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Free T4 Index (T7)

Free T4 Index (T7) is a calculated laboratory metric that estimates the amount of biologically active thyroid hormone by adjusting total T4 for variations in thyroid‑binding proteins. Higher values generally indicate increased thyroid activity, which can be seen in conditions such as hyperthyroidism or excess iodine intake, while lower values suggest reduced thyroid function, as occurs with hypothyroidism, protein‑loss states, or certain medications. Levels are influenced by factors that alter binding protein concentrations, including pregnancy, estrogen therapy, liver disease, and acute illness, as well as by overall metabolic demand and nutritional status.

The free T4 index, often labeled T7, is a calculated estimate of how much active thyroid hormone is circulating in your blood, adjusting your total T4 measurement for changes in the proteins that carry it. For most adults the value sits around 1.5 to 4.5, a unitless index rather than a concentration.

MeasureTypical range
Free T4 index (T7)About 1.5 to 4.5 (unitless index)

Ranges vary between labs, so compare to the range on your own lab report.

What the free T4 index measures

The free T4 index is an older, calculated way to estimate free thyroxine, the fraction of thyroid hormone that is not bound to carrier proteins and can act on your tissues. Your lab multiplies your total T4 by a T3 uptake result, which corrects for how much thyroid binding protein you carry. This math matters because pregnancy, estrogen therapy, and illness can shift binding proteins and make total T4 alone misleading.

What high or low levels mean

A high free T4 index points toward an overactive thyroid, or hyperthyroidism, and often shows up with symptoms such as weight loss, a racing heart, tremor, or heat intolerance. A low index suggests an underactive thyroid, or hypothyroidism, which can bring fatigue, weight gain, constipation, and cold sensitivity. Keep in mind that most modern labs now measure free T4 directly with immunoassays, so the calculated index appears mainly in older records or specific panels. Your result means more when you read it together with your TSH, since that pituitary hormone reflects how your body is responding overall. If your index falls outside the reference range, or if your symptoms and numbers disagree, follow up with your clinician for repeat or confirmatory testing.

Free T4 index FAQ

Is the free T4 index the same as free T4?

Not exactly. The index is a calculation that estimates free T4 from total T4 and a T3 uptake test, while a free T4 assay measures the unbound hormone directly. Both aim to describe active thyroid hormone.

Why is it called T7?

The label T7 is an older shorthand for the free thyroxine index, produced by combining a total T4 measurement with a T3 resin uptake test. The numbering dates back to how labs once catalogued thyroid panel components.

Do I need to fast before this test?

No special fasting is usually required for thyroid testing. Because the index combines total T4 and T3 uptake from one blood draw, tell your clinician about any biotin supplements or medications that can affect thyroid results. Read alongside your thyroid-stimulating hormone, total T4, and T3 uptake for the full thyroid picture.

References

  • Mayo Clinic Laboratories. Free Thyroxine Index (FTI), Serum.
  • HealthMatters.io. Free Thyroxine Index, Thyroid Panel.

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