Hormones
T4 (Thyroxine), Total
What This Marker Tells Us
Total T4 measures all the thyroxine hormone in your blood, including both the small amount of active free hormone and the large majority that's bound to transport proteins and temporarily inactive.
Why It Matters
Reflects overall thyroid hormone production. Changes in binding proteins due to pregnancy, birth control, liver disease, or malnutrition can dramatically alter total T4 without affecting actual thyroid function. Despite this limitation, total T4 helps screen for thyroid disorders and monitors replacement therapy. When elevated, it suggests hyperthyroidism (overactive thyroid) causing weight loss, anxiety, rapid heart rate, and heat intolerance. When low, it indicates hypothyroidism (underactive thyroid) causing fatigue, weight gain, cold intolerance, and cognitive sluggishness.
How to Interpret Your Trends
Total T4 typically ranges from 4.5-12 μg/dL. Low levels (below 4.5 μg/dL) may indicate hypothyroidism, but must be interpreted with TSH—low T4 with high TSH confirms primary hypothyroidism, while low T4 with low TSH suggests pituitary dysfunction. Levels at the lower end of normal with symptoms may still warrant intervention. High T4 (above 12 μg/dL) suggests hyperthyroidism when TSH is suppressed, but may simply reflect elevated binding proteins from estrogen if TSH is normal. Optimal T4 levels support stable energy, healthy metabolism, and temperature regulation without symptoms of excess or deficiency.
What Influences This Marker
Total T4 rises with hyperthyroidism (Graves' disease, toxic nodules), excessive thyroid medication, high estrogen states (pregnancy, birth control, estrogen therapy), and acute illness. It decreases with hypothyroidism, iodine deficiency, severe caloric restriction, chronic illness, certain medications (lithium, amiodarone), and conditions reducing binding proteins. Thyroid medication dose directly affects T4 levels. Biotin supplements can interfere with lab testing, causing falsely abnormal results, so it should be stopped 2-3 days before testing.
How Your Team Uses It
Your coach addresses lifestyle factors affecting thyroid health including adequate iodine intake through seafood, dairy, or iodized salt (without excess), selenium-rich foods like Brazil nuts and fish, stress management practices, sleep optimization, and avoiding excessive caloric restriction.
Related Signals We Also Review
TSH, T3 uptake, Free T4 Index, free T4 and T3 (when available), thyroid antibodies, and symptoms for complete thyroid assessment.