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 thyroid hormone to make. When doctors order a TSH blood test, they’re checking whether your thyroid is working properly, since TSH levels shift in predictable ways when the thyroid produces too much or too little hormone. It’s one of the most commonly ordered blood tests and often the first step in diagnosing thyroid problems.
How TSH Controls Your Thyroid
Your thyroid gland, located in the front of your neck, produces hormones that regulate metabolism, energy, heart rate, and body temperature. But it doesn’t decide on its own how much hormone to release. That signal comes from TSH.
The system works through a feedback loop involving three parts of the body: the hypothalamus (a region deep in the brain), the pituitary gland (just below it), and the thyroid itself. The hypothalamus releases a signaling hormone that prompts the pituitary to produce TSH. TSH then travels through the bloodstream and tells the thyroid to make its hormones, known as T3 and T4. When T3 and T4 levels rise high enough, the hypothalamus and pituitary detect that and dial back their signals, reducing TSH. When thyroid hormone levels drop, TSH goes up to push the thyroid to work harder.
This is why TSH moves in the opposite direction of thyroid hormone levels. A high TSH usually means the thyroid isn’t making enough hormone, so the pituitary is sending a louder signal. A low TSH usually means the thyroid is making too much, so the pituitary has gone quiet.
What Normal TSH Levels Look Like
For adults, a normal TSH level generally falls between 0.4 and 4.2 mIU/L (milliunits per liter). Labs may report slightly different ranges depending on the testing method, so your result will typically come with a reference range printed alongside it.
During pregnancy, the ranges shift. The Endocrine Society recommends TSH stay between 0.2 and 2.5 mIU/L in the first trimester, and between 0.3 and 3.0 mIU/L in the second and third trimesters. This matters because thyroid hormones play a critical role in fetal brain development, and even mildly abnormal levels can affect pregnancy outcomes.
What High TSH Means
A TSH level above the normal range signals that your thyroid is underperforming, a condition called hypothyroidism. The pituitary is producing extra TSH because it’s trying to coax a sluggish thyroid into working harder.
The most common cause is Hashimoto’s disease, an autoimmune condition where your immune system attacks the thyroid and gradually damages it. Other causes include thyroid inflammation (thyroiditis), surgical removal of part or all of the thyroid, radiation treatment to the head or neck area, and certain medications, including some heart and bipolar disorder drugs that can interfere with thyroid hormone production. Too little iodine in the diet is a less common cause in developed countries.
When TSH is elevated but thyroid hormone levels are still within the normal range (typically TSH between 4.5 and 10.0 mIU/L), the condition is called subclinical hypothyroidism. You may have no symptoms at all, or you might notice subtle ones. When thyroid hormone levels actually drop below normal, symptoms become more noticeable:
- Fatigue that doesn’t improve with rest
- Weight gain that’s hard to explain by diet alone
- Cold sensitivity, feeling chilled when others are comfortable
- Dry skin and thinning hair
- Joint and muscle pain
- Depression or low mood
- Heavy or irregular periods and fertility difficulties
- Slowed heart rate
What Low TSH Means
A TSH level below the normal range typically means your thyroid is producing too much hormone, known as hyperthyroidism. Because thyroid hormone levels are already high, the pituitary stops sending TSH. The most common causes include Graves’ disease (an autoimmune condition that overstimulates the thyroid), noncancerous thyroid nodules that produce excess hormone, and thyroid inflammation that causes stored hormone to leak into the bloodstream.
One of the most frequent causes of low TSH is actually taking too much thyroid medication. People already being treated for hypothyroidism with hormone replacement can end up with suppressed TSH if their dose is higher than needed.
Symptoms of an overactive thyroid include rapid heartbeat or palpitations, unexplained weight loss despite a normal or increased appetite, feeling shaky or nervous, frequent bowel movements or diarrhea, warm and moist skin, and menstrual changes. In mild cases (subclinical hyperthyroidism), you may not notice any symptoms at all.
Why Doctors Test TSH First
TSH is the most sensitive early indicator of thyroid dysfunction. It shifts before thyroid hormone levels themselves move outside the normal range. This means a TSH test can catch a developing thyroid problem while it’s still subclinical, before symptoms appear.
If your TSH comes back abnormal, your doctor will typically follow up by measuring free T4 (the active thyroid hormone circulating in your blood). The combination of these two results clarifies the picture. High TSH with low free T4 confirms overt hypothyroidism. High TSH with normal free T4 indicates subclinical hypothyroidism. Low TSH with high free T4 points to hyperthyroidism. In rare cases, both TSH and free T4 can be low, which suggests the problem originates in the pituitary gland rather than the thyroid itself.
What Can Affect Your Results
TSH levels naturally fluctuate throughout the day, peaking in the early morning hours and dipping in the afternoon. Most labs don’t require fasting for a TSH test, but the time of your blood draw can influence results slightly.
Biotin supplements are a more significant concern. Biotin, commonly found in multivitamins, prenatal vitamins, and supplements marketed for hair, skin, and nail health, can directly interfere with the lab assay used to measure TSH. Depending on the testing method, biotin can cause falsely low or falsely high results. Products containing 150 micrograms or more of biotin per dose have been reported to cause this interference, and the risk increases with higher doses. If you take any biotin-containing supplement, mention it to your doctor before the test. Most labs recommend stopping biotin at least two to three days before a blood draw.
Pregnancy, as noted above, naturally lowers TSH in the first trimester because a pregnancy hormone (hCG) directly stimulates the thyroid, reducing the need for TSH. This is normal and doesn’t indicate hyperthyroidism on its own, which is why trimester-specific reference ranges exist.