TSH
TSH is made by the pituitary gland and tells the thyroid how much hormone to produce. A high TSH usually means an underactive thyroid. A low TSH usually means an overactive thyroid. It is the most sensitive first test.
Free T4 and free T3
These are the thyroid hormones themselves. They are read together with TSH to decide whether a problem is mild or definite.
Antibodies
- Anti-TPO antibodies point to autoimmune (Hashimoto) thyroid disease.
- TSH-receptor antibodies (TRAb) point to Graves disease, a common cause of overactive thyroid.
Why results can confuse
- Reference ranges differ between labs and in pregnancy.
- A slightly high TSH with a normal T4 (subclinical hypothyroidism) is often repeated after 6 to 8 weeks before deciding on treatment.
- Biotin supplements can distort some thyroid tests. Tell your doctor if you take them.
- A short illness can temporarily alter results.
You do not need to fast for thyroid tests. If you take thyroid tablets, ask your doctor whether to take them before or after the blood test.
Scans
An ultrasound looks at the thyroid’s size and any lumps. A thyroid scan is used for specific questions, such as a nodule with a low TSH.
When to see a specialist
Consider an opinion if symptoms persist despite normal tests, results are abnormal and the plan is unclear, you are pregnant or planning pregnancy, you have a lump, or your child has abnormal tests.
This guide is general information and is not a substitute for a consultation.