A good TSH level for most healthy adults falls between 0.5 and 4.0 mIU/L, though the number that’s “good” for you depends on your age, whether you’re pregnant, and whether you’re being treated for a thyroid condition. TSH, or thyroid-stimulating hormone, is produced by your pituitary gland to tell your thyroid how much hormone to make. When your thyroid is sluggish, TSH rises to push it harder. When your thyroid is overactive, TSH drops because less stimulation is needed.
The Standard Reference Range
Most labs flag TSH results as abnormal if they fall outside 0.5 to 5.0 mIU/L. If you’re being treated for a thyroid disorder, your provider will generally aim to keep your TSH between 0.5 and 4.0 mIU/L. But “within range” and “optimal” aren’t always the same thing.
A large meta-analysis that pooled individual patient data found that the lowest risk of cardiovascular disease and death was associated with TSH levels between roughly 1.9 and 2.9 mIU/L. Interestingly, people with very low TSH levels (still technically within the normal range) had a higher risk of dying from cardiovascular causes than those in that middle zone. This doesn’t mean a TSH of 1.0 is dangerous, but it does suggest that the sweet spot for long-term health may sit closer to the middle of the reference range rather than the low end.
Why Your Age Changes the Target
TSH naturally drifts upward as you get older. After age 40, the upper limit of what’s considered normal increases by about 0.3 mIU/L for every additional decade of life. That means a TSH of 6.0 in a 75-year-old may be perfectly normal, while the same number in a 30-year-old would prompt further investigation. Some labs already adjust for this and set the upper limit as high as 7.0 mIU/L for older adults.
This matters because overtreating a mildly elevated TSH in an older person can push them into a hyperthyroid state, which raises the risk of heart rhythm problems and bone loss. If you’re over 65 and your TSH is slightly above the standard range with no symptoms, your provider may reasonably decide to monitor rather than treat.
TSH Targets During Pregnancy
Pregnancy shifts the goalposts significantly. The Endocrine Society recommends keeping TSH between 0.2 and 2.5 mIU/L during the first trimester, then between 0.3 and 3.0 mIU/L for the second and third trimesters. These tighter ranges exist because even mild thyroid underactivity in early pregnancy is linked to a higher risk of miscarriage and developmental problems.
If you’re planning to become pregnant and already take thyroid medication, your dose will likely need to increase early in pregnancy. Many providers check TSH as soon as pregnancy is confirmed and then recheck it every four to six weeks through at least the first half of pregnancy.
What High TSH Means
A TSH above the normal range signals that your thyroid isn’t producing enough hormone on its own. Your pituitary is essentially shouting louder to get a response. When TSH is elevated but your actual thyroid hormone levels are still normal, the condition is called subclinical hypothyroidism. It’s common, affecting up to 10% of adults, and it doesn’t always need treatment.
Treatment is more clearly recommended when TSH reaches 10 mIU/L or higher, or when younger and middle-aged adults have symptoms like fatigue, weight gain, cold intolerance, or brain fog alongside a moderately elevated TSH. For people with levels between 5 and 10 who feel fine, the decision to treat is more nuanced and often involves retesting in a few months to see if the elevation persists.
What Low TSH Means
A TSH below 0.5 mIU/L suggests your thyroid is producing too much hormone, so your pituitary has backed off. Levels between 0.1 and 0.4 mIU/L are considered low, while anything below 0.1 mIU/L is considered suppressed. A suppressed TSH paired with elevated thyroid hormones points to overt hyperthyroidism, which can cause a racing heart, unintentional weight loss, anxiety, tremors, and heat intolerance.
When TSH is low but thyroid hormone levels remain in the normal range, that’s subclinical hyperthyroidism. It’s less dramatic in terms of symptoms, but over time it can increase the risk of atrial fibrillation and bone thinning, particularly in older adults and postmenopausal women.
Factors That Skew Your Results
TSH follows a strong daily rhythm. Levels peak around 2 to 4 a.m. and hit their lowest point in the late afternoon, between about 4 and 8 p.m. The swing is substantial: TSH can vary by as much as 72% from its lowest to its highest point in a single day. A blood draw at 8 a.m. will typically give a higher reading than one at 3 p.m. If you’re monitoring TSH over time, try to get your blood drawn at roughly the same time of day each visit so the numbers are comparable.
Biotin supplements are another common source of error. Biotin, found in many hair, skin, and nail supplements (sometimes at doses of 5,000 to 10,000 mcg), interferes with the lab chemistry used to measure TSH and can produce falsely low results. This means your TSH could look normal or even suppressed when it’s actually elevated. If you take biotin, let your provider know before any thyroid blood work. Stopping the supplement for at least two to three days before the test is a common recommendation, though some labs suggest a longer washout for high-dose supplements.
Illness, stress, certain medications (especially steroids and some anti-seizure drugs), and even skipping a meal before the test can also nudge results. A single abnormal TSH reading almost always gets rechecked before any diagnosis is made.
Making Sense of Your Number
If your TSH falls between 1.0 and 3.0 mIU/L and you feel well, your thyroid is almost certainly functioning normally. If your result is slightly outside the standard range, context matters enormously: your age, symptoms, pregnancy status, and whether you’re already on medication all shape what “good” looks like for you. A TSH of 6.0 in a symptomatic 35-year-old tells a very different story than the same number in an 80-year-old with no complaints.
When tracking your thyroid over time, pay more attention to the trend than to any single result. A TSH that has climbed from 2.0 to 4.5 over two years, even though both values are technically normal, is more informative than a one-time snapshot. Keeping copies of your lab results and noting the time of day you were tested makes it much easier to spot meaningful changes.