroidism (e.g., Graves' disease, toxic adenoma), early-phase thyroiditis, or excessive thyroid hormone intake. Low FT3 is typically seen in hypothyroidism (e.g., Hashimoto's thyroiditis), after thyroid surgery, or in severe illness (non-thyroidal illness syndrome). Importantly, FT3 alone is insufficient for diagnosis – TSH and fT4 must be measured together. Interpretation depends on age, medications (e.g., beta-blockers, glucocorticoids), and comorbidities. A MyBody-X DNA test can indicate genetic predispositions for thyroid disorders but cannot measure current hormone levels. If abnormalities are suspected, a doctor's visit with blood draw is necessary.