Free T4 (Free Thyroxine)
The active form of thyroid hormone available to your cells. Essential for diagnosing and managing thyroid conditions.
What is Free T4?
Free T4 (free thyroxine) measures the amount of unbound, biologically active thyroid hormone in your blood.
Your thyroid gland produces T4, which exists in two forms:
- Bound T4 — Attached to proteins, inactive (~99%)
- Free T4 — Unbound and active (~1%)
Only free T4 can enter cells and affect metabolism. This makes free T4 a more accurate measure of thyroid function than total T4.
Free T4 works together with TSH to assess thyroid health. TSH tells you if your brain thinks you need more or less thyroid hormone; free T4 tells you how much active hormone you actually have.
Normal Ranges
| Free T4 Level | Interpretation |
|---|---|
| 0.8–1.8 ng/dL | Normal (varies by lab) |
| Below 0.8 | Low (hypothyroidism) |
| Above 1.8 | High (hyperthyroidism) |
Optimal free T4 is typically in the middle to upper part of the normal range, though individual needs vary.
Why It Matters
Metabolism Regulation
Free T4 controls your metabolic rate — how fast your cells convert nutrients to energy. It affects:
- Energy levels
- Body temperature
- Heart rate
- Weight management
- Digestive function
Thyroid Diagnosis
Free T4 helps distinguish between different thyroid conditions:
- High TSH + Low Free T4 = Primary hypothyroidism (thyroid not producing enough)
- Low TSH + High Free T4 = Primary hyperthyroidism (thyroid producing too much)
- Abnormal TSH + Normal Free T4 = Subclinical thyroid dysfunction
Medication Dosing
For people taking thyroid hormone replacement, free T4 helps determine if the dose is right. Too much or too little medication can cause symptoms.
Low Free T4 (Hypothyroidism)
Symptoms
- Fatigue and sluggishness
- Weight gain
- Cold intolerance
- Dry skin and hair
- Constipation
- Depression
- Brain fog
Common Causes
- Hashimoto's thyroiditis (autoimmune)
- Thyroid surgery or radiation
- Iodine deficiency
- Pituitary problems
High Free T4 (Hyperthyroidism)
Symptoms
- Anxiety and irritability
- Weight loss despite normal eating
- Heat intolerance
- Rapid heartbeat
- Tremors
- Difficulty sleeping
- Diarrhea
Common Causes
- Graves' disease (autoimmune)
- Toxic nodular goiter
- Excessive thyroid medication
- Thyroiditis
Free T4 and TSH Together
These two markers work as a feedback system:
- Normal TSH + Normal Free T4: Thyroid working well
- High TSH + Low Free T4: Hypothyroidism (thyroid failing)
- Low TSH + High Free T4: Hyperthyroidism (overactive thyroid)
- Normal TSH + High/Low Free T4: Less common patterns requiring investigation
How Often to Test
- No thyroid issues: Not routinely tested unless symptoms
- Subclinical thyroid dysfunction: Every 6–12 months
- On thyroid medication: Every 6–12 weeks until stable, then annually
- Pregnant with thyroid condition: Every 4–6 weeks
Related Biomarkers
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