Patient Guide

Complete Guide to Thyroid Blood Tests

Name: Complete Guide to Thyroid Blood Tests: TSH, T3 & T4
Creator: Suman Konda
Published: 2026-07-05

TSH is just one of several thyroid tests. This guide explains which tests are ordered when, what each result means, and how to interpret your full thyroid panel.¹

Written by Suman Konda, PharmD, Clinical Pharmacist · Editorial policy · Not medical advice

Last updated: 01 August 2026 · How we check our content

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

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?

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. Thyroid. 2012;22(12):1200–1235. PMID 23246686 · doi:10.1089/thy.2012.0205

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).
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). 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 95% of Graves' patients and are the only cause of hyperthyroidism with eye involvement (Graves' ophthalmopathy). 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.