What Is TSH in Blood Work and How It Checks Thyroid Function
Medically reviewed by Medical Advisory Board Last reviewed 2026-10-04
A TSH blood test measures thyrotropin to evaluate thyroid hormone levels
A TSH blood test measures thyroid stimulating hormone to show how well your thyroid works and whether you have too much or too little thyroid hormone. The test cannot explain why levels are high or low, so follow-up blood work may be needed. Review the specific reference range printed on your lab report to evaluate your result.
TSH in blood work stands for thyroid stimulating hormone, a substance measured to show how well your thyroid works and whether you have too much or too little thyroid hormone in your blood. At The Metabolic Journal, we summarize what MedlinePlus says about how this lab test evaluates thyroid health. When your doctor checks what is TSH in blood work, the test measures this hormone, which is also known as thyrotropin.
The test can tell if you have hyperthyroidism (too much thyroid hormone) or hypothyroidism (too little thyroid hormone). It cannot explain why your levels may be too high or too low, so additional tests may be needed. Because reference ranges vary across testing facilities, use the reference range printed on your lab report to understand your numbers. For more details on this marker, see our guide on what is TSH.
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A TSH blood test measures thyroid stimulating hormone to show how well your thyroid gland is working. Another name for this hormone is thyrotropin. The test can determine if you have hyperthyroidism, meaning too much thyroid hormone in your blood, or hypothyroidism, meaning too little thyroid hormone.
However, the test cannot explain why your TSH levels are too high or too low. Because it does not reveal the underlying cause of an abnormal result, further testing may be needed. To learn how multiple lab markers work together, explore our review of the thyroid panel.
Symptoms of High and Low Thyroid Hormone Levels
Symptoms of thyroid hormone imbalances depend on whether your blood has too much or too little thyroid hormone. MedlinePlus lists specific symptoms for each condition. To evaluate your physical changes, you can read our overview of thyroid symptoms.
| Condition | Reported Symptoms |
|---|---|
| Hyperthyroidism (too much thyroid hormone) | Weight loss, rapid heartbeat, nervousness, insomnia, fatigue, tremors, sweating, frequent bowel movements, goiter |
| Hypothyroidism (too little thyroid hormone) | Fatigue, weight gain, sensitivity to cold, joint or muscle pain, dry skin, hair loss, irregular periods, fertility problems, constipation |
Fatigue appears as a symptom under both conditions. Because symptoms overlap and a TSH test cannot identify why levels are abnormal, healthcare providers evaluate symptoms alongside lab testing.
Variation in What Is TSH in Blood Work with a Healthy Gland
Your TSH level may be high or low even when your thyroid gland is healthy. An atypical result does not automatically mean your thyroid gland has a disease.
TSH levels are often higher in people over 80 years old. In contrast, TSH levels are often lower during early pregnancy. Because results can vary with age and pregnancy, check the range printed on your report. Reference ranges vary by laboratory, so check the reference range printed directly on your lab report. For more context on standard lab ranges, see our guide on what is normal TSH.
Follow-Up Testing for What Is TSH in Blood Work
Further testing may be needed because a TSH test cannot explain why your hormone levels are too high or too low. When an initial test shows atypical levels, healthcare providers may order follow-up tests to investigate the cause.
These follow-up evaluations may include T4, T3, and thyroid antibody tests. Clinicians use these additional tests to help diagnose specific conditions, such as Graves' disease or Hashimoto's disease. To learn more about antibody testing, see our guide on thyroid antibodies or read our overview of hypothyroidism.
Preparing for a TSH Blood Draw
Preparing for a TSH blood test requires reviewing your current medications with your healthcare provider. You may need to stop taking certain medicines before the test, so tell your provider about everything you take.
If your healthcare provider orders additional blood work at the same time, fasting may be required. Talk with your provider about your medicines and instructions before you complete what is TSH in blood work testing.
How to test thyroid at home
You don't need a lab appointment to check your own thyroid — a few at-home kits mail you a collection kit and return physician-reviewed results online within days:
Prefer to order direct? The Everlywell Thyroid Test uses a CLIA-certified lab, physician-reviewed results, and prepaid return shipping.
What Makes a Good Selenium Supplement
Selenium is studied in thyroid health, typically ~200 mcg/day as selenomethionine. It has a narrow safe range — more is not better, and long-term high doses can be harmful — so don't stack multiple products, and never add iodine without testing. Pick a modest, third-party-tested dose.
Frequently Asked Questions
What is TSH in blood work?
TSH stands for thyroid stimulating hormone, also known as thyrotropin. A TSH test is a blood test that measures this hormone to find out how well your thyroid is working. It can tell if you have hyperthyroidism (too much thyroid hormone) or hypothyroidism (too little thyroid hormone) in your blood, but it cannot explain why your levels are high or low.
What does a high TSH mean?
A high TSH means your thyroid stimulating hormone level is elevated on your blood test, but the test cannot explain why your levels are high. High levels can occur even when your thyroid gland is healthy, such as in people over 80 years old. Your provider may order additional testing, including T4, T3, and thyroid antibody tests, to find the underlying cause.
What does a low TSH mean?
A low TSH means your thyroid stimulating hormone level is reduced on your lab report, though the test cannot explain the reason for the low level. TSH levels are also often lower during early pregnancy even when the thyroid gland is healthy. Your doctor may order follow-up tests with T4, T3, and thyroid antibodies to investigate further.
Do I need to fast for a TSH test?
Fasting may be required if your healthcare provider orders additional blood work alongside your TSH test. You may also need to stop taking certain medicines before the blood draw. Tell your healthcare provider about everything you take so they can give you proper instructions for your test.
Can TSH be abnormal with a healthy thyroid?
Yes, your TSH level may be high or low even when your thyroid gland is healthy. For example, TSH levels are often higher in people over 80 years of age and lower during early pregnancy. Because normal ranges vary by laboratory, always check the reference range printed on your lab report.
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