What it measures.
Free T4 (thyroxine) is the storage form of thyroid hormone, serving as a reservoir for conversion to active T3. 'Free' refers to the unbound portion that's biologically available, rather than the 99%+ bound to proteins. Low Free T4 with elevated TSH confirms primary hypothyroidism.
The concentration of unbound thyroxine (T4) in blood. T4 contains four iodine atoms and has a long half-life of about 8 days, making it a stable marker of thyroid hormone production.
Why it matters.
Free T4 indicates how much thyroid hormone your thyroid gland is producing. It's essential for evaluating thyroid dysfunction and monitoring replacement therapy. Most T4 converts to the more active T3 in tissues.
Physiology.
The thyroid gland produces T4 under TSH stimulation. T4 circulates bound to proteins (TBG, albumin, transthyretin) with only 0.03% free. Peripheral tissues convert T4 to T3 via deiodinase enzymes. The liver and kidneys perform most of this conversion.
Testing & preparation.
How to prepare
- No special preparation needed
- Can be drawn anytime; no fasting required
- Continue thyroid medication unless instructed otherwise
- Biotin supplements may interfere—stop 72 hours before testing
When to test
When TSH is abnormal, evaluating thyroid symptoms, or monitoring thyroid replacement therapy. Test together with TSH for complete picture.
How often
Every 6-8 weeks during dose adjustments, then annually when stable.
Interpretation.
High free t4
Common causes:
- Hyperthyroidism (Graves' disease, toxic nodules)
- Excessive thyroid medication
- Thyroiditis (temporary release from inflamed gland)
- TSH-secreting pituitary tumor (rare)
Implications:
- Accelerated metabolism and weight loss
- Anxiety, tremor, rapid heart rate
- Risk of atrial fibrillation and bone loss
- Requires prompt evaluation and treatment
Low free t4
Common causes:
- Primary hypothyroidism (Hashimoto's, iodine deficiency)
- Secondary hypothyroidism (pituitary dysfunction)
- Insufficient thyroid medication dose
- Severe illness affecting thyroid function
Implications:
- Fatigue, weight gain, cold intolerance
- Depression, cognitive slowing
- Elevated cholesterol and cardiovascular risk
- Requires thyroid hormone replacement
Optimization.
Diet
- Ensure adequate iodine (seafood, dairy, iodized salt)
- Include selenium-rich foods (Brazil nuts, fish, eggs)
- Adequate zinc for T4 to T3 conversion
- Avoid excessive raw cruciferous vegetables if hypothyroid
Lifestyle
- Manage chronic stress (impairs T4 to T3 conversion)
- Prioritize quality sleep for thyroid function
- Address gut health for optimal hormone metabolism
- Regular moderate exercise supports thyroid function
Supplements
- Selenium 200mcg if deficient
- Zinc 15-30mg if needed
- Iron if deficient (required for thyroid synthesis)
FAQs.
Why test Free T4 instead of Total T4?
Free T4 measures only the biologically active, unbound hormone. Total T4 includes protein-bound hormone, which can be affected by pregnancy, estrogen therapy, liver disease, and other factors that alter binding proteins. Free T4 gives a more accurate picture of actual thyroid status.
Can Free T4 be normal while TSH is abnormal?
Yes, this defines 'subclinical' thyroid disease. Subclinical hypothyroidism shows elevated TSH with normal Free T4—the thyroid is struggling but still producing adequate hormone. Subclinical hyperthyroidism shows suppressed TSH with normal Free T4. Both may warrant monitoring or treatment depending on severity.