Hypothyroidism is diagnosed with a simple blood test, most commonly by measuring thyroid-stimulating hormone (TSH). A normal TSH generally falls between about 0.5 and 4.0 mIU/L. Values above that range suggest your thyroid is underperforming, and your pituitary gland is working harder to compensate. The test itself is a standard blood draw, and results typically come back within a day or two.
The TSH Test: First and Most Important
TSH is the single most useful screening test for hypothyroidism. Your pituitary gland releases TSH to tell your thyroid to produce more hormone. When the thyroid falls behind, TSH rises to pick up the slack. A high TSH is the earliest and most sensitive signal that your thyroid is struggling, often catching the problem before you feel symptoms.
If your TSH comes back above the normal range but below 10 mIU/L, you fall into a category called subclinical hypothyroidism. This is the mild form, and it accounts for roughly 90% of subclinical cases. Many people in this range feel fine and don’t need treatment right away. When TSH climbs above 10 mIU/L, the case for treatment becomes much stronger because the risk of cholesterol problems, cardiovascular issues, and worsening symptoms increases significantly.
When Free T4 Gets Checked
If your TSH comes back high, the next step is usually a free T4 test. Free T4 measures the actual thyroid hormone circulating in your blood and available for your body to use. This test confirms what the elevated TSH is suggesting and helps determine how severe the problem is.
A high TSH with a normal free T4 means subclinical hypothyroidism: your thyroid is slipping, but it’s still producing enough hormone for now. A high TSH paired with a low free T4 confirms overt hypothyroidism, meaning your body isn’t getting enough thyroid hormone to function properly. This is the combination that most clearly points to a diagnosis and the need for treatment.
There’s also a less common pattern worth knowing about. If both your TSH and free T4 come back low, the issue may not be with your thyroid at all. This pattern can signal a problem with the pituitary gland, which controls TSH production. This is called secondary or central hypothyroidism, and it requires different follow-up.
What About Free T3?
You may see free T3 included in thyroid panels offered by direct-to-consumer labs or requested by some practitioners. For diagnosing hypothyroidism, though, free T3 has limited utility for the vast majority of patients. T3 testing is most useful when TSH is very low and free T4 is normal or decreased, a scenario that points toward hyperthyroidism (an overactive thyroid) rather than an underactive one. For a straightforward hypothyroidism evaluation, TSH and free T4 give you the information you need.
Antibody Tests to Find the Cause
Once hypothyroidism is confirmed, your provider may order an antibody test to figure out why your thyroid is underperforming. The most common cause is Hashimoto’s disease, an autoimmune condition where your immune system gradually attacks the thyroid. Most people with Hashimoto’s will have antibodies to thyroid peroxidase (TPO) in their blood. TPO is a protein the thyroid needs to produce its hormones, so antibodies against it directly interfere with hormone production.
TPO antibody testing also matters for people in the subclinical range. If your TSH is elevated but below 10 mIU/L, having positive TPO antibodies is one of the factors that can tip the decision toward starting treatment. Positive antibodies suggest the problem is autoimmune and more likely to progress over time rather than resolve on its own. Other antibodies associated with Hashimoto’s, such as thyroglobulin antibodies, may also be tested if the clinical picture isn’t clear from TPO alone.
Timing and Preparation Matter
How and when you get your blood drawn can meaningfully shift your TSH results. TSH follows a natural daily rhythm, peaking overnight between midnight and early morning and dropping to its lowest point in the late morning and afternoon. A study examining this variation found that TSH values declined significantly when blood was drawn around 10 a.m. compared to earlier in the morning. For the most accurate reading, an early morning draw is ideal.
Eating before the test also makes a difference. Research found that TSH drops after a meal regardless of what the fasting value was. In one study, 75% of people who had subclinical hypothyroidism based on their fasting TSH were reclassified as normal when tested after eating. Their thyroid function hadn’t actually changed; the timing of the meal suppressed TSH just enough to push it below the diagnostic cutoff. Free T4 levels, on the other hand, weren’t significantly affected by food. To avoid this kind of false reassurance, getting your blood drawn in the morning before eating gives the most reliable result.
Stop Biotin Before Testing
If you take biotin supplements (sometimes labeled as vitamin B7 or vitamin H, commonly found in hair, skin, and nail formulas), you need to stop them before thyroid testing. Biotin interferes with the lab assays used to measure thyroid hormones and can produce wildly misleading results. In documented cases, people taking 20 to 30 mg of biotin per day were falsely diagnosed with thyroid conditions they didn’t have.
The interference can take time to clear. In case studies, TSH sometimes returned to normal within 24 hours of stopping biotin, but free T4 and free T3 took 48 to 72 hours to normalize. Some antibody markers took even longer. The recommended approach is to stop biotin for at least 48 to 72 hours before having your thyroid levels checked. Even if you’re taking a lower dose in a multivitamin, mention it to your provider so they can advise you.
Home Thyroid Tests
Several companies now offer finger-prick thyroid tests you can do at home and mail to a lab. A preliminary study comparing TSH measured from finger-prick dried blood spots against standard venous blood draws found very high agreement between the two methods, with a correlation of 0.988. The test correctly identified hypothyroidism in six out of seven confirmed cases with no false positives.
These results are promising, but finger-prick testing still has practical limitations. The blood volume is much smaller, which can make it harder to run a full panel. Results still need clinical interpretation, especially if your TSH falls in the subclinical range where context matters. A home test can be a reasonable first step if you want to check your thyroid function without scheduling an appointment, but an abnormal result will need follow-up with a standard blood draw to confirm the finding and guide any treatment decisions.
What the Full Picture Looks Like
For most people, the diagnostic path is straightforward: a TSH test, possibly followed by free T4 and TPO antibodies if TSH is elevated. The whole process usually involves one or two blood draws. Here’s how the common results break down:
- Normal TSH, normal free T4: Thyroid function is fine. No further testing needed.
- Elevated TSH, normal free T4: Subclinical hypothyroidism. Antibody testing helps determine whether it’s likely to progress.
- Elevated TSH, low free T4: Overt hypothyroidism. Treatment is typically recommended.
- Low TSH, low free T4: Possible pituitary problem. Further evaluation of pituitary function is needed.
If your results are borderline, your provider may recommend repeating the test in six to eight weeks rather than jumping to treatment. TSH can fluctuate temporarily due to illness, stress, or medication changes, so a single borderline result doesn’t always mean a lasting problem. Getting retested under the same conditions (early morning, fasting, biotin-free) gives the clearest comparison.