The TSH test measures thyroid-stimulating hormone produced by the pituitary gland. The tsh test is the most sensitive single screening test for thyroid disorders and is typically the first test ordered when thyroid disease is suspected. Despite its name, TSH levels move opposite to thyroid hormone levels in primary thyroid disease — high TSH indicates underactive thyroid, low TSH indicates overactive thyroid. This guide covers what TSH measures, normal ranges, and how to interpret abnormal results.
How TSH works
TSH is produced by the pituitary gland in response to circulating thyroid hormone levels. When thyroid hormone is low, the pituitary increases TSH to stimulate the thyroid; when thyroid hormone is high, the pituitary decreases TSH. This negative feedback loop makes TSH highly sensitive to small changes in thyroid hormone — TSH often shows abnormalities before free T4 levels move outside normal range.
This is why TSH is the preferred initial screening test rather than free T4 or T3.
Reference ranges
- Normal: 0.4-4.0 mIU/L (some labs use 0.45-4.5)
- Subclinical hypothyroidism: TSH elevated, free T4 normal
- Overt hypothyroidism: TSH elevated, free T4 low
- Subclinical hyperthyroidism: TSH low, free T4 normal
- Overt hyperthyroidism: TSH low, free T4 elevated
Elevated TSH (high TSH)
Most commonly indicates hypothyroidism. Causes:
- Autoimmune (Hashimoto’s) thyroiditis — most common in adults
- Iodine deficiency (rare in iodized salt regions)
- Post-thyroidectomy or post-radioactive iodine treatment
- Medications (lithium, amiodarone, certain immunotherapies)
- Recovery phase of subacute thyroiditis
Mildly elevated TSH (4-10 mIU/L) with normal free T4 is “subclinical hypothyroidism.” Treatment depends on TSH level, symptoms, age, and other factors. Many patients with mild subclinical hypothyroidism don’t need treatment immediately.
Low TSH (suppressed TSH)
Most commonly indicates hyperthyroidism. Causes:
- Graves’ disease — most common in younger adults
- Toxic nodular goiter or toxic adenoma — more common in older adults
- Subacute thyroiditis (transient)
- Medication effects (excess thyroid hormone replacement)
- Iodine excess
- Pregnancy first trimester (hCG-mediated suppression, transient)
Subclinical hyperthyroidism (low TSH with normal free T4) often warrants observation and rechecking. Persistent suppression may need workup and treatment.
TSH in monitoring thyroid replacement
For patients on levothyroxine for hypothyroidism, TSH is the standard monitoring test. Target ranges:
- Most patients: 0.5-2.5 mIU/L (some endocrinologists target wider range)
- Pregnancy: 0.1-2.5 first trimester, 0.2-3.0 second/third trimester
- Older adults: slightly higher targets may be appropriate
- Patients with thyroid cancer: variable suppression depending on risk
Target individualization is important. Some patients feel better at lower TSH within normal range; others tolerate higher TSH. The American Thyroid Association publishes detailed guidance.
Frequently Asked Questions
Do I need to fast for TSH?
Generally no. TSH doesn’t significantly fluctuate with eating, though some studies suggest morning fasting samples may produce slightly higher values.
What’s a “normal” TSH level?
0.4-4.0 mIU/L is the standard range. Optimal range debated; some endocrinologists target 0.5-2.5 for treated patients.
Why is TSH elevated when I feel fine?
Subclinical hypothyroidism is common. Treatment decisions depend on TSH level, symptoms, age, and other factors.
Can stress affect TSH?
Severe acute illness can transiently affect TSH. Routine stress doesn’t significantly change TSH.
How quickly does TSH change after starting thyroid medication?
4-6 weeks for full equilibration. Repeat TSH testing at 6-8 weeks after dose changes is standard.
The bottom line on the TSH test
TSH is the most sensitive single screening test for thyroid disorders. Elevated TSH suggests hypothyroidism; suppressed TSH suggests hyperthyroidism. For follow-up of abnormal TSH, free T4 testing helps distinguish overt from subclinical disease. Treatment of subclinical disease depends on multiple factors. Discuss results with your healthcare provider in clinical context. For more detail, see our thyroid panel guide.