EndocrinologyEndocrinology

Free T3, Free T4

Normal Range
FT3: 2.3-4.2 pg/mL; FT4: 0.8-1.8 ng/dL
Sample Type
Serum
Fasting
No
Unit
pg/dL

Function

Free T3 (Triiodothyronine) and Free T4 (Thyroxine) are the 'active' forms of thyroid hormones that circulate in the blood unbound to proteins. T4 is the primary hormone produced by the thyroid gland, while T3 is the more potent hormone, largely created by the conversion of T4 in the liver and other tissues. These hormones regulate the body's metabolic rate, heart function, digestion, muscle control, and brain development.

Why it is Ordered

These tests are ordered to evaluate thyroid function, specifically when TSH (Thyroid Stimulating Hormone) levels are abnormal. Measuring 'Free' hormones is more accurate than 'Total' hormones because it isn't affected by changes in protein levels caused by pregnancy, liver disease, or certain medications.

Associated Conditions

  • Hyperthyroidism: Overactive thyroid (Graves' disease, toxic nodules), characterized by high FT3/FT4.
  • Hypothyroidism: Underactive thyroid (Hashimoto's thyroiditis), characterized by low FT3/FT4.
  • T3 Toxicosis: A specific form of hyperthyroidism where only T3 is elevated.
  • Central Hypothyroidism: A pituitary issue where TSH is low and FT4 is also low.

Panel Components

This test is a profile comprising the following specific markers:

Free Triiodothyronine (FT3)
pg/mL
Free Thyroxine (FT4)
ng/dL

Why Context Matters

Biotin (Vitamin B7) is a major confounding factor and can falsely show high T3/T4 and low TSH, mimicking Graves' disease. Patients should stop biotin 48-72 hours before the test. Other factors include acute illness ('Euthyroid Sick Syndrome'), certain drugs like amiodarone or heparin, and the timing of thyroid medication intake before the blood draw.

Lab ranges are statistical averages, not biological laws. "Normal" for a 20-year-old male isn't normal for a 60-year-old female.

Clinical References

Source-of-truth databases and clinical guidelines for Free T3, Free T4:

Research & Guidelines

Related Indicators

Thyroid Function Panel (Free T3, Free T4, TSH)

View Range

24-Hour Urinary VMA

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17-Alpha Hydroxy Progesterone

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DR

Medically Reviewed by Dr. Binoy Babu, MBBS

Board Certified Doctor • 10+ Years Clinical Experience

Dr. Babu is a practicing physician dedicated to empowering patients with clear, actionable medical information. He founded 2opi to bridge the gap between complex lab reports and patient understanding, ensuring everyone has access to a reliable second opinion.

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