For most healthy adults, TSH should fall between 0.5 and 4.0 mIU/L. That said, the “right” number depends on your age, whether you’re pregnant, and whether you’re being treated for a thyroid condition. A TSH that’s perfectly normal for a 75-year-old could signal a problem in a 30-year-old who’s trying to conceive.
The Standard Reference Range
Laboratories typically report a normal TSH range of 0.5 to 5.0 mIU/L, though many clinicians use 0.5 to 4.0 mIU/L as the working range for diagnosing and managing thyroid disorders. The difference exists because experts don’t fully agree on where to draw the upper boundary. Normal ranges also vary slightly between labs depending on the testing platform they use.
That “normal” range is built by measuring TSH in a large group of healthy adults with no thyroid problems and capturing the middle 95% of results. The 2.5% of people above and below that window are flagged as abnormal. This means some perfectly healthy people will naturally sit near the edges without having any thyroid dysfunction at all.
How Age Shifts the Target
TSH naturally rises as you get older, especially after 50 in women and 60 in men. In one large dataset, the upper normal limit for 50-year-old women was about 4.0 mIU/L, but by age 90 it had climbed roughly 50% to 6.0 mIU/L. Some labs now set the upper cutoff as high as 7.0 mIU/L for older adults. This matters because a slightly elevated TSH in someone over 70 may simply reflect normal aging rather than a thyroid problem that needs treatment.
TSH Ranges in Children
Children, especially newborns, run much higher than adults. In the first few days of life, TSH can range from 0.5 to 18.2 mIU/L. By one to two months, the upper end drops to around 6.5 mIU/L, and by two months through the teenage years it settles into a range close to the adult window: roughly 0.47 to 4.17 mIU/L. Newborn screening programs flag critically high values (above about 38.5 mIU/L in the first days) to catch congenital hypothyroidism early.
TSH Targets During Pregnancy
Pregnancy increases demand on the thyroid, and the acceptable TSH range tightens considerably. The Endocrine Society recommends keeping TSH below 2.5 mIU/L during the first trimester and below 3.0 mIU/L for the remainder of pregnancy. A reading above 2.5 in the first trimester that would be unremarkable in a non-pregnant adult may prompt further evaluation or a dose adjustment for women already taking thyroid medication.
If you’re planning a pregnancy and you’re on thyroid hormone replacement, your provider will likely check your TSH early and adjust your dose to stay within these tighter limits. Thyroid hormone needs often increase by 20 to 30 percent during pregnancy, so expect more frequent blood work.
TSH Goals During Hypothyroidism Treatment
If you’re taking thyroid hormone replacement for an underactive thyroid, the goal is to keep your TSH between 0.4 and 4.0 mIU/L while your symptoms resolve. In practice, many people feel best when their TSH sits in the lower half of that range, though there’s no single “optimal” number that works for everyone. Your provider will adjust your dose based on both your lab results and how you feel.
Subclinical Hypothyroidism
If your TSH is above normal but your thyroid hormone levels are still within range, that’s called subclinical hypothyroidism. It’s common and often doesn’t require treatment. The general threshold for starting medication is a TSH of 10 mIU/L or higher. Below that, the approach for most people is to monitor with repeat testing every few months. Younger and middle-aged adults who have symptoms or other risk factors may be treated at lower TSH levels.
TSH Targets After Thyroid Cancer
Thyroid cancer survivors have different TSH goals depending on how well they responded to treatment. For those with an excellent response, the long-term target is 0.3 to 2.0 mIU/L, which is close to the normal range. People with an intermediate response are kept lower, between 0.1 and 0.5 mIU/L. Those with persistent or recurrent disease need their TSH suppressed below 0.1 mIU/L, which requires a higher dose of thyroid hormone and closer monitoring for side effects like bone loss and heart rhythm changes.
Why Your Result Can Fluctuate
TSH isn’t a fixed number. It follows a daily rhythm, peaking in the early morning hours and dropping through the afternoon. Real-world data shows that people who test before 8 a.m. average a TSH of 2.92 mIU/L, while those testing between 10 a.m. and 5 p.m. come in about 24% lower at 2.09 mIU/L. That’s a big enough swing to push a borderline result from one side of the reference range to the other.
For consistent, reliable results, test in the morning before 10 a.m. after an overnight fast. If you’re tracking your TSH over time, try to test at roughly the same time of day each visit. If you take thyroid medication, take it after the blood draw rather than before.
Biotin Can Skew Results
High-dose biotin supplements, popular for hair and nail growth, can interfere with the assay used to measure TSH and produce falsely low or falsely high readings. The standard dietary intake of 30 micrograms per day doesn’t cause problems, but many over-the-counter supplements contain 5,000 to 10,000 micrograms per capsule. If you’re taking biotin at those levels, let your provider know before your blood draw. Stopping the supplement for a few days beforehand is usually enough to eliminate interference.
What Your Number Means in Practice
A TSH below 0.5 mIU/L suggests your thyroid is overactive (or your medication dose is too high). Common symptoms include a racing heart, weight loss, anxiety, tremor, and heat intolerance. A TSH above 4.0 to 5.0 mIU/L suggests an underactive thyroid, which can cause fatigue, weight gain, cold sensitivity, dry skin, and sluggish thinking.
But TSH alone doesn’t tell the whole story. When your result is abnormal or borderline, your provider will typically check free T4 (and sometimes free T3) to see how much thyroid hormone is actually circulating. You can have a TSH at 3.5, well within the reference range, and still feel lousy for reasons that have nothing to do with your thyroid. Conversely, a mildly elevated TSH in an older adult with no symptoms may need nothing more than periodic rechecking.
The most useful way to interpret your TSH is in context: your age, symptoms, medication history, pregnancy status, and the trend over multiple tests. A single snapshot matters less than the pattern over time.