TSH, Free T4, Free T3, Antibodies: Which Thyroid Blood Tests to Ask For

TSH is the single most sensitive screening test for thyroid disease and is what most guidelines start with. Free T4 confirms whether the thyroid is actually producing hormone. TPO antibodies identify autoimmune thyroiditis. Free T3 and reverse T3 are not recommended for routine monitoring in primary hypothyroidism.

Why the right thyroid panel matters

Most people with thyroid symptoms walk into a primary-care visit and get one test: TSH. That is correct for first-line screening, but it can also miss the diagnosis story when symptoms and labs don't agree. Knowing what each test actually measures — and when to ask for more — is the difference between being told "your thyroid is fine" and getting an answer that explains your symptoms [C1][C5].

TSH — what it is, what it shows

Thyroid-stimulating hormone (TSH) is made by the pituitary gland, not the thyroid. It is the signal the brain sends to push the thyroid to produce more hormone [C5]. In a healthy person, TSH rises when thyroid hormone is low and falls when thyroid hormone is high — like a thermostat. That makes TSH the most sensitive single marker for primary thyroid disease: tiny shifts in thyroid output produce relatively large changes in TSH [C1][C6].

Typical adult reference ranges run from about 0.4 to 4.0 mIU/L, though specific labs and assays vary [C5][C9]. For patients on levothyroxine, the American Thyroid Association recommends targeting TSH within the reference range, and some specialists aim for the lower half (0.5 to 2.5 mIU/L) in patients who remain symptomatic at the higher end [C1].

Free T4 — the actual hormone output

Free T4 (free thyroxine) measures the unbound, biologically active fraction of T4 circulating in the blood. It tells you whether the thyroid is making hormone — independent of how loud the pituitary is shouting [C5][C9]. ATA-endorsed guidelines recommend ordering free T4 whenever TSH is abnormal, when pituitary disease is suspected, or in pregnancy [C1][C3].

A common pattern: in primary hypothyroidism, TSH rises before free T4 falls. This is called subclinical hypothyroidism — a high TSH with a normal free T4 [C6]. In overt hypothyroidism, both are abnormal.

Free T3 — the active hormone, but not routinely useful

T3 is the biologically active hormone — about three to four times more potent than T4 at the cellular level [C5]. Most T3 is made by peripheral conversion from T4 in the liver, kidney, and muscle, not by the thyroid itself [C1].

Patients often ask for free T3 testing because wellness sites describe it as the "real" thyroid test. The 2014 ATA hypothyroidism guideline is explicit: free T3 is not recommended for routine monitoring in primary hypothyroidism on levothyroxine, because TSH is more sensitive to dose changes and free T3 levels fluctuate with conversion rates that change throughout the day [C1]. Free T3 is useful in specific situations — suspected hyperthyroidism, particularly T3 toxicosis, and follow-up of treated Graves' disease — but is not a first-line test in hypothyroidism [C4][C5].

TPO antibodies — the marker that names the disease

Anti-thyroid peroxidase (TPO) antibodies are the lab finding that converts a vague "low thyroid function" into a specific diagnosis: Hashimoto's thyroiditis, the autoimmune cause of most hypothyroidism in iodine-sufficient regions [C7]. Elevated TPO antibodies indicate the immune system is producing antibodies against the enzyme the thyroid uses to make hormone [C7].

Why this matters even if your TSH is normal:

• People with elevated TPO antibodies and normal TSH have a higher annual risk of progressing to overt hypothyroidism than antibody-negative patients [C6].
• The presence of antibodies changes pregnancy monitoring — ATA pregnancy guidelines recommend more aggressive TSH targets in antibody-positive women [C3].
• It changes the answer to "is this autoimmune or just a sluggish thyroid?" — which affects how patients understand the disease and which dietary discussions actually apply [C7].

Thyroglobulin antibodies (TgAb) often rise alongside TPO antibodies and are tested in some labs as part of a Hashimoto's panel. TgAb is also tracked in patients who have had thyroid cancer surgery, where thyroglobulin is used as a recurrence marker [C7].

TSI / TRAb — the test for Graves' disease

Thyroid-stimulating immunoglobulins (TSI) and TSH receptor antibodies (TRAb) are markers of Graves' disease — the autoimmune cause of hyperthyroidism [C4][C8]. They are not part of routine hypothyroidism workup, but should be tested when TSH is low and free T4 or T3 are elevated, particularly in pregnant women with a history of Graves' [C3][C4].

Reverse T3 — popular online, not recommended by guidelines

Reverse T3 (rT3) is an inactive form of T3 produced when the body shunts T4 conversion away from active T3. Wellness sites describe high rT3 as evidence of "thyroid hormone resistance" or stress-induced thyroid dysfunction. The 2014 ATA guideline does not recommend reverse T3 testing for diagnosis or monitoring of hypothyroidism, citing lack of evidence that levels meaningfully change clinical management [C1]. It can be elevated in serious illness, fasting, and after cardiac surgery — situations where treating the underlying problem matters far more than the rT3 number itself.

What to ask for, by situation

1. First-time screening (no diagnosis yet): TSH. If abnormal, add free T4 [C1][C5].
2. Persistent hypothyroid symptoms with normal TSH: TSH, free T4, and TPO antibodies. If antibody-positive, you have Hashimoto's even with normal labs and warrant closer monitoring [C6][C7].
3. Known hypothyroidism on levothyroxine — routine check: TSH alone is sufficient for dose adjustment. Recheck 6 to 8 weeks after any dose change [C1].
4. Pregnant or trying to conceive: TSH and free T4, with TPO antibodies in women with prior thyroid disease or recurrent miscarriage. TSH targets are tighter in pregnancy [C3].
5. Suspected hyperthyroidism (palpitations, weight loss, heat intolerance): TSH, free T4, free T3, and TRAb/TSI [C4][C8].
6. Suspected pituitary disease (low TSH with low free T4, or unusual lab patterns): TSH, free T4, free T3, and referral to endocrinology [C1].

Frequently asked questions

Why won't my doctor order free T3? Because guidelines do not recommend it for routine hypothyroidism monitoring [C1]. In primary hypothyroidism on levothyroxine, TSH responds to dose changes more reliably than free T3, and free T3 fluctuates with diet, stress, and time of day. It's appropriate to test in specific situations — your endocrinologist will use it when the clinical picture asks for it [C1][C4].

How often should I get my thyroid checked if I have Hashimoto's? With normal TSH and on no medication, annually is typical. After starting levothyroxine or changing dose, recheck at 6 to 8 weeks. Once stable, every 6 to 12 months [C1][C7].

Do I need to fast before a thyroid blood test? No. Routine TSH and free T4 do not require fasting. If you are on levothyroxine, take your dose after the blood draw — not before — because freshly absorbed hormone can transiently raise free T4 [C1].

What's a "normal" TSH? Adult reference ranges are typically 0.4 to 4.0 mIU/L, but they vary by lab assay [C5][C9]. Pregnancy-specific ranges are lower [C3]. For patients on levothyroxine, the ATA recommends targeting within the reference range, with some specialists aiming for the lower half if symptoms persist [C1].

My antibodies are sky-high but my TSH is normal — am I sick? You have autoimmune thyroiditis, which is a real diagnosis even with normal hormone levels [C7]. Annual TSH monitoring is appropriate, because progression to overt hypothyroidism is more common in antibody-positive patients [C6][C7].

Bottom line

A well-targeted thyroid panel answers a specific question, not "everything that could possibly be wrong." TSH is the right first test for almost everyone [C1][C5]. Free T4 confirms the thyroid's actual output. TPO antibodies name the autoimmune cause when symptoms or family history suggest Hashimoto's [C7]. Free T3, reverse T3, and TSI/TRAb are situational, not screening, tests [C1][C4]. The shortest path to a useful answer is a clear conversation with your prescriber about why each test is being ordered — and what would change in your care depending on the result.

Sources

1. [C1] Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670–1751. PubMed: 25266247
2. [C2] Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012;22(12):1200–1235. PubMed: 23246686
3. [C3] Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27(3):315–389. PubMed: 28056690
4. [C4] Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26(10):1343–1421. PubMed: 27521067
5. [C5] American Thyroid Association. Thyroid Function Tests — Patient Information. thyroid.org
6. [C6] Surks MI, Ortiz E, Daniels GH, et al. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. JAMA. 2004;291(2):228–238. PubMed: 14722150
7. [C7] American Thyroid Association. Hashimoto's Thyroiditis — Patient Information. thyroid.org
8. [C8] American Thyroid Association. Graves' Disease — Patient Information. thyroid.org
9. [C9] NIH MedlinePlus. Thyroid Tests. medlineplus.gov

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For educational purposes only. Not medical advice. Always consult your healthcare provider.