TSH, or thyroid-stimulating hormone, is a hormone produced by a small gland at the base of your brain called the pituitary gland. Its job is to tell your thyroid how much hormone to make. Doctors use TSH as the primary blood test to check whether your thyroid is working properly, and a normal level for most adults falls between 0.4 and 4.0 mIU/L.
How TSH Controls Your Thyroid
Your body regulates thyroid function through a feedback loop involving three players: the hypothalamus (a region deep in the brain), the pituitary gland (just below it), and the thyroid gland in your neck. The hypothalamus kicks things off by releasing a signaling hormone that tells the pituitary to produce TSH. TSH then travels through the bloodstream and binds to receptors on the thyroid, prompting it to produce two key hormones: T4 and T3. These thyroid hormones influence your metabolism, energy levels, heart rate, body temperature, and more.
The system is self-correcting. When T4 and T3 levels rise high enough, they signal the hypothalamus and pituitary to ease off, reducing TSH output. When thyroid hormone levels drop, TSH climbs to push the thyroid harder. This is why TSH works so well as a screening test: it reflects what’s happening with your thyroid before you might notice symptoms. Even small dips in thyroid hormone output trigger a proportionally larger rise in TSH, making it a sensitive early indicator.
What a Normal TSH Level Looks Like
For most adults, the standard reference range is 0.4 to 4.0 mIU/L. But “normal” shifts depending on your age and whether you’re pregnant.
Older adults tend to run slightly higher. Research on elderly populations found a reference range of roughly 0.55 to 5.14 mIU/L, and a French endocrine guideline suggests the upper limit can reasonably be estimated by dividing your age (in decades) by 10. So for a 70-year-old, a TSH up to 7.0 may be considered acceptable, and for an 80-year-old, up to 8.0.
During pregnancy, targets are lower because the developing baby depends on maternal thyroid hormones, especially early on. The American Thyroid Association has recommended trimester-specific ranges: 0.1 to 2.5 in the first trimester, 0.2 to 3.0 in the second, and 0.3 to 3.5 in the third.
What a High TSH Means
A TSH above the normal range usually signals that your thyroid isn’t producing enough hormone, a condition called hypothyroidism. Your pituitary is essentially yelling louder at a thyroid that isn’t keeping up. The most common cause is Hashimoto’s disease, an autoimmune condition where the immune system gradually damages the thyroid gland. Other causes include thyroid inflammation (thyroiditis), surgical removal of part or all of the thyroid, radiation treatment to the head or neck, and certain medications used for heart conditions, bipolar disorder, or cancer.
When your thyroid is underactive, you may notice fatigue, unexplained weight gain, sensitivity to cold, joint and muscle pain, dry skin, thinning hair, heavy or irregular periods, a slower heart rate, or depression. These symptoms often develop gradually, so many people attribute them to stress or aging before getting tested.
Sometimes TSH is elevated but thyroid hormone levels (T4) remain in the normal range. This is called subclinical hypothyroidism. You may have mild symptoms or none at all. Whether it needs treatment depends on how high the TSH is, whether you have symptoms, and other individual factors.
What a Low TSH Means
A TSH below the normal range suggests your thyroid is making too much hormone, known as hyperthyroidism. In this case, the pituitary detects the excess and dials TSH down to near zero. Graves’ disease, another autoimmune condition, is the most common cause. Thyroid inflammation can also temporarily flood the bloodstream with stored hormone, pushing TSH down.
Hyperthyroidism tends to speed things up. Common symptoms include unintentional weight loss, a fast or irregular heartbeat, heart palpitations, increased hunger, nervousness and irritability, trembling in the hands, sweating, and brittle hair. In older adults, the signs can be subtler: fatigue, depression, weight loss, or an irregular heartbeat without the more obvious signs of feeling “revved up.” A family history of thyroid disease, particularly Graves’ disease, raises your risk.
TSH Compared to Other Thyroid Tests
TSH is typically the first test ordered because it’s the most sensitive. Small changes in thyroid hormone output cause much larger swings in TSH, so TSH often shifts before T4 or T3 levels leave their normal ranges. If your TSH comes back abnormal, your doctor will usually follow up with a free T4 test (and sometimes free T3) to get a fuller picture of what the thyroid is actually producing. Together, these results clarify whether the problem is overt or subclinical, and they help guide treatment decisions.
What Can Affect Your Results
TSH levels aren’t static throughout the day. They start rising in the late afternoon, peak during the early part of the night, and drop to their lowest point during daytime hours. A blood draw at 8 a.m. and one at 3 p.m. can produce meaningfully different numbers. If your doctor is tracking your TSH over time, getting blood drawn at roughly the same time of day improves the comparison.
Sleep matters too. After a sleepless night, morning TSH levels can be roughly twice as high as they would be after a normal night of sleep. This is because TSH release, which is normally suppressed during sleep, keeps going when you stay awake.
Biotin supplements are a less obvious factor. Biotin (vitamin B7), commonly found in hair, skin, and nail supplements, can interfere with the laboratory method used to measure TSH. The result is a falsely low reading, which could make it look like you have hyperthyroidism when you don’t. If you take biotin, stop it at least 48 to 72 hours before your blood draw to avoid misleading results.