TSH, or thyroid-stimulating hormone, is a blood test that measures how well your thyroid is functioning. The normal range for adults is 0.27 to 4.2 mIU/L, though slight variations exist between labs. It’s the single most common screening test for thyroid problems and often the first one your doctor orders if symptoms suggest your thyroid might be off.
What TSH Actually Measures
TSH doesn’t come from the thyroid itself. It’s produced by the pituitary gland, a pea-sized structure at the base of your brain. The pituitary acts like a thermostat: when thyroid hormone levels in your blood drop too low, it releases more TSH to signal the thyroid to produce more. When thyroid hormone levels rise high enough, the pituitary backs off and releases less TSH.
This feedback loop is why TSH works so well as a screening tool. It reflects what your thyroid is doing indirectly, by showing how hard your pituitary gland is working to keep things balanced. A TSH level that’s too high means your pituitary is shouting at a sluggish thyroid. A TSH level that’s too low means your thyroid is already flooding the bloodstream with hormone, so the pituitary has gone quiet.
What High TSH Means
A TSH above the normal range points toward hypothyroidism, an underactive thyroid. The most common cause is Hashimoto’s disease, an autoimmune condition where the immune system gradually damages the thyroid until it can’t produce enough hormone. Other causes include previous thyroid surgery, radiation treatment, thyroid inflammation, and certain medications (some heart, bipolar, and cancer drugs can interfere with thyroid hormone production).
Symptoms of an underactive thyroid tend to creep in slowly:
- Fatigue and feeling sluggish
- Unexplained weight gain
- Feeling cold when others are comfortable
- Dry skin and thinning hair
- Joint and muscle pain
- Heavy or irregular periods
- Depression
- Slowed heart rate
Because these symptoms overlap with many other conditions, a high TSH reading is often what confirms the thyroid is the culprit.
Subclinical Hypothyroidism
Sometimes TSH is mildly elevated (roughly 5 to 10 mIU/L) but the thyroid hormone levels themselves are still in the normal range. This is called subclinical hypothyroidism, and it’s a gray area. You may have no symptoms at all, or you may notice vague fatigue. Doctors categorize it as grade 1 when TSH falls between 4.5 and 9.9 mIU/L and grade 2 when TSH reaches 10 mIU/L or higher. Whether to treat grade 1 is a judgment call that depends on your symptoms, age, and other risk factors. Grade 2 is more likely to progress to full hypothyroidism and is typically treated.
What Low TSH Means
A TSH below the normal range suggests hyperthyroidism, an overactive thyroid that’s producing too much hormone. The most common cause is Graves’ disease, another autoimmune condition, though thyroid inflammation and thyroid nodules can also be responsible. A family history of thyroid disease and recent pregnancy both raise the risk.
An overactive thyroid tends to speed things up. Common symptoms include:
- Unintentional weight loss with increased appetite
- Rapid or irregular heartbeat, sometimes with palpitations
- Nervousness, anxiety, and irritability
- Trembling in the hands and fingers
- Sensitivity to heat and warm, moist skin
- More frequent bowel movements
- Muscle weakness and sleep problems
- Fine, brittle hair
In older adults, these classic symptoms are often subtle. The main signs may be an irregular heartbeat, unexplained weight loss, depression, or simply feeling weak and tired during normal activities, which makes the condition easy to miss without a blood test.
How to Prepare for a TSH Test
You don’t need to fast. Eat and drink normally before the blood draw. The test itself is a simple venous blood draw, typically from your arm, and results usually come back within a day or two.
One thing worth knowing: biotin supplements can throw off your results. Multivitamins contain small amounts (30 to 300 micrograms), which are generally not a problem. But the high-dose biotin supplements marketed for hair and nail growth often contain 5,000 to 10,000 micrograms, enough to produce misleading readings. The American Thyroid Association recommends stopping biotin at least two days before thyroid testing.
Certain prescription medications can also affect TSH independently of actual thyroid function. Glucocorticoids (commonly prescribed for inflammation and autoimmune conditions) can suppress TSH, making it appear lower than it should be. Dopamine-related medications can have a similar effect. If you’re taking any of these, your doctor will factor that into how they interpret your results.
What Happens After an Abnormal Result
A single abnormal TSH reading doesn’t always mean you have a thyroid disorder. Temporary shifts can happen during illness, stress, or pregnancy. Doctors typically order a repeat test to confirm the result, along with additional blood work measuring the actual thyroid hormones (called free T4 and sometimes free T3). TSH tells you something is off; the follow-up tests help clarify what’s happening and how severe it is.
If your TSH is high and your thyroid hormones are low, that confirms hypothyroidism. Treatment is a daily pill that replaces the missing thyroid hormone. Most people take it long-term, and once the dose is dialed in, TSH is rechecked periodically (usually every 6 to 12 months) to make sure levels stay in range.
If your TSH is low and thyroid hormones are elevated, further testing helps determine the cause of hyperthyroidism, since treatment differs depending on whether it’s Graves’ disease, an inflamed thyroid, or a nodule. Options range from medication that slows hormone production to more permanent approaches like radioactive iodine treatment.
If your TSH is only slightly outside the normal range and your thyroid hormone levels are fine, your doctor may simply recommend monitoring with repeat testing in a few months rather than starting treatment right away.