Thyroxine or T4 is a thyroid hormone responsible for regulation of the body's basal metabolic rate and essential for growth, development and heart and nervous system function. Free and total T4 serum levels are measured by LC-MS/MS for determination of hyper- or hypothyroidism as well as for monitoring treatment with synthetic thyroid hormones. This Certified Spiking Solution® is suitable for use in LC-MS/MS testing methods as starting materials in preparation of linearity standards, calibrators, and controls.