A Patient Information Guide
Your thyroid blood tests help your medical team monitor how well your thyroid gland is regulating your body’s metabolism.
The information below explains the most common thyroid markers, how they interact, and what your results mean.
Normal Range: 0.4–4.5 mIU/L (may vary slightly by laboratory)
TSH is produced by the pituitary gland in your brain, not by the thyroid itself. It acts as a messenger, telling your thyroid gland how much hormone to make.
Along with Free T4 (below), this is the most important thyroid blood test. Because of the brain’s natural feedback loop, TSH moves in the opposite direction of your thyroid function:
A high TSH usually indicates an underactive thyroid (hypothyroidism). The brain is screaming at the thyroid to work harder.
A low TSH usually indicates an overactive thyroid (hyperthyroidism). The brain has stopped sending signals because there are already too many thyroid hormones in the blood.
Normal Range: 10.0–22.0 pmol/L (0.8–1.8 ng/dL)
Thyroxine (T4) is the main chemical hormone manufactured and released directly by your thyroid gland. The word “Free” means it is not bound to proteins in your blood, leaving it active and available for your body to use.
As thyroid function declines, TSH rises and Free T4 falls. Your doctor will always look at your Free T4 alongside your TSH to get an accurate picture of your health. For example, if you are taking levothyroxine replacement therapy, your doctor will adjust your dose based on these two numbers combined.
Normal Range: 3.5–6.5 pmol/L (2.3–4.2 pg/mL)
T3 is the highly active form of thyroid hormone. While your thyroid gland produces some T3 directly, most of it is created when your body tissue converts T4 into T3 by removing an iodine molecule.
Doctors rarely test Free T3 on its own. It is most useful when evaluating an overactive thyroid (hyperthyroidism), where T3 levels can sometimes shoot up much faster and higher than T4 levels. It is generally not reliable or necessary for diagnosing an underactive thyroid.
Normal Range: Less than 35 IU/mL (depends heavily on the lab assay)
Antibodies are proteins created by your immune system to fight off infections. Sometimes, the immune system mistakenly creates antibodies that target your own healthy tissues, such as your thyroid. TPO is an enzyme essential for making thyroid hormones.
A highly raised TPO antibody level indicates an autoimmune response. It is the definitive marker for Hashimoto’s thyroiditis, the most common cause of an underactive thyroid. If your antibodies are high but your TSH is currently normal, it means you have an increased risk of developing hypothyroidism in the future.
Normal Range: Less than 115 IU/mL
Similar to TPO, thyroglobulin is a protein produced by your thyroid gland where thyroid hormones are stored. TgAb are antibodies that mistakenly attack this protein structure.
A raised TgAb level is another sign of autoimmune thyroid disease. Your doctor may check these alongside TPO antibodies to confirm a diagnosis, or use them as a baseline tracking marker if you are being treated for structural thyroid nodules.
Normal Range: Negative or Less than 1.75 IU/L
TRAb are specific antibodies that mimic the action of TSH. Instead of destroying tissue, they latch onto the thyroid gland and falsely command it to pump out massive amounts of extra hormone.
A positive TRAb blood test confirms a diagnosis of Graves’ disease, which is the primary cause of an overactive thyroid. Measuring these antibodies helps doctors distinguish Graves’ disease from other temporary forms of thyroid inflammation (thyroiditis).
Normal Range: 3.0–40.0 ug/L (in people with an intact thyroid gland)
Thyroglobulin is a protein matrix produced exclusively by normal, healthy thyroid tissue, as well as by some types of thyroid cancer cells.
This test is not used to check if your thyroid is underactive or overactive. Instead, it is primarily used as a medical tracking tool after a patient has undergone surgery to remove a thyroid tumor. If the thyroid has been completely removed, your thyroglobulin level should ideally drop near zero.
Typical Signs: Bradycardia (slow heart rate) or Tachycardia (fast heart rate)
While vital signs are not blood tests, doctors will assess your pulse rate and weight changes at every clinical consultation because thyroid hormones completely dictate your metabolic rate.
Why Your Pulse Matters: Thyroid hormone controls how fast your heart beats. An underactive thyroid slows everything down, causing a slow heart rate, fatigue, and feeling cold. An overactive thyroid accelerates your metabolism, causing a racing heart, anxiety, tremors, and unexplained weight loss.
How frequently you require a blood draw depends on your diagnosis, your symptoms, and whether your medication dose is currently being altered:
| Clinical Situation | Recommended Monitoring Frequency |
| Healthy / No Symptoms (Routine Screening) | Only if symptomatic, or every 3 to 5 years over age 60 |
| Starting Levothyroxine or Dose Changes | Every 6 to 8 weeks until your TSH level stabilizes |
| Stable on a Long-Term Hypothyroid Dose | Annually |
| Active Overactive Thyroid (Antithyroid Drugs) | Every 4 to 6 weeks during early treatment phases |
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