Hormones
T3 Uptake
What This Marker Tells Us
T3 uptake measures the availability of binding sites on proteins that transport thyroid hormones through your bloodstream, indirectly reflecting how much thyroid-binding globulin (TBG) is present in your blood.
Why It Matters
T3 uptake is primarily used to calculate the Free T4 Index (T7), which estimates free thyroid hormone levels when direct free T4 testing isn't available. The test measures how much labeled thyroid hormone binds to your blood proteins versus a resin in the lab; higher uptake means fewer available binding sites (lower TBG), while lower uptake means more binding sites (higher TBG). Conditions that increase TBG (like pregnancy, birth control pills, or estrogen therapy) decrease uptake, while conditions that decrease TBG (like testosterone therapy, severe illness, or protein malnutrition) increase uptake. This marker helps distinguish between changes in total thyroid hormone levels due to binding protein changes versus actual thyroid dysfunction, preventing misdiagnosis and unnecessary treatment.
How to Interpret Your Trends
T3 uptake typically ranges from 24-39%, with the exact range depending on the lab. Low uptake (below range) indicates elevated thyroid-binding proteins, which can occur with high estrogen states, pregnancy, or estrogen therapy. This doesn't indicate hypothyroidism but rather increased protein binding. High uptake (above range) suggests decreased binding proteins from androgens, malnutrition, severe illness, or protein-losing conditions. Isolated T3 uptake changes without corresponding total T4 changes usually reflect binding protein alterations rather than thyroid dysfunction. The most important use is calculating Free T4 Index when interpreted alongside total T4.
What Influences This Marker
T3 uptake decreases with estrogen therapy, birth control pills, pregnancy, estrogen-producing tumors, and sometimes liver disease. It increases with androgen therapy, testosterone replacement, severe illness, malnutrition, nephrotic syndrome, active acromegaly, and high-dose corticosteroids. Weight loss and protein deficiency lower binding protein production. Changes in body composition and hormonal status influence this marker independently of actual thyroid function.
How Your Team Uses It
Your coach addresses nutritional deficiencies through meal planning and may support protocols around hormone therapy adjustments when needed, though T3 uptake itself doesn't typically drive specific coaching interventions.
Related Signals We Also Review
Total T4, Free T4 Index (T7), TSH, free T4 (when available), and sex hormone status for comprehensive thyroid assessment.