What this test measures
TSH — Thyroid Stimulating Hormone — is produced by your pituitary gland, a pea-sized gland at the base of your brain. Its job is to tell your thyroid gland (the butterfly-shaped gland in your neck) how much hormone to make.
Here's the key insight: TSH works inversely to thyroid function. When your thyroid is sluggish, your pituitary shouts louder (high TSH). When your thyroid is overactive, the pituitary barely whispers (low TSH). So TSH is really measuring how hard your brain is working to control your thyroid — not the thyroid output itself.
TSH is the most sensitive screening test for thyroid disorders and is routinely checked during annual physicals, fatigue workups, and monitoring of thyroid conditions.
Normal reference range
0.4 – 4.0 mIU/L (adults)
Some labs use 0.5–4.5 mIU/L. Ranges shift during pregnancy (lower is normal) and with age (slightly higher is acceptable in elderly adults). Always compare your result to your lab's printed range, not a number from the internet.
Symptoms associated with abnormal TSH
Lab results alone don't diagnose thyroid disease — symptoms matter too. Here's what each pattern typically looks like:
↑ High TSH (Hypothyroid)
- Fatigue and sluggishness
- Weight gain despite no diet change
- Feeling cold all the time
- Constipation
- Dry skin and hair loss
- Brain fog / poor memory
- Depression or low mood
- Slow heart rate
- Muscle weakness or aches
- Heavy or irregular periods
↓ Low TSH (Hyperthyroid)
- Unintentional weight loss
- Rapid or irregular heartbeat
- Feeling hot / excessive sweating
- Anxiety or nervousness
- Trembling hands
- Trouble sleeping
- Increased appetite
- Frequent bowel movements
- Bulging eyes (Graves' disease)
- Muscle weakness
Frequently asked questions
My TSH is slightly out of range but I feel fine. Should I worry?
Mild TSH elevation (4.0–10 mIU/L) without symptoms is called "subclinical hypothyroidism." Many doctors monitor rather than treat immediately. Your doctor will consider your symptoms, trends over time, and other thyroid tests (Free T4, TPO antibodies) before recommending treatment.
Do I need to fast before a TSH test?
No fasting is required for TSH. However, if you take thyroid medication (levothyroxine), some doctors ask you to delay your morning dose until after the blood draw, so the result reflects your baseline level rather than the spike after dosing.
Can stress affect my TSH?
Acute physical stress (illness, surgery) can temporarily suppress TSH. Chronic psychological stress has a less direct effect on TSH itself but can worsen symptoms of underlying thyroid conditions. If you were sick during your test, your doctor may want to retest when you've recovered.
What's the difference between TSH and Free T4?
TSH tells you how urgently the pituitary is signaling the thyroid. Free T4 measures the actual hormone the thyroid is producing. Doctors often check both together — TSH is more sensitive for detecting early problems, while Free T4 confirms what's actually happening at the thyroid level.
Is TSH the same as T3 or T4?
No — TSH, T3, and T4 are three different hormones. TSH comes from the pituitary (the "commander"). T4 (thyroxine) is the main hormone the thyroid produces. T3 (triiodothyronine) is the active form, converted from T4 in tissues throughout the body. TSH controls T4 output; T3 is what actually affects your cells.
How often should TSH be checked?
For healthy adults with no thyroid history, screening every 5 years starting around age 35 is common. People with known thyroid disease or on thyroid medication typically check every 6–12 months, or after any dose change (usually 6–8 weeks after adjustment).
Related tests your doctor may order