Checking for hypothyroidism starts with a simple blood test that measures thyroid stimulating hormone, or TSH. A TSH level between roughly 0.4 and 4.0 mIU/L is considered normal for most labs, though the exact range can vary slightly depending on the testing method used. If your TSH comes back high, it means your thyroid isn’t producing enough hormone on its own, and your brain is sending stronger signals to compensate. That single number is the most reliable first step, but it’s rarely the whole picture.
What the TSH Test Actually Tells You
TSH works like a thermostat. When your thyroid output drops, your pituitary gland releases more TSH to try to push the thyroid harder. So a high TSH level is the earliest sign that your thyroid is underperforming, often catching problems before you even feel symptoms. Most labs flag anything above 4.0 to 5.0 mIU/L as elevated, though some professional organizations have recommended a tighter upper limit of 2.5 mIU/L.
A mildly elevated TSH (between roughly 4.5 and 10 mIU/L) with normal thyroid hormone levels is called subclinical hypothyroidism. You may feel fine, or you may have vague symptoms that are easy to attribute to stress or aging. Both the American Thyroid Association and the American Association of Clinical Endocrinology recommend starting medication when TSH rises above 10 mIU/L, or at lower levels if you have symptoms, positive antibody tests, or cardiovascular risk factors.
Follow-Up Blood Tests After TSH
If your TSH is elevated, the next step is usually a free T4 test. T4 is the main hormone your thyroid produces, and measuring the “free” (unbound) portion tells your doctor how much is actually available for your body to use. High TSH combined with low free T4 confirms hypothyroidism. If your free T4 is still in the normal range despite elevated TSH, that’s the subclinical category described above.
T3, the other thyroid hormone, is less useful for diagnosing hypothyroidism. Your body converts T4 into T3, and T3 levels tend to drop later in the disease process. Other thyroid tests can catch hypothyroidism earlier, so T3 testing is typically reserved for more complex situations or when hyperthyroidism is suspected instead.
Testing for the Underlying Cause
Once hypothyroidism is confirmed, your doctor will likely want to know why it’s happening. Hashimoto’s thyroiditis, an autoimmune condition where the immune system gradually attacks the thyroid, is the most common cause. A thyroid antibody blood test checks for this. High levels of thyroid peroxidase antibodies (TPO antibodies) are a strong indicator of Hashimoto’s. Some people also test positive for thyroglobulin antibodies. Most people with Hashimoto’s have elevated levels of one or both types.
Knowing the cause matters because Hashimoto’s tends to be progressive. If your TSH is only mildly elevated now but you have high antibody levels, your doctor may monitor you more closely or start treatment sooner, anticipating that thyroid function will continue to decline over time.
The Physical Exam
Blood work does the heavy lifting, but a physical exam still plays a role. Your doctor will visually inspect and then feel your thyroid gland, which sits at the front of your neck just below the Adam’s apple. They’ll ask you to swallow while pressing gently against the gland to check its size, texture, tenderness, and whether any lumps (nodules) are present. An enlarged thyroid, called a goiter, can sometimes be visible to the naked eye.
Beyond the neck, doctors look for physical signs consistent with low thyroid function: dry or coarse skin, puffiness around the eyes, thinning hair, and slow reflexes. A classic test involves tapping the Achilles tendon and watching how quickly the foot relaxes back. In hypothyroidism, that relaxation phase is noticeably sluggish. None of these signs alone confirm a diagnosis, but combined with lab results, they help build the full picture.
When Ultrasound Is Needed
A thyroid ultrasound isn’t part of routine hypothyroidism testing. It’s typically ordered only if your doctor feels a lump or enlarged gland during the physical exam. That said, research suggests ultrasound may be more useful than it gets credit for. In one study, ultrasound identified the autoimmune pattern of Hashimoto’s as the cause in 19% of patients whose antibody blood tests had come back negative. It also detected clinically significant nodules (larger than 9mm) in a meaningful number of patients. Some endocrinologists now recommend an ultrasound as part of the initial workup for newly diagnosed hypothyroidism, even without a palpable goiter.
Symptoms That Prompt Testing
Clinical scoring systems developed for research have helped quantify which symptoms most reliably point to hypothyroidism. The patterns that emerge from these tools are useful for knowing when to ask for a blood test. The symptoms with the strongest association include persistent fatigue and lethargy, difficulty thinking clearly or finding the right word, memory problems, constipation, dry skin, weight gain, and feeling cold when others are comfortable. Physical signs like slow movements, a hoarse voice, puffy face, and delayed ankle reflexes carry even more weight.
One validated scoring system, the Zulewski clinical score, assigns points for 12 symptoms and signs. A score above 5 suggests hypothyroidism, while 0 to 2 points suggests normal thyroid function. Notably, dry skin and constipation become less specific in women over 55 because those symptoms are common in that age group regardless of thyroid status. This is why symptoms alone can’t diagnose hypothyroidism. They’re the reason to get tested, not a substitute for it.
At-Home Thyroid Test Kits
Several companies now sell mail-order thyroid test kits that use a finger-prick blood sample. These can measure TSH, free T4, and sometimes antibodies. The appeal is obvious: no doctor’s visit, no lab appointment. But accuracy varies. Rapid TSH tests perform well as a screening tool for ruling out significant hypothyroidism. One validation study of a rapid TSH test found 100% sensitivity at a TSH cutoff of 10 mIU/L, meaning it caught every case above that threshold. However, specificity was only about 77%, meaning roughly one in four positive results was a false alarm.
In practical terms, if an at-home test says your TSH is normal, that result is quite reliable. If it flags your TSH as elevated, you’ll need a standard lab draw to confirm it before making any treatment decisions. These kits work best as a first screening step, not as a final answer.
What Can Throw Off Your Results
Biotin supplements are the biggest hidden interference with thyroid blood tests. If you take biotin for hair, skin, or nail health, the doses in many supplements (often 5 to 10 mg, sometimes up to 30 mg) far exceed the recommended daily intake of 30 to 70 micrograms. At those levels, biotin can dramatically skew immunoassay results. In one documented case, a patient’s TSH read as 0.02 while on high-dose biotin, then normalized to 2.0 after stopping for just one week. The interference can make thyroid levels look abnormally high or low depending on the test.
If you take any biotin-containing supplement, stop it at least one week before thyroid blood work. Even a single 30 mg dose can affect results for at least 24 hours, with the distortion peaking around two hours after ingestion. This includes many multivitamins and “hair, skin, and nails” formulas that contain biotin without prominently advertising it. Check your supplement labels before your blood draw.
Other factors that can influence TSH readings include the time of day (TSH is highest in the early morning and lowest in the afternoon), recent illness, pregnancy, and certain medications. For the most consistent results, schedule your blood draw in the morning.