What is Free T4?
Free T4 (thyroxine) is the primary hormone the thyroid gland releases. It acts largely as a prohormone, converted into the more active T3 in the tissues. The 'free' measurement reflects the unbound, available portion. Free T4 is paired with TSH to diagnose and stage thyroid disease.
Why it matters
Free T4 confirms and grades thyroid dysfunction found on TSH testing, distinguishes overt from subclinical disease, and helps monitor thyroid hormone replacement.
What it measures
The unbound fraction of T4 in serum, interpreted alongside TSH.
Reference & optimal ranges
Reference ranges vary by lab, assay, age, and sex. The ranges below reflect commonly published adult intervals and are for education only always interpret results with the range printed on your own lab report and a clinician.
High free T4 with low TSH indicates hyperthyroidism.
Common symptoms
- Weight loss
- Palpitations
- Anxiety, tremor
- Heat intolerance
Potential causes
- Graves' disease
- Toxic nodular goiter
- Excess thyroid medication
- Thyroiditis (early phase)
Low free T4 with high TSH indicates overt hypothyroidism.
Common symptoms
- Fatigue
- Weight gain
- Cold intolerance
- Dry skin
- Depression
Potential causes
- Hashimoto's thyroiditis
- Thyroid surgery/radiation
- Pituitary dysfunction (with low TSH)
- Iodine deficiency
How to improve your Free T4
Lifestyle
Consistent medication timing
If on levothyroxine, take it consistently and retest per your clinician.
Nutrition
Balanced iodine intake
Both deficiency and excess iodine impair thyroid function.
Exercise
Regular activity
Supports metabolism while thyroid status is optimized.
Sleep
Adequate sleep
Improves as thyroid status normalizes.
Frequently asked questions
Scientific references
- T4 (Thyroxine) Tests Testing.com
- T4 Test MedlinePlus, U.S. National Library of Medicine
Educational information, not medical advice. This page is for general education and does not replace diagnosis or treatment by a licensed clinician. Do not start, stop, or change any medication, supplement, or treatment based on this content. Reference and optimal ranges vary between laboratories interpret your results with the range on your own report and a qualified professional.