Test To Table
Educational Guide

What Does Free T4 (Thyroxine) Mean?

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

Normal0.8–1.8 ng/dL
Low< 0.8 ng/dL
High> 1.8 ng/dL

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.

Related Biomarkers

TSH (Thyroid Stimulating Hormone)

what does high TSH mean

Free T3 (Triiodothyronine)

what does low free T3 mean

Ferritin

what does low ferritin mean

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Educational content only · Not medical advice