What is free T4?
Free T4 is a blood test that measures the thyroid hormone your body can actually use. It’s usually ordered alongside a TSH test, and together they tell your doctor how well your thyroid is working.
The short version
- T4 is thyroxine, the main hormone your thyroid gland makes. It controls how your body uses energy.
- “Free” means the part of T4 not attached to proteins in your blood, so it’s available to your tissues. That’s the part that matters, which is why the free T4 test has largely replaced the older total T4 test.
- High can point to an overactive thyroid. Low can point to an underactive one. Only your doctor can say which, and usually only together with TSH.
- Medicare: covered under a national rule for diagnosing and monitoring thyroid problems, up to twice a year when your condition is stable. You pay $0. Medicare pays the lab $9.02.
T4, free T4, and TSH: three names, one system
Your thyroid is a small gland in your neck. It makes thyroxine, called T4, which travels through your blood and sets the pace for nearly everything: breathing, heart rate, weight, digestion, and mood.
- Total T4 counts all the thyroxine in your blood. Most of it is bound to proteins and can’t be used.
- Free T4 counts only the unbound part. The National Library of Medicine notes that medical experts consider it more accurate than total T4, so it’s the one doctors use more often.
- TSH is a different hormone, made by the pituitary gland in your brain. It’s the signal that tells the thyroid to make more or less T4. TSH is usually the first thyroid test ordered, and free T4 is the follow-up.
The two work as a pair. In the words of MedlinePlus: “A T4 test alone can’t provide enough information to diagnose thyroid problems. So, it’s usually done with a TSH blood test.”
What a high or low free T4 can mean
These are the conditions the National Library of Medicine lists. A result outside the normal range does not by itself mean you have any of them. Your doctor reads free T4 together with TSH, your symptoms, and your history.
| Result | Can point to |
|---|---|
| Higher than normal | An overactive thyroid (hyperthyroidism), including Graves’ disease or a toxic goiter. Less often, too much iodine, or a pituitary problem. |
| Lower than normal | An underactive thyroid (hypothyroidism), including Hashimoto’s disease or thyroiditis. |
| Normal | With a normal TSH, the thyroid is usually working as it should. Some patterns, such as a normal free T4 with an abnormal TSH, need your doctor’s interpretation. |
There is no fixed “normal” number to quote here. Each lab sets its own range and prints it next to your result. Compare your number to that range, not to one you find online. Why that range is not the whole story →
Why the test gets ordered
- To follow up an abnormal TSH result
- To diagnose an overactive or underactive thyroid
- To check a thyroid nodule or goiter
- To see whether thyroid medication is at the right dose
- To look into a pituitary problem
Before the test
No fasting is needed. Two things do matter. If you take thyroid medication, many doctors want the blood drawn before your morning dose, so ask how yours wants it done. And tell the lab about supplements you take. High-dose biotin, found in many hair and nail supplements, can throw off thyroid results. More on preparing for blood work →
What Medicare pays
Free T4 falls under Medicare’s national thyroid rule (NCD 190.22), the same in every state. It’s covered for diagnosing thyroid disease, monitoring treatment, and working up a long list of symptoms. The rule to know is frequency: for someone whose thyroid treatment is stable, Medicare covers testing up to twice a year. More often is covered when a dose has changed or symptoms have returned.
The free T4 test page has the full coverage details and what the test costs to buy directly if it won’t be covered. Free T4 cost and Medicare coverage →
Sources
- MedlinePlus (National Library of Medicine): Thyroxine (T4) Test
- Medicare National Coverage Determination 190.22, Thyroid Testing
- Medicare payment amounts: CMS Clinical Laboratory Fee Schedule, 2026 Q3
Checked September 23, 2026. This page explains what the test measures. It is not medical advice and cannot interpret your result. Your doctor can.
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