Marlene Busko 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...