EndocrinologyEndocrinology

Thyroid Function Test

Normal Range
0.4 - 4.0 mIU/L (TSH Primary Marker)
Sample Type
Serum
Fasting
No
Unit
mIU/L

Function

The Thyroid Function Test (TFT) is a panel of blood tests used to evaluate how well the thyroid gland is working. The thyroid, a small butterfly-shaped gland in the neck, produces hormones—primarily Thyroxine (T4) and Triiodothyronine (T3)—that regulate the body's metabolism, energy production, and sensitivity to other hormones. The most sensitive marker in this panel is the Thyroid-Stimulating Hormone (TSH), produced by the pituitary gland to signal the thyroid to produce more or less T4/T3.

Why it is Ordered

Physicians order TFTs to diagnose or monitor several conditions:

  • Hypothyroidism: An underactive thyroid that slows metabolism.
  • Hyperthyroidism: An overactive thyroid that accelerates metabolism.
  • Thyroiditis: Inflammation of the thyroid gland.
  • Pituitary Gland Disorders: To see if the signal from the brain is reaching the gland.
  • Treatment Monitoring: Checking the efficacy of synthetic thyroid hormone or anti-thyroid medications.

Associated Conditions

Abnormal results are associated with various autoimmune and physiological states:

  • Hashimoto’s Disease: The leading cause of hypothyroidism in iodine-sufficient areas.
  • Graves’ Disease: An autoimmune cause of hyperthyroidism.
  • Goiters and Nodules: Physical enlargements that may or may not impact hormone output.
  • Postpartum Thyroiditis: Temporary dysfunction following pregnancy.

Panel Components

This test is a profile comprising the following specific markers:

Free Thyroxine (FT4)
ng/dL
Thyroid Stimulating Hormone (TSH)
mIU/L
T3 Uptake
%
Free Triiodothyronine (FT3)
pg/mL
Total Triiodothyronine (T3)
ng/dL
Total Thyroxine (T4)
µg/dL

Why Context Matters

Results can be significantly confounded by the consumption of Biotin (Vitamin B7) supplements, which can falsely elevate T3/T4 and falsely lower TSH. Additionally, the time of day (diurnal rhythm) causes TSH levels to peak at night and dip in the late afternoon. Acute illness, extreme stress, and certain medications (like glucocorticoids or amiodarone) can lead to 'Euthyroid Sick Syndrome,' where results look abnormal despite no underlying thyroid disease.

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 Thyroid Function Test:

Research & Guidelines

Related Indicators

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

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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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