The primary blood test for thyroid function is the TSH test, which measures thyroid stimulating hormone. This single test is the standard first step doctors use to screen for both overactive and underactive thyroid conditions, with normal values generally falling between 0.5 and 5.0 µU/mL. Depending on your results and symptoms, your doctor may order additional tests to get a fuller picture.
The TSH Test: Where Screening Starts
TSH is a hormone produced by your pituitary gland, a small structure at the base of your brain. Its job is to signal your thyroid to produce hormones. When your thyroid isn’t making enough hormone, your pituitary pumps out more TSH to compensate, pushing levels higher. When your thyroid is overproducing, your pituitary dials TSH back down. This inverse relationship is why a single TSH reading can flag problems in either direction.
The standard reference range is 0.5 to 5.0 µU/mL, though labs vary slightly and experts don’t fully agree on the upper cutoff. TSH levels naturally shift with age. In adults, levels tend to rise after age 50 in women and 60 in men. Research from the American Thyroid Association shows that a 50-year-old woman might have an upper normal limit of 4.0, while by age 90, that number climbs by about 50% to 6.0. Some labs adjust their reference range upward for older patients, using a ceiling as high as 7.0. Children naturally run higher than adults as well.
If you’re already being treated for a thyroid condition, your doctor will typically aim to keep your TSH between 0.5 and 4.0. There are exceptions: people with certain thyroid cancers may need their TSH kept deliberately low to reduce the risk of recurrence.
Free T4 and T3: Measuring Actual Thyroid Hormones
TSH tells you how hard your brain is working to regulate the thyroid, but it doesn’t directly measure the hormones your thyroid produces. That’s where T4 (thyroxine) and T3 (triiodothyronine) tests come in. These are the hormones that actually enter your cells and control your metabolism, energy, and body temperature.
Both T4 and T3 exist in two forms in your blood: “free” and “bound.” Free hormones are the active versions that can enter your tissues and do their work. Bound hormones are attached to proteins and essentially inactive. Doctors usually measure free T4, since it gives the most useful snapshot of available thyroid hormone. For T3, a total T3 test (which captures both free and bound) is generally considered more accurate than free T3 alone.
Your doctor will often order free T4 alongside TSH when your TSH comes back abnormal. This pairing helps distinguish between mild and more significant thyroid problems. In some cases, T3 is also measured, particularly when hyperthyroidism is suspected but T4 looks normal. A condition called T3 toxicosis produces high T3 levels with normal T4, and it would be missed without that specific test.
What Different Result Patterns Mean
The combination of your TSH and free T4 results tells a clearer story than either number alone. Here’s how the common patterns break down:
- High TSH, low free T4: This is the classic pattern for hypothyroidism (underactive thyroid). Your pituitary is working overtime because the thyroid isn’t keeping up.
- High TSH, normal free T4: Often called subclinical hypothyroidism. Your thyroid is struggling but still producing enough hormone for now. Your doctor may monitor this over time or recommend treatment depending on your symptoms and how high the TSH is.
- Low TSH, high free T4: This pattern points to hyperthyroidism (overactive thyroid). Your thyroid is flooding the body with hormone, so the pituitary has stopped sending signals.
- Low TSH, normal free T4, high T3: This suggests T3 toxicosis, a form of hyperthyroidism driven specifically by excess T3.
- Low or normal TSH, low free T4: This unusual combination can indicate a problem with the pituitary gland itself rather than the thyroid. The pituitary isn’t sending enough TSH, so the thyroid underproduces.
Antibody Tests for Autoimmune Thyroid Disease
If your initial blood work suggests a thyroid problem, your doctor may order antibody tests to find out why. The most common causes of thyroid dysfunction are autoimmune, meaning your immune system is attacking your own thyroid tissue. Antibody tests identify this.
Three antibodies are commonly measured:
- TPO antibodies (TPOAb): High levels are the hallmark of Hashimoto’s disease, the most common cause of hypothyroidism. Your immune system targets an enzyme the thyroid needs to produce hormones.
- Thyroglobulin antibodies (TgAb): Also associated with Hashimoto’s disease. Most people with Hashimoto’s have elevated levels of TPOAb, TgAb, or both.
- Thyrotropin receptor antibodies (TRAb): These are a sign of Graves’ disease, the most common cause of hyperthyroidism. These antibodies mimic TSH, essentially forcing the thyroid to overproduce.
Higher antibody levels correlate with a greater likelihood of autoimmune thyroid disease. Some people test positive for thyroid antibodies without having symptoms yet. In those cases, it signals increased risk, and your doctor may want to monitor your thyroid function periodically.
TSH Ranges During Pregnancy
Pregnancy shifts what counts as a normal TSH level. Your body’s demand for thyroid hormone increases significantly, especially in the first trimester when the developing baby depends entirely on the mother’s supply. The Endocrine Society recommends keeping TSH between 0.2 and 2.5 in the first trimester, and between 0.3 and 3.0 in the second and third trimesters. These are tighter windows than the standard adult range, and thyroid screening is common in early pregnancy for this reason.
Preparing for a Thyroid Blood Test
You generally don’t need to fast or do anything special before a thyroid blood draw. You can eat and drink normally. Fasting only matters for thyroid scans, which are imaging tests, not blood tests.
There is one important exception: biotin supplements. Biotin (vitamin B7) is found in many hair, skin, and nail supplements, sometimes at high doses. It can interfere with the lab technology used to measure thyroid hormones, producing falsely low TSH readings and potentially skewing other results. Even a single 10 mg dose of biotin has caused interference when blood was drawn within 24 hours. The FDA has flagged this as a real source of lab error. If you take any supplement containing biotin, mention it to your doctor before your blood draw. Stopping the supplement for a few days beforehand is a common recommendation.
TSH levels also fluctuate throughout the day, running higher in the early morning and lower in the afternoon. For the most consistent results, especially if you’re tracking levels over time, an early morning draw is ideal.