Thyroid Panel (Thyroid) Blood Test Explained — Normal Ranges & Results

MD

Medically reviewed by Dr. Elena Marsh, MD — Board-Certified Internal Medicine
Last reviewed: March 2026 · Reviewed for US reference ranges

A thyroid panel measures how well your thyroid gland — the butterfly-shaped gland in your neck — is controlling your metabolism. The key test is TSH (thyroid-stimulating hormone), often paired with Free T4 and sometimes Free T3 and antibodies.

Thyroid disorders are very common in the US, especially in women over 40. An underactive thyroid (hypothyroidism) causes fatigue, weight gain, and cold intolerance; an overactive thyroid (hyperthyroidism) causes weight loss, anxiety, and a racing heart.

Thyroid reference ranges (US standard)

These ranges are aligned with Quest Diagnostics and LabCorp. Your own report prints the exact range your lab used — always defer to that range, since methods differ slightly between labs.

Parameter	Normal Range	What it measures
TSH	0.4–4.0 mIU/L	Master thyroid control hormone
Free T4	0.8–1.8 ng/dL	Main thyroid hormone (storage form)
Free T3	2.3–4.2 pg/mL	Active thyroid hormone
TPO Antibodies	Under 35 IU/mL	Marker for Hashimoto's
Thyroglobulin Ab	Under 20 IU/mL	Autoimmune thyroid marker

USA Insurance Note

The Thyroid is billed under CPT 84443. At an annual physical it is usually covered as preventive screening with little or no out-of-pocket cost. When ordered to investigate a symptom, it may be applied to your deductible. Decode a specific value →

How to read your thyroid results

TSH — the most important number

TSH works in reverse: when your thyroid is underactive, TSH goes UP (the brain shouts louder to stimulate it); when overactive, TSH goes DOWN. A TSH above 4.0–4.5 mIU/L with low T4 suggests hypothyroidism; a TSH below 0.4 with high T4 suggests hyperthyroidism.

Hashimoto's thyroiditis

Hashimoto's is the most common cause of hypothyroidism in the US. It's an autoimmune condition confirmed by elevated TPO antibodies. Many people have antibodies for years before their thyroid slows down.

Graves' disease

Graves' is the most common cause of hyperthyroidism, driven by antibodies that overstimulate the thyroid. It often comes with eye changes and a noticeably fast heartbeat.

Age & pregnancy note

In adults over 65, a slightly higher TSH (up to ~6 mIU/L) can be acceptable. In pregnancy, TSH targets are lower and thyroid testing is part of standard prenatal care.

Thyroid — Frequently Asked Questions

What is a normal TSH level?+

A normal TSH is generally 0.4–4.0 mIU/L. A TSH above this range suggests an underactive thyroid (hypothyroidism); below it suggests an overactive thyroid (hyperthyroidism). Targets shift slightly in pregnancy (lower) and older adults (a bit higher is acceptable).

My TSH is high but I feel fine — what does that mean?+

A mildly high TSH with normal T4 and no symptoms is called subclinical hypothyroidism. It's common and doesn't always need treatment. Your doctor may simply recheck it in a few weeks to months, sometimes with TPO antibodies, before deciding on treatment.

What is the difference between hypothyroidism and Hashimoto's?+

Hypothyroidism means an underactive thyroid — a state, defined by high TSH and low thyroid hormone. Hashimoto's thyroiditis is the most common cause of that state in the US: an autoimmune condition where the immune system attacks the thyroid, confirmed by elevated TPO antibodies.

Do I need to fast for a thyroid test?+

Generally no — fasting isn't required for TSH and thyroid hormone levels. However, biotin supplements (often taken for hair and nails) can interfere with thyroid test results, so many providers ask you to stop biotin for 2–3 days beforehand.

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