Thyroxine (T4), Total

Total T4 is a blood test that measures the amount of thyroxine, a hormone made by your thyroid gland, in your bloodstream. Your thyroid uses this hormone to help regulate your metabolism, energy levels, heart rate, and many other body functions. Most of the T4 in your blood is attached to proteins that carry it through the body, so this test measures both the attached and unattached portions together. Your doctor will usually look at this result alongside other thyroid tests—especially TSH and free T4—to get a complete picture of how your thyroid is working. Certain medications, pregnancy, and other health conditions can affect this test result, so always discuss your results with your healthcare provider.

When elevated: Elevated total T4 may indicate primary or secondary hyperthyroidism, thyroiditis, or increased thyroid hormone-binding protein concentrations rather than true hormone excess. Clinical correlation with TSH and free T4 is essential to distinguish true hyperthyroid states from binding protein alterations. When low: Low total T4 may reflect primary hypothyroidism, secondary (pituitary) hypothyroidism, or decreased thyroid hormone-binding proteins. Distinguishing true hypothyroidism from reduced binding protein states requires concurrent TSH and free T4 measurement.

Total T4 is moderately relevant to athletes because thyroid hormones regulate metabolic rate, energy availability, and recovery capacity. Overtraining or severe caloric restriction can suppress T4 levels, potentially impairing endurance, strength gains, and immune function—though T4 alone is insufficient for diagnosis; TSH and free T4 provide better clinical context.