T4

Also known as: Thyroxine, Total T4, Free T4 (FT4)

Diabetes & Hormones Blood
Patient Friendly Summary

T4 confirms what TSH hints at. Doctors usually treat the pattern (TSH + T4), not a single number, and recheck after 6–8 weeks of any dose change.

What this test means

T4 is the thyroid's main output, later converted to active T3. Free T4 (FT4) measures the usable portion and is widely used with TSH.

Why it is done

It is done with TSH to confirm and grade thyroid underactivity or overactivity and to fine-tune treatment.

Understanding your value

If your value is high

High T4 supports a diagnosis of hyperthyroidism when TSH is low.

If your value is low

Low T4 with high TSH supports hypothyroidism; treatment is usually straightforward and monitored with repeat tests.

About the normal range

Free T4 commonly runs about 0.8–1.8 ng/dL, varying by lab. Pregnancy uses trimester-specific ranges.

When to consult a doctor

Consult your doctor for abnormal results and for regular monitoring if you take thyroid medication.

Frequently Asked Questions

It is done with TSH to confirm and grade thyroid underactivity or overactivity and to fine-tune treatment. Your doctor will decide if this test is right for your situation.

High T4 supports a diagnosis of hyperthyroidism when TSH is low. This needs clinical correlation — always discuss your report with your doctor.

Low T4 with high TSH supports hypothyroidism; treatment is usually straightforward and monitored with repeat tests. This needs clinical correlation — always discuss your report with your doctor.

A single value rarely tells the whole story. Results need to be read together with your symptoms, history, and other tests. Please consult your doctor for a proper interpretation.

Quick Facts

  • TestT4
  • Short formsThyroxine, Total T4, Free T4 (FT4)
  • Sample typeBlood
  • CategoryDiabetes & Hormones

Related Terms

High Low Borderline

Specialists

Endocrinology & Diabetology

Related Tests

Test
TSH

The master signal that controls your thyroid gland.

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Test
T3

One of the two active thyroid hormones.

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Related Organs

Organ
Thyroid

The thyroid gland controls metabolism, energy, and weight. TSH, T3, and T4 blood tests show whether it is unde…

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Disclaimer: This information is for patient education only and does not replace medical advice, diagnosis, or treatment. Always consult a qualified doctor.