Hypothyroidism is diagnosed primarily through a simple blood test that measures thyroid-stimulating hormone (TSH). If your TSH is above the normal range (roughly 0.4 to 4.0 mIU/L), your thyroid is underperforming. From there, additional blood work, and sometimes antibody testing, helps your doctor determine how severe the problem is and what’s causing it.
The TSH Test: The Starting Point
TSH is the single most important number in a hypothyroidism diagnosis. Your pituitary gland produces TSH to tell your thyroid to make more hormone. When the thyroid isn’t keeping up, the pituitary compensates by cranking out more TSH, like turning up a thermostat when the furnace is struggling. A high TSH reading signals that your thyroid is underactive.
If TSH comes back elevated, the next step is measuring free T4, the main hormone your thyroid produces. The combination of these two values tells the full story:
- High TSH with low free T4: This is overt (clinical) hypothyroidism. Your thyroid is clearly failing to produce enough hormone, and you likely have noticeable symptoms.
- High TSH with normal free T4: This is subclinical hypothyroidism. Your thyroid is struggling but still producing adequate hormone for now. You may or may not feel symptoms.
The distinction matters because treatment decisions differ. Both the American Thyroid Association and the American Association of Clinical Endocrinology recommend starting medication when TSH rises above 10 mIU/L. Below that threshold, treatment depends on whether you have symptoms, test positive for thyroid antibodies, have cardiovascular risk factors, or are a woman of reproductive age.
What Antibody Tests Reveal
Once hypothyroidism is confirmed through TSH and free T4, your doctor may order a thyroid peroxidase antibody test (TPOAb). This test identifies whether your immune system is attacking your thyroid, which is the hallmark of Hashimoto’s thyroiditis, the most common cause of hypothyroidism in developed countries.
High levels of TPO antibodies confirm Hashimoto’s as the underlying cause. Your doctor may also check thyroglobulin antibodies (TgAb). Most people with Hashimoto’s have elevated levels of one or both types. The higher the antibody levels, the more likely an autoimmune process is driving the problem.
Antibody testing also has predictive value. If you have subclinical hypothyroidism with positive TPO antibodies, you’re more likely to progress to full-blown hypothyroidism over time. This is one reason positive antibodies can tip the decision toward starting treatment even when TSH is below 10.
Physical Exam Signs
Blood tests carry the diagnosis, but a physical exam can provide supporting clues. Doctors look for dry skin, a puffy face, thinning hair, a slow heart rate, and an enlarged thyroid gland (goiter). One classic finding is called Woltman’s sign: when the doctor taps the Achilles tendon at your ankle, the reflex is noticeably slow to relax. In a healthy person the reflex snaps back quickly, but in hypothyroidism the relaxation phase is sluggish. It’s a low-tech finding that has been documented in the New England Journal of Medicine and can prompt a doctor to order thyroid labs they might not have considered.
When TSH Alone Can Be Misleading
In rare cases, hypothyroidism originates not in the thyroid itself but in the pituitary gland or hypothalamus. This is called central hypothyroidism. The key difference: free T4 is low, but TSH may be low, normal, or only slightly elevated rather than high. A standard screening that looks only at TSH would miss this entirely, because the pituitary isn’t sending the right signal. Central hypothyroidism is uncommon, but it’s an important reason that doctors measure free T4 alongside TSH when symptoms are present but initial numbers don’t add up.
Another scenario that mimics hypothyroidism on lab work is non-thyroidal illness syndrome, sometimes called “sick euthyroid syndrome.” Seriously ill or hospitalized patients often show low thyroid hormone levels even though their thyroid gland is healthy. In these cases, TSH is typically low or normal rather than elevated, and the abnormalities resolve once the underlying illness improves. Finding an elevated TSH in a sick patient points toward genuine hypothyroidism rather than this temporary lab pattern.
Diagnosis During Pregnancy
Pregnancy changes what counts as a normal TSH. The American Thyroid Association recommends tighter upper limits: 2.5 mIU/L in the first trimester and 3.0 mIU/L in the second and third trimesters. These thresholds are lower than the standard reference range because untreated hypothyroidism during pregnancy carries risks for both mother and baby, including preterm birth and impaired fetal brain development. If you’re pregnant or planning to become pregnant and have a history of thyroid problems, early and trimester-specific testing is important.
Getting Accurate Results
The timing of your blood draw can shift TSH values enough to affect a borderline diagnosis. TSH follows a natural daily rhythm, peaking between midnight and early morning, then dropping to its lowest levels around midday. Research published in the Indian Journal of Endocrinology and Metabolism found that TSH values decline significantly by 10 a.m. regardless of whether you’ve eaten. Eating itself also lowers TSH compared to fasting levels. For the most consistent and representative result, an early morning fasting blood draw is ideal, particularly if you’re being evaluated for subclinical hypothyroidism where the numbers are close to the cutoff.
Biotin supplements are another common source of inaccurate thyroid results. Biotin is found in many hair, skin, and nail supplements, sometimes in high doses. Doses of 20 mg or more can interfere with the lab assays used to measure TSH and free T4, producing falsely abnormal results. If you take biotin, stop it at least 48 to 72 hours before your blood test. In some cases, it can take up to 72 hours for free T4 values to return to their true baseline after stopping.
When Imaging Is and Isn’t Needed
A thyroid ultrasound is not part of a routine hypothyroidism workup. The American Academy of Family Physicians specifically advises against ordering an ultrasound in patients with abnormal thyroid function tests unless there’s a palpable lump, a large goiter, or an irregular-feeling gland on physical exam. The ultrasound is a tool for evaluating thyroid nodules, not for confirming an underactive thyroid. If your doctor diagnoses hypothyroidism based on blood work alone and your neck exam is normal, that’s standard practice.