Free T4 (thyroxine) is the main hormone produced by your thyroid. It controls metabolism, energy, and virtually every cell in your body.
Free T4 (free thyroxine) measures the unbound, biologically active form of thyroxine — the primary hormone produced by your thyroid gland. About 99% of T4 in blood is bound to proteins; Free T4 measures the remaining 1% that's available to enter cells and exert effects.
T4 is often called the "storage" or "prohormone" form of thyroid hormone. Your tissues convert T4 into T3 (triiodothyronine), which is the more active form. According to the American Thyroid Association, Free T4 is a key test for evaluating thyroid function.
Understanding the TSH-T4 relationship is key to interpreting thyroid tests:
| TSH | Free T4 | Interpretation |
|---|---|---|
| Normal | Normal | Euthyroid (normal function) |
| High ↑ | Low ↓ | Primary Hypothyroidism |
| Low ↓ | High ↑ | Primary Hyperthyroidism |
| High ↑ | Normal | Subclinical Hypothyroidism |
| Low ↓ | Normal | Subclinical Hyperthyroidism |
* TSH and T4 typically move in opposite directions. When T4 is low, the pituitary increases TSH to stimulate the thyroid. When T4 is high, TSH is suppressed.
Healthy thyroid hormone production
Underactive thyroid, insufficient hormone
Severe hypothyroidism, treatment needed
Overactive thyroid, excess hormone
Significant hyperthyroidism, evaluation needed
Select your Free T4 value for a detailed explanation of what it means and what to do next.
Low Free T4 (typically <0.8 ng/dL) with elevated TSH indicates hypothyroidism. The American Thyroid Association identifies these common causes:
Most common cause. Autoimmune destruction of the thyroid gland. Often runs in families. TPO antibodies are usually positive.
After thyroid surgery, radioactive iodine treatment, or radiation therapy for head/neck cancers.
Lithium, amiodarone, interferon, and certain cancer immunotherapies can cause hypothyroidism.
Both deficiency and excess iodine can impair thyroid function. Rare in the US due to iodized salt.
High Free T4 (typically >1.8 ng/dL) with suppressed TSH indicates hyperthyroidism. According to the Mayo Clinic, common causes include:
Most common cause. Autoimmune — antibodies stimulate thyroid to overproduce hormones. May include eye symptoms (Graves' ophthalmopathy).
Thyroid nodules that produce excess hormone autonomously, independent of TSH control.
Inflammation (viral, postpartum, medication-induced) causes stored hormone to leak into blood. Usually temporary.
Taking too much thyroid hormone replacement (levothyroxine). Dose adjustment resolves it.
Upload your thyroid panel results and monitor TSH and Free T4 trends. See how your treatment is working.
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