Free T4 (Thyroxine)
Free T4 is the inactive thyroid hormone that gets converted to active T3 as needed. Low free T4 with high TSH confirms hypothyroidism.
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What Free T4 (Thyroxine) measures
T4 (thyroxine) is the main hormone made by the thyroid gland. It's relatively inactive until tissues convert it to T3. The 'free' portion is what's available for cells to use. Free T4 is interpreted alongside TSH for the most accurate thyroid assessment.
Normal ranges
Reference ranges may vary slightly by lab. Always use the range provided on your specific test report.
What affects your free t4 (thyroxine) level
- Hashimoto's thyroiditis (most common cause of low T4)
- Iodine deficiency
- Pregnancy (T4 needs rise)
- Severe illness or surgery
- Medications: lithium, amiodarone
- Pituitary disorders (rare)
- Surgical thyroid removal or radioactive iodine treatment
Foods that may help
Seafood and seaweed (in moderation)
Iodine sources — but excess can also cause problems
Eggs and dairy
Reliable everyday iodine plus tyrosine
Brazil nuts
Selenium supports T4 production and conversion
Oysters and beef
Zinc — required throughout the thyroid hormone pathway
Cooked cruciferous vegetables
Cooking inactivates the goitrogens that can interfere in iodine deficiency
When to see your doctor
Low free T4 paired with elevated TSH confirms hypothyroidism and usually requires hormone replacement. Subclinical patterns (TSH slightly elevated, T4 normal) may or may not need treatment depending on symptoms and antibodies — discuss with an endocrinologist.
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Educational content only · Not medical advice