About This Test

The TSH test measures the amount of thyroid stimulating hormone in your blood. TSH is produced by the pituitary gland and regulates thyroid hormone production. It is the primary screening test for thyroid disorders.

Why Get Tested?

  • Screen for thyroid disorders
  • Diagnose hypothyroidism or hyperthyroidism
  • Monitor thyroid treatment effectiveness
  • Investigate symptoms like fatigue, weight changes, or hair loss
  • Check thyroid function during pregnancy

Reference Ranges

  • Normal: 0.4 - 4.0 mIU/L (ranges vary by lab)
  • Hypothyroidism: Above 4.0-5.0 mIU/L
  • Hyperthyroidism: Below 0.4 mIU/L

When Is Testing Recommended?

  • Symptoms of thyroid dysfunction
  • Family history of thyroid disease
  • Autoimmune disorders (type 1 diabetes, celiac disease)
  • Taking medications affecting thyroid (lithium, amiodarone)
  • Pregnancy planning or during pregnancy
  • Goiter or thyroid nodules
  • Unexplained high cholesterol

Test Preparation

Test Preparation

Fasting: Not required.

Before Your Test:

  • Take medications as usual unless instructed otherwise
  • Inform doctor of all medications, especially:
- Biotin supplements (can cause false results) - Thyroid medications - Steroids - Dopamine agonists
  • Time of day matters: TSH is highest in early morning

What to Expect:

  • Simple blood draw
  • Takes 5 minutes
  • Results in 1-3 days
  • No special aftercare needed

Understanding Your Results

Understanding TSH Results

High TSH (Above 4.0-5.0 mIU/L)

Indicates hypothyroidism (underactive thyroid)
  • Pituitary producing more TSH to stimulate thyroid
  • Common causes: Hashimoto's thyroiditis, thyroid surgery, radiation

Low TSH (Below 0.4 mIU/L)

Indicates hyperthyroidism (overactive thyroid)
  • Pituitary producing less TSH because thyroid hormones are high
  • Common causes: Graves' disease, thyroid nodules, thyroiditis

Normal TSH

  • Thyroid function appears normal
  • May need Free T4 testing if symptoms persist
  • Subclinical thyroid disease possible with normal TSH

Additional Testing

If TSH is abnormal, your doctor may order:
  • Free T4: Measures available thyroid hormone
  • Free T3: Active thyroid hormone (less commonly measured)
  • Thyroid antibodies: TPO antibodies, Tg antibodies
  • Thyroid ultrasound: If nodules suspected

Special Situations

Pregnancy

  • TSH targets are lower during pregnancy
  • First trimester: TSH should be below 2.5 mIU/L
  • Second/third trimester: Below 3.0 mIU/L

Elderly

  • Slightly higher TSH may be normal in adults over 80
  • Individualized treatment targets

Subclinical Thyroid Disease

  • TSH abnormal but T4 normal
  • May or may not require treatment
  • Monitoring every 6-12 months

Related Tests

  • Free T4 and Free T3
  • Thyroid peroxidase (TPO) antibodies
  • Thyroglobulin antibodies
  • Thyroid stimulating immunoglobulin (TSI)
  • Reverse T3

Sources

Last Reviewed: April 2026