Complete Guide to Thyroid Blood Tests: TSH, T3 & T4

Quick answer

Thyroid testing always starts with TSH, the most sensitive single test; abnormal TSH is then paired with free T4, and antibodies (anti-TPO, TRAb) identify autoimmune causes like Hashimoto's or Graves' disease. The core takeaway: test in the morning before levothyroxine if treated, and read TSH with free T4 together, since either one alone can mislead.

Overview: which thyroid tests exist?

Test	What it measures	When ordered
TSH (thyroid stimulating hormone)	Pituitary signal to the thyroid: first-line screening test	All thyroid screening; monitoring treatment
Free T4 (FT4)	Unbound thyroxine: main thyroid hormone	If TSH abnormal; monitoring levothyroxine
Free T3 (FT3)	Unbound triiodothyronine (active form)	If T4 normal but symptoms persist; suspected T3 toxicosis
Total T4 / Total T3	Bound + unbound hormone	Rarely used; affected by protein binding changes
Anti-TPO antibodies	Antibodies against thyroid peroxidase enzyme	Suspected Hashimoto thyroiditis (high TSH)
Anti-TG antibodies	Antibodies against thyroglobulin	Hashimoto; differentiated thyroid cancer monitoring
TSH receptor antibodies (TRAb)	Stimulating or blocking antibodies	Graves disease (low TSH, high T4)
Thyroglobulin (Tg)	Protein made by thyroid	Monitoring for thyroid cancer recurrence after surgery/RAI
Calcitonin	Hormone from C cells	Medullary thyroid cancer

Key points

TSH is the best first test; it reacts to small hormone changes.
Pair abnormal TSH with free T4; add T3 only in specific situations.
Anti-TPO confirms Hashimoto's; TRAb confirms Graves' disease.
Test in the morning, and before levothyroxine if you take it.

Step 1: Always start with TSH

TSH is the single best initial test for thyroid function. It is sensitive to even small changes in thyroid hormone levels. The pituitary acts like a thermostat: if T4 is low, TSH rises to stimulate more production; if T4 is high, TSH falls to suppress the thyroid. A normal TSH (0.4–4.0 mIU/L) makes significant thyroid dysfunction very unlikely and usually no further testing is needed.

Step 2: Interpreting TSH + free T4 patterns

TSH	Free T4	Diagnosis
High TSH	Low T4	Overt hypothyroidism: treat with levothyroxine
High TSH	Normal T4	Subclinical hypothyroidism: monitor or treat
Normal TSH	Normal T4	Euthyroid: normal thyroid function
Low TSH	High T4	Overt hyperthyroidism: investigate cause
Low TSH	Normal T4	Subclinical hyperthyroidism: monitor or treat
Low TSH	Low T4	Secondary hypothyroidism: pituitary problem; check ACTH

When to check antibodies

Anti-TPO antibodies: Hashimoto thyroiditis

When to order: if TSH is elevated (hypothyroid or subclinical). A positive anti-TPO confirms autoimmune thyroiditis (Hashimoto) as the cause. High anti-TPO with normal TSH in someone with thyroid symptoms may help predict future hypothyroidism. Anti-TPO levels do not predict disease severity or need for treatment, TSH does.

TRAb (TSH receptor antibodies): Graves disease

When to order: if TSH is low (hyperthyroid). Positive TRAb confirms Graves disease as the cause of hyperthyroidism. They can also be used to predict remission after antithyroid drug treatment, persistently elevated TRAb after 12–18 months of treatment predicts relapse.

Timing matters: when to test

Levothyroxine monitoring: test TSH 6–8 weeks after any dose change (T4 has a 7-day half-life, TSH takes weeks to stabilise)
Take levothyroxine in the morning on an empty stomach, and arrange blood test before that day's dose for accurate monitoring
Antithyroid drugs (carbimazole/methimazole): test TSH and free T4 every 4–6 weeks until stable
Pregnancy: TSH targets change each trimester; test at least once per trimester

Questions to ask your doctor

Should I test free T3 as well as T4?
What is my anti-TPO level?
Do I have Graves disease or another cause of hyperthyroidism?
Am I taking my levothyroxine correctly?
What TSH level should I aim for?

The TSH cascade, when to add T4 and T3

Thyroid-stimulating hormone (TSH) is the pituitary hormone that controls thyroid hormone production. It is measured first because it integrates the net effect of thyroid hormone on the body, a feedback system so sensitive that TSH changes by a factor of 10 for every 2-fold change in free T4. TSH is the single most sensitive marker of thyroid function:

TSH normal (0.4–4.0 mU/L): Thyroid function is almost certainly normal. No further thyroid tests needed in an asymptomatic person.
TSH elevated: Add free T4. If TSH elevated + free T4 low → overt hypothyroidism. If TSH elevated + free T4 normal → subclinical hypothyroidism (treat if TSH consistently above 10 mU/L, or if symptomatic or TSH 5–10 with elevated TPO antibodies, NICE NG145).¹
TSH suppressed: Add free T4. If TSH suppressed + free T4 high → overt hyperthyroidism. If TSH suppressed + free T4 normal → add free T3 (T3 thyrotoxicosis, the thyroid is secreting excess T3 but not excess T4; seen particularly in early Graves' disease and toxic nodules), one of several discordant patterns that warrant a closer look rather than assuming assay error.² If TSH suppressed + both T4 and T3 normal → subclinical hyperthyroidism.

Graves' disease versus Hashimoto's, distinguishing hyperthyroid causes

Once hyperthyroidism is confirmed (suppressed TSH + elevated free T4), the cause determines treatment:

Graves' disease: Autoimmune stimulation of TSH receptors by TSH receptor antibodies (TRAb, also called thyroid-stimulating immunoglobulin, TSI). TRAb are present in around 95% of Graves' patients and are the only cause of hyperthyroidism with eye involvement (Graves' ophthalmopathy), per the American Thyroid Association's evidence-based guideline on thyrotoxicosis.³ Treated with antithyroid drugs (carbimazole, propylthiouracil), radioactive iodine, or thyroidectomy. TRAb measurement predicts relapse risk after antithyroid drug therapy and is essential in pregnancy (TRAb crosses the placenta, neonatal Graves' thyrotoxicosis requires monitoring).
Toxic nodular goitre / toxic adenoma: No TRAb detectable. Autonomous thyroid nodule(s) secreting excess hormone independent of TSH control. Diagnosed by thyroid ultrasound and radionuclide scan (the nodule shows increased uptake, "hot nodule"). Treatment: radioactive iodine or surgery. Antithyroid drugs control but do not cure autonomous nodules.
Thyroiditis: Subacute (De Quervain's, painful, viral) or silent/postpartum thyroiditis. Thyroid hormone leaks from inflamed follicles causing temporary thyrotoxicosis. TRAb negative, nuclear scan shows suppressed uptake. Self-limiting, antithyroid drugs not indicated.

Your next steps for thyroid testing

Always start with TSH
TSH is the single best initial test — it reacts to small hormone changes like a thermostat. A normal TSH (0.4–4.0 mU/L) in an asymptomatic person usually ends the investigation.
Time the draw right
Test in the morning, and if you take levothyroxine, arrange the blood test before that day’s dose on an empty stomach. Afternoon draws and post-tablet draws are the commonest source of confusing TSH results.
Read TSH with free T4 as a pattern
High TSH + low T4 is overt hypothyroidism; high TSH + normal T4 is subclinical; low TSH + high T4 is overt hyperthyroidism; low TSH + normal T4 may need free T3 added. The pair diagnoses what neither alone can.
Check antibodies when results are abnormal
Anti-TPO confirms Hashimoto’s as the cause of a raised TSH; TRAb confirms Graves’ disease as the cause of a suppressed TSH. The cause determines the treatment.
Ask about your personal TSH target
Targets change with treatment, age and pregnancy (each trimester has its own target). After any levothyroxine dose change, retesting is done at 6–8 weeks, not sooner — TSH takes weeks to stabilise.

TSH and free T4 patterns at a glance

The TSH-plus-free-T4 matrix below is the core of thyroid interpretation. Antibodies and T3 then answer the follow-up question: what is causing it.

TSH	Free T4	Interpretation
High	Low	Overt hypothyroidism
High	Normal	Subclinical hypothyroidism — monitor or treat
Normal	Normal	Euthyroid — normal thyroid function
Low	High	Overt hyperthyroidism — investigate the cause
Low	Normal	Subclinical hyperthyroidism — monitor or treat; consider free T3
Low	Low	Secondary hypothyroidism — pituitary problem, not the thyroid

Practical notes

Keep your tablet routine consistent around test day. Many doctors advise taking your thyroid tablet after the blood draw rather than before, and testing at roughly the same time of day each time — morning is common — so results stay comparable. Ask your own doctor what timing they prefer rather than changing the routine on your own.

Tell the lab about biotin and supplements. High-dose biotin, common in hair-and-nail multivitamins, can interfere with the immunoassay methods used for thyroid tests and produce misleading results; your doctor may ask you to pause it for a couple of days before testing. Also mention calcium, iron and any recent contrast scans or illness.

Read the cascade in order: TSH first, then free T4, then antibodies or T3 only if the picture needs them. Because TSH moves roughly tenfold for every twofold change in thyroid hormone, small TSH shifts carry more meaning than they look like. Compare with previous results from the same lab rather than judging one reading alone.

Once levels are stable, testing becomes infrequent — often every 6–12 months — but any new symptoms, a new medication, pregnancy or a planned pregnancy are reasons to recheck sooner. If your dose or brand changes, ask when the follow-up test should be; it is usually several weeks later, once levels have settled.

In India

A TSH test at a large Indian laboratory typically costs around ₹300–₹600, while a full thyroid profile (TSH with free T4 and free T3) typically costs around ₹700–₹1,500 — though prices vary by city and lab.

Home sample collection for thyroid tests is standard in major cities, and most large labs are NABL-accredited. Because different analysers use slightly different methods, try to repeat follow-up tests at the same lab — and the reference range printed on your own report is the one that counts for your result.

Frequently Asked Questions

Why is TSH always the first thyroid test?

TSH is the single best initial test because it is sensitive to even small changes in thyroid hormone levels. The pituitary acts like a thermostat: if T4 is low, TSH rises to stimulate more; if T4 is high, TSH falls to suppress the thyroid. A normal TSH (0.4–4.0 mU/L) in an asymptomatic person means thyroid function is almost certainly normal and no further tests are needed.

When should thyroid antibodies be checked?

Anti-TPO is ordered when TSH is elevated: a positive result confirms Hashimoto thyroiditis as the cause, and high anti-TPO with a still-normal TSH can predict future hypothyroidism. TRAb is ordered when TSH is suppressed: a positive result confirms Graves' disease as the cause of hyperthyroidism and helps predict remission prospects after antithyroid treatment.

How does the TSH cascade decide when to add T4 and T3?

TSH changes roughly tenfold for every twofold change in thyroid hormone, so it is measured first. If TSH is elevated, add free T4 (low T4 means overt hypothyroidism, normal T4 means subclinical). If TSH is suppressed, add free T4 first, then free T3 when T4 is normal — T3 thyrotoxicosis means the thyroid is secreting excess T3 without excess T4.

How are Graves' disease and Hashimoto's distinguished?

Once hyperthyroidism is confirmed (suppressed TSH with elevated free T4), the cause decides treatment. Graves' disease shows positive TSH receptor antibodies (TRAb) in around 95% of cases. Toxic nodular goitre and thyroiditis are TRAb-negative and diagnosed by ultrasound and radionuclide scanning instead. Anti-TPO positivity, by contrast, points to Hashimoto's on the hypothyroid side.

Should I take my thyroid tablet before my TSH blood test?

Ask your doctor — many advise taking the tablet after the blood draw so the result reflects your steady state rather than the day's dose, and keeping the same routine each time. What matters most is consistency: test at roughly the same time of day and in the same relation to your tablet as last time.

My TSH is normal but I still have symptoms — what should I ask?

Ask whether free T4 and thyroid antibodies have been checked, whether the result should be compared with your previous trend rather than judged alone, and whether your symptoms could point to something beyond the thyroid. A normal TSH rules out most thyroid disease, but it doesn't explain every symptom — keep the conversation going with your doctor.

Can the biotin in my multivitamin affect my thyroid results?

Yes. High-dose biotin can interfere with the laboratory methods used for TSH, T4 and T3 and produce falsely high or low readings. Tell your doctor about any hair, skin and nail supplements; they may ask you to stop biotin for a few days before the test. Don't adjust anything without their advice.

How often does my TSH need rechecking once it's stable?

When levels are stable and nothing has changed, many doctors recheck every 6–12 months. Recheck sooner if symptoms return, a new medication is started, or pregnancy occurs or is planned. Your doctor sets the interval based on your history — ask what it is so you don't have to guess.

References

Sources cited on this page. PubMed links open the original abstract.

Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults. Endocr Pract. 2012;18(6):988–1028. PMID 23246686 · doi:10.4158/EP12280.GL
Koulouri O, Moran C, Halsall D, Chatterjee K, Gurnell M. "Pitfalls in the measurement and interpretation of thyroid function tests." Best Pract Res Clin Endocrinol Metab. 2013;27(6):745–762. PMID 24275187 · doi:10.1016/j.beem.2013.10.003
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. PMID 27521067 · doi:10.1089/thy.2016.0229

Medical Disclaimer: This page is for general educational purposes only and does not constitute medical advice. Always consult a qualified doctor for diagnosis and treatment.