Thyroid Stimulating Hormone
TSH· also: Thyroid Stimulating Hormone
Clinical Overview
TSH is secreted by the pituitary gland to control thyroid hormone production. It is the single most sensitive test for thyroid dysfunction. High TSH indicates the pituitary is working harder to stimulate an underactive thyroid (hypothyroidism); low TSH indicates the pituitary is suppressed because the thyroid is overactive (hyperthyroidism) or the patient is receiving excess thyroid hormone.
Why This Test Matters
TSH is the best first-line test for thyroid disease because it is exquisitely sensitive — small changes in thyroid hormone levels cause much larger TSH changes (logarithmic relationship). TSH is used to screen for thyroid dysfunction, diagnose hypothyroidism and hyperthyroidism, monitor thyroid replacement therapy (levothyroxine), and suppress thyroid cancer recurrence. A single TSH result in the normal range effectively rules out primary thyroid disease.
Reference RangesWHO/IFCC standards
| Age Group | Sex | Reference Range | Unit | Notes |
|---|---|---|---|---|
| Adults (18–64) | All | 0.4 – 4 | mIU/L | — |
| Elderly (65+) | All | 0.5 – 6 | mIU/L | Higher range acceptable in elderly |
| All ages | Female | 0.1 – 2.5 | mIU/L | Pregnancy first trimester |
Also reported in: µIU/mL.
Critical (Panic) Values
Critical Low: < 0.01 mIU/L. Critical High: > 100 mIU/L. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High TSH Causes
- Primary hypothyroidism (Hashimoto's thyroiditis, post-radioiodine, post-surgery)
- Subclinical hypothyroidism (TSH high, FT4 normal)
- Inadequate levothyroxine dose in treated hypothyroidism
- TSH-secreting pituitary adenoma (very rare)
- Recovery phase after non-thyroidal illness
- Iodine deficiency
Low TSH Causes
- Graves' disease (most common cause of hyperthyroidism)
- Toxic multinodular goiter
- Solitary toxic thyroid nodule
- Excess levothyroxine therapy
- Subclinical hyperthyroidism (TSH low, FT4 normal)
- Secondary hypothyroidism (pituitary failure — low TSH with low FT4)
- Non-thyroidal illness (sick euthyroid syndrome)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required. Ideally drawn in the morning — TSH has a slight diurnal variation (higher at night). Do not take biotin supplements for at least 24 hours before the test, as high-dose biotin interferes with immunoassays.
Factors That Can Affect Results
- Biotin supplements (high doses >10 mg/day interfere with many TSH assays, causing falsely low TSH)
- Non-thyroidal illness (sick euthyroid syndrome — TSH can be low or high during severe illness without true thyroid disease)
- Dopamine and corticosteroids (suppress TSH secretion)
- Pregnancy (first trimester lowers TSH due to hCG cross-reactivity with TSH receptors)
- Heterophilic antibodies (rare — can falsely elevate or lower TSH)
Related Topics
Frequently Asked Questions
What TSH level confirms hypothyroidism?
A TSH above 10 mIU/L on two separate occasions confirms overt hypothyroidism and warrants treatment. TSH of 4–10 mIU/L with normal FT4 is called subclinical hypothyroidism — treatment is recommended when TSH is above 10, or above 4 mIU/L in pregnant women, women trying to conceive, patients with symptoms, or those with anti-TPO antibodies (higher risk of progression).
Can TSH be normal in thyroid disease?
TSH can be normal in central hypothyroidism (pituitary or hypothalamic disease where the pituitary fails to produce adequate TSH despite low thyroid hormone), in early or subclinical thyroid disease, or during certain medications. If central hypothyroidism is suspected, FT4 must also be measured alongside TSH.
How often should TSH be monitored in treated hypothyroidism?
After starting or changing a levothyroxine dose, TSH should be rechecked 6–8 weeks later (the time needed for TSH to reach a new steady state). Once the dose is stable and TSH is in the target range, annual monitoring is appropriate for most patients.