To assess thyroid function, clinicians typically determine blood levels of thyroid-stimulating hormone (TSH) and thyroxine (T4) and do not directly measure triiodothyronine (T3).

However, findings from a large observational study of a US sample of adults with normal thyroid function suggested that subclinical variation in the hypothalamic-pituitary-thyroid (HPT)–axis effector hormone T3 "is an important and overlooked factor linking socioeconomic forces, human biology, and aging," researchers reported. https://www.medscape.com/viewarticle/direct-measurement-t3-likely-vital-say-researchers-2024a10000xd