Getting tested for hypothyroidism starts with a simple blood draw, usually ordered by your primary care doctor. The core test measures thyroid-stimulating hormone (TSH), and results typically come back within a day or two. Depending on what that first test shows, your doctor may order additional blood work to confirm the diagnosis and identify the cause.
The First Test: TSH
TSH is the single most sensitive screening test for thyroid dysfunction. Your pituitary gland produces TSH to tell your thyroid to make hormones. When your thyroid is underperforming, your pituitary compensates by pumping out more TSH, so a high TSH level is the earliest signal that something is off.
Most labs consider a TSH between roughly 0.4 and 4.5 mIU/L normal, though reference ranges vary slightly between laboratories. A TSH above that range with normal thyroid hormone levels is called subclinical hypothyroidism, a milder form where the thyroid is starting to struggle but hasn’t fully fallen behind yet. When TSH climbs higher and your actual thyroid hormone levels drop below normal, that’s overt hypothyroidism.
Treatment recommendations differ based on severity. Major endocrinology guidelines generally recommend treatment when TSH is persistently at or above 10 mIU/L. For levels between the upper limit of normal and 10, the decision is more nuanced and depends on your symptoms, age, and other health factors.
Follow-Up Blood Work: Free T4 and Free T3
If your TSH comes back abnormal, your doctor will likely check your free T4 level, and possibly free T3. These measure the actual thyroid hormones circulating in your blood. TSH alone can sometimes be misleading. Certain conditions, including pituitary problems and acute illness, can push TSH into a range that looks abnormal even when the thyroid itself is fine. Checking free T4 alongside TSH gives a much clearer picture.
Many labs in the UK and increasingly in the US now run TSH and T4 together as a standard combination. If your doctor suspects a pituitary or hypothalamus issue (called central hypothyroidism), TSH becomes unreliable on its own, and free T4 is essential for guiding any decisions about treatment.
Antibody Tests for Hashimoto’s Disease
Once blood work confirms your thyroid is underactive, the next question is why. The most common cause is Hashimoto’s disease, an autoimmune condition where your immune system attacks the thyroid. To check for this, your doctor can order thyroid antibody tests, specifically thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb).
High levels of one or both of these antibodies point strongly toward Hashimoto’s. Most people with the condition have elevated levels of at least one type. Knowing the cause matters because Hashimoto’s tends to be progressive. If your antibodies are positive but your thyroid levels are only mildly off now, your doctor may want to monitor you more closely over time since subclinical hypothyroidism from autoimmune thyroid disease can gradually worsen into overt hypothyroidism.
Do You Need an Ultrasound?
Probably not. The American Academy of Family Physicians specifically advises against routinely ordering a thyroid ultrasound for abnormal thyroid function tests unless there’s a palpable lump or visible enlargement in your neck. Ultrasound is designed to evaluate thyroid nodules and structural abnormalities, not to diagnose or monitor hypothyroidism. If your doctor feels something unusual during a neck exam, they may order imaging at that point, but it’s not a standard part of the workup.
How to Prepare for Your Blood Test
There are no strict fasting requirements for a standalone TSH test, but timing and preparation can affect your results in meaningful ways.
TSH follows a natural daily rhythm, peaking between 11 p.m. and 5 a.m. and dropping to its lowest point in the late afternoon. Levels measured in an early morning fasting state tend to be higher than those drawn later in the day after eating. Research from the Indian Journal of Endocrinology and Metabolism found that fasting morning samples gave more consistent readings, and suggested that a fasting morning draw could reduce the need for repeat testing due to borderline results. If your doctor has ordered other blood tests alongside thyroid labs, you may need to fast for 8 to 12 hours beforehand anyway.
Two things to flag for your doctor before the test: medications and supplements. Several medications can affect thyroid test results, so tell your provider about everything you’re taking. Don’t stop any medication on your own. Biotin supplements deserve special attention. Even moderate doses of 5 mg per day can interfere with the lab equipment used to measure thyroid hormones, potentially producing false results. The interference lasts about 8 hours after a 5 mg dose and up to two days after a 10 mg dose, depending on the lab’s testing platform. If you take biotin (common in hair, skin, and nail supplements), stop it at least two days before your blood draw to be safe.
At-Home Thyroid Test Kits
Several companies sell finger-prick thyroid test kits you can use at home. These can be a convenient first step if you want a quick screening result, but they have real limitations. The kits aren’t standardized, and there isn’t enough data from large studies to confirm how well home test results match traditional lab values. As one Columbia University thyroid surgeon put it, an abnormal home test result should always be repeated with a standard blood draw sent to a lab with verified accuracy.
Home kits can be useful as a screening tool, particularly if getting to a doctor’s office is difficult or you want preliminary data before scheduling an appointment. But they’re not reliable enough to base treatment decisions on, especially since managing thyroid medication requires detecting very small changes in hormone levels.
What to Tell Your Doctor
Before your appointment, it helps to write down specific symptoms you’ve been experiencing and how long they’ve lasted. Common hypothyroidism symptoms include persistent fatigue, unexplained weight gain, feeling cold when others don’t, constipation, dry skin, hair thinning, and brain fog. Note any family history of thyroid disease or autoimmune conditions, since these significantly increase your risk.
If you’ve been struggling with your weight, energy, or mood and suspect your thyroid, say so directly. Thyroid conditions can overlap with many other issues, and giving your doctor a clear picture of your symptoms helps them decide which tests to order and how aggressively to investigate. Most of the time, a single blood draw covering TSH and free T4 is enough to either confirm or rule out hypothyroidism within a couple of days.