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.

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?