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Free T4
Thyroid prohormone
Also known as: FT4,Thyroxine Free,T4 (Thyroxine) Free,Free Thyroxine
Reference Ranges
Clinical Reference
0.8 – 1.8 ng/dL
Optimal Range
1.1 – 1.5 ng/dL
Prohormone converted to T3
Related Conditions
Hyperthyroidism
High
Hypothyroidism
Low
Food & Lifestyle Recommendations
🍎 Food
Adequate iodine, tyrosine (protein), and selenium support thyroid hormone production
T4 synthesis requires iodine + tyrosine
🍎 Food
Ensure adequate iodine: iodized salt, seaweed, fish, dairy
Iodine is essential for T4 synthesis; deficiency directly reduces FT4 production
🍎 Food
Include selenium-rich foods: Brazil nuts (1-2/day), fish, eggs
Selenium is required for deiodinase enzymes that activate thyroid hormones
🍎 Food
Avoid excessive raw cruciferous vegetables (goitrogens) if iodine-deficient
Cruciferous vegetables contain goitrogens that can interfere with thyroid function when iodine is low
Evidence-Backed Supplements
When Low
-
Strong evidence; iodine is essential for T4 synthesis. Deficiency directly reduces Free T4. Supplementation corrects deficiency-induced low FT4.
-
Moderate evidence; selenium is required for T4-to-T3 conversion and thyroid hormone metabolism. May support FT4 levels in deficiency.
Selenomethionine 100–200 mcg/day Morning -
Limited evidence; zinc is involved in thyroid hormone synthesis. Deficiency may impair FT4 production.
Zinc picolinate or bisglycinate 15–30 mg/day Evening, away from iron/calcium
Questions to Ask Your Doctor
- Is my low T4 due to a thyroid problem, or could it be related to pituitary function? Central hypothyroidism = low T4 with normal/low TSH
- Does my low Free T4 indicate hypothyroidism, and should I be started on thyroid medication? Low FT4 with elevated TSH confirms hypothyroidism. Levothyroxine is standard treatment.
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