Key Benefits

• Estimate your usable thyroid hormone level, adjusted for protein binding effects.
• Spot thyroid imbalance when binding proteins make total T4 misleading.
• Clarify fatigue, weight change, hair loss, constipation, or palpitations linked to thyroid.
• Guide levothyroxine treatment by reflecting hormone availability beyond total T4 shifts.
• Protect fertility by flagging thyroid issues that disrupt ovulation and menstrual cycles.
• Support pregnancy care when rising binding proteins elevate total T4 artificially.
• Track recovery or relapse during treatment, illness, or estrogen-containing therapy changes.
• Interpret results best with TSH and your symptoms for clear decisions.

What is Free T4 Index (T7)?

Free T4 Index (T7) is a calculated estimate of how much unbound thyroid hormone thyroxine is circulating in your blood. It starts with the total thyroxine made by your thyroid gland and adjusts for how much of that hormone is carried by blood proteins. Most thyroxine travels attached to carrier proteins—chiefly thyroxine‑binding globulin, with support from transthyretin and albumin—while only a small fraction remains unattached and ready to act (free thyroxine, or free T4).

The index’s purpose is to approximate the biologically available portion of thyroxine, the part that can enter cells and be converted to the more active hormone triiodothyronine (T3). In doing so, it offers a binding‑adjusted view of thyroid hormone supply to tissues. This free hormone availability underpins many essential functions driven by thyroid signaling, including energy use, heat production, heart rhythm, brain performance, and growth and repair. By accounting for variations in protein binding, the Free T4 Index aims to approximate the availability of thyroxine to the body rather than just the total amount circulating.

Why is Free T4 Index (T7) important?

Free T4 Index (T7) estimates the amount of thyroxine that is truly available to cells, correcting total T4 for changes in binding proteins like TBG. Because thyroid hormone sets the body’s metabolic pace, this index reflects how energy production, heart rhythm, temperature regulation, brain speed, muscle performance, bone turnover, and reproductive function are running. It is especially useful when binding proteins shift, such as in pregnancy, estrogen therapy, liver disease, or genetic TBG variants. Reference intervals vary by lab, and values near the middle usually align with a clinically euthyroid state.

When the index is below range, it signals insufficient biologically available thyroxine. Metabolism slows: fatigue, cold intolerance, weight gain, dry skin, constipation, slowed heart rate, and low mood are common. LDL cholesterol may rise, reflexes slow, and thinking can feel foggy. Women may notice heavy or irregular periods and reduced fertility; men may experience low libido. Children can show poor growth and delayed puberty. In pregnancy, low availability of T4 increases risks to maternal health and fetal neurodevelopment.

When the index is above range, it reflects thyrotoxicosis. Metabolism accelerates: heat intolerance, unintentional weight loss, tremor, anxiety, diarrhea, and palpitations. The heart is vulnerable to atrial fibrillation; bones lose density; muscles weaken. Women may have light or absent menses; men can see reduced fertility. Children may grow rapidly with advanced bone age; pregnancy risks increase.

Big picture, the Free T4 Index complements TSH to clarify thyroid status when binding proteins confound other measures. It links the thyroid–pituitary axis with liver-derived binding proteins and informs risks spanning cardiovascular health, cognition, bone strength, fertility, and pregnancy outcomes.

What Insights Will I Get?

The Free T4 Index (T7) estimates how much active thyroid hormone (free thyroxine) is available to your tissues by mathematically correcting total T4 for binding proteins. It approximates free T4 and helps gauge the thyroid system’s control of energy use, heart rate and rhythm, body temperature, cognition and mood, menstrual cycles and fertility, lipid metabolism, gut motility, and immune tone.

Low values usually reflect too little thyroid hormone available to cells (hypothyroxinemia). This most often comes from an underactive thyroid (primary hypothyroidism) and less commonly from pituitary or hypothalamic disease (central hypothyroidism). Severe illness can also lower it. System effects include slower metabolic rate, reduced cardiac output and heart rate, higher LDL cholesterol, constipation, heavier or irregular menses, fertility challenges, and cognitive slowing. In pregnancy, a low T7 suggests true thyroid hormone deficiency despite high binding proteins and has fetal neurodevelopment implications.

Being in range suggests a euthyroid state with stable hypothalamic–pituitary–thyroid feedback, steady energy production, balanced cardiovascular function, appropriate bone turnover, and regular reproductive and cognitive function. In healthy adults, results often sit near the mid-portion of the reference range.

High values usually reflect excess thyroid hormone action (thyrotoxicosis) from an overactive gland, thyroiditis, or excess hormone exposure. Expect faster metabolism, heat intolerance, weight loss, anxiety, tremor, rapid or irregular heartbeat (including atrial fibrillation in older adults), diarrhea, and increased bone loss; in children, growth may accelerate.

Notes: T7 is calculated from total T4 and T3 uptake (or THBR) and is less affected by binding protein shifts (e.g., pregnancy, estrogen therapy, liver or kidney disease) than total T4. Modern practice often favors direct free T4 assays. Acute illness, assay method, biotin, and some medications can artifactually alter results; pairing with TSH improves interpretation.