EndocrinologyEndocrinology

Free T4 and TSH

Normal Range
TSH: 0.45-4.5 mIU/L; FT4: 0.8-1.8 ng/dL
Sample Type
Serum
Fasting
No
Unit
mIU/L, ng/dL

Function

This combined panel measures the two most critical indicators of thyroid health. Thyroid Stimulating Hormone (TSH) is produced by the pituitary gland to signal the thyroid to produce hormones. Free T4 (Thyroxine) is the active, unbound form of the primary hormone produced by the thyroid gland itself. Together, they form a feedback loop: when T4 levels drop, TSH rises, and vice versa.

Why it is Ordered

This panel is the gold standard for evaluating thyroid function. It is ordered for patients experiencing symptoms of an overactive or underactive thyroid, or to monitor those on thyroid replacement therapy.

  • Symptoms of Hypothyroidism: Weight gain, fatigue, cold intolerance, dry skin, and depression.
  • Symptoms of Hyperthyroidism: Weight loss, rapid heartbeat, anxiety, heat intolerance, and tremors.

Associated Conditions

  • Hashimoto's Thyroiditis: The most common cause of hypothyroidism (High TSH, Low FT4).
  • Graves' Disease: A common cause of hyperthyroidism (Low TSH, High FT4).
  • Central Hypothyroidism: A rarer condition involving pituitary failure (Low TSH, Low FT4).
  • Subclinical Hypothyroidism: An early stage where TSH is elevated but FT4 is still within the normal range.

Panel Components

This test is a profile comprising the following specific markers:

Thyroid Stimulating Hormone (TSH)
mIU/L
Free Thyroxine (FT4)
ng/dL

Why Context Matters

One of the most significant confounding factors for this test is Biotin (Vitamin B7). High doses of biotin, common in hair and nail supplements, can interfere with the laboratory immunoassay, often causing results that falsely mimic hyperthyroidism (low TSH and high FT4). Additionally, severe acute illness ('Euthyroid Sick Syndrome') can temporarily cause abnormal thyroid levels that do not reflect a true thyroid disorder. Pregnancy and certain medications (like amiodarone or lithium) also significantly alter these levels.

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 T4 and TSH:

Research & Guidelines

Related Indicators

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

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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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