TSH (Thyroid-Stimulating Hormone)

A blood test that measures how well the thyroid gland is working by checking the level of thyroid-stimulating hormone produced by the pituitary gland.

Last medically reviewed: April 10, 2026 | Reviewed by LabResult.MD Editorial Team

What is TSH?

TSH (thyroid-stimulating hormone) is a hormone produced by the pituitary gland, a small gland at the base of the brain. TSH tells the thyroid gland to make and release thyroid hormones (T3 and T4) into the bloodstream.

Thyroid hormones control the body's metabolism, which is how the body uses energy. They affect heart rate, body temperature, digestion, brain development, and bone health.

The TSH test works on an inverse relationship: when thyroid hormone levels are low, the pituitary gland releases more TSH to stimulate the thyroid. When thyroid hormone levels are high, the pituitary gland produces less TSH. This is why a high TSH often signals an underactive thyroid, and a low TSH often signals an overactive thyroid.

Why Is It Tested?

A healthcare provider may order a TSH test to:

  • Screen for thyroid disorders (hypothyroidism or hyperthyroidism)
  • Evaluate symptoms such as unexplained weight changes, fatigue, anxiety, or sensitivity to cold or heat
  • Monitor treatment in patients taking thyroid medication
  • Screen newborns for congenital hypothyroidism
  • Evaluate fertility issues, as thyroid problems can affect menstrual cycles and pregnancy
  • Follow up on abnormal results from other thyroid tests

Normal Reference Ranges

Reference ranges may vary by laboratory, testing method, age, and other factors. Always use the ranges provided on your lab report.

Population Normal Range (mIU/L)
Adults (general) 0.4 – 4.0 mIU/L
Pregnancy (1st trimester) 0.1 – 2.5 mIU/L
Pregnancy (2nd trimester) 0.2 – 3.0 mIU/L
Pregnancy (3rd trimester) 0.3 – 3.0 mIU/L
Older adults (over 70) 0.4 – 5.0 mIU/L (may be slightly higher)

What Does a High TSH Level Mean?

A high TSH level usually means the thyroid is not making enough thyroid hormones (hypothyroidism). The pituitary gland responds by producing more TSH to try to stimulate the thyroid. High TSH could indicate:

  • Hypothyroidism (underactive thyroid) — the most common cause
  • Hashimoto's thyroiditis — an autoimmune condition where the immune system attacks the thyroid
  • Thyroid damage from radiation treatment or surgery
  • Certain medications (such as lithium or amiodarone)
  • Iodine deficiency (rare in developed countries)
  • Pituitary gland disorders (rare) that cause excess TSH production

Common symptoms of hypothyroidism include fatigue, weight gain, cold sensitivity, dry skin, constipation, and depression.

What Does a Low TSH Level Mean?

A low TSH level usually means the thyroid is making too much thyroid hormone (hyperthyroidism). The pituitary gland responds by reducing TSH production. Low TSH could indicate:

  • Hyperthyroidism (overactive thyroid) — the most common cause
  • Graves' disease — an autoimmune condition that overstimulates the thyroid
  • Thyroid nodules producing excess thyroid hormone
  • Thyroiditis (inflammation of the thyroid, which can cause temporary hormone release)
  • Taking too much thyroid medication
  • Early pregnancy (TSH normally decreases in the first trimester)

Common symptoms of hyperthyroidism include rapid heartbeat, unexplained weight loss, anxiety, tremors, increased sweating, and difficulty sleeping.

Common Next Steps

Depending on the result, a healthcare provider may recommend:

  • Additional thyroid tests: free T4 (thyroxine) and free T3 (triiodothyronine) to get a fuller picture
  • Thyroid antibody tests (TPO antibodies, TSI) to check for autoimmune thyroid disease
  • Thyroid ultrasound if nodules or structural changes are suspected
  • Medication such as levothyroxine for hypothyroidism, or anti-thyroid drugs for hyperthyroidism
  • Regular monitoring with repeat TSH tests every 6–12 weeks after starting or adjusting medication
  • Referral to an endocrinologist for complex thyroid conditions

Frequently Asked Questions

What is a normal TSH level?

The typical reference range for TSH is 0.4 to 4.0 mIU/L for most adults, according to the American Thyroid Association. However, some endocrinologists consider 0.5–2.5 mIU/L to be an optimal range. TSH levels can be affected by age, pregnancy, and time of day.

Can TSH levels change throughout the day?

Yes, TSH levels follow a circadian rhythm and are typically highest in the early morning hours and lowest in the late afternoon. This is why many healthcare providers recommend testing TSH in the morning for the most consistent results, especially when monitoring thyroid medication.

What is the difference between TSH, T3, and T4?

TSH is a pituitary hormone that tells the thyroid how much hormone to produce. T4 (thyroxine) and T3 (triiodothyronine) are the actual thyroid hormones that regulate metabolism. TSH is usually tested first because it is the most sensitive indicator of thyroid dysfunction — if TSH is abnormal, T3 and T4 tests help determine the cause.

Related Tests and Biomarkers

Thyroid Panel Cholesterol Glucose

Sources

  • American Thyroid Association. Thyroid Function Tests (2024)
  • MedlinePlus. TSH Test. U.S. National Library of Medicine
  • Endocrine Society. Clinical Guidelines on Hypothyroidism

Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice. Reference ranges vary by laboratory. Always discuss your results with a qualified healthcare provider.