T4 Thyroxine Test: Free vs Total and What Results Mean
T4 is the main hormone produced by the thyroid gland. Free T4 measures the unbound portion that tissues can use, while total T4 includes hormone attached to blood proteins. Free T4 is the version clinicians rely on most, and typical adult ranges are about 0.8 to 1.8 ng/dL. A low free T4 with a high TSH suggests an underactive thyroid; a high free T4 with a low TSH suggests an overactive one. Walk-in testing makes it simple to order free or total T4. This guide explains the difference and how to read the result.
Key Takeaways
- T4 is the main hormone made by the thyroid and is measured as free T4 or total T4.
- Free T4 reflects the unbound, active fraction and is generally preferred.
- Typical adult free T4 ranges are about 0.8 to 1.8 ng/dL, varying by laboratory.
- Low free T4 with high TSH points toward an underactive thyroid.
- High free T4 with low TSH points toward an overactive thyroid.
Frequently Asked Questions
What is the difference between free T4 and total T4?
Free T4 measures the small unbound fraction the body can use, while total T4 includes hormone bound to blood proteins. Free T4 is the version clinicians rely on most.
What is a normal T4 level?
Typical adult ranges are about 0.8 to 1.8 ng/dL for free T4 and about 5 to 12 mcg/dL for total T4, but ranges vary by laboratory.
Do I need to fast for a T4 test?
Fasting is generally not required for thyroid testing.
Can I order a T4 test without a doctor?
Yes. Direct-access lab testing lets you order free or total T4 alone or as part of a thyroid panel and visit a walk-in collection site.
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