A good TSH level for most adult women falls between 0.27 and 4.2 mIU/L. That’s the standard reference range used by most labs, though the ideal number within that range depends on your age, whether you’re pregnant or trying to conceive, and whether you have a thyroid condition. Many women feel their best when TSH sits in the lower half of that range, roughly between 0.5 and 2.5 mIU/L, but “optimal” is personal.
The Standard Reference Range
For women aged 21 to 99, most laboratories define normal TSH as 0.27 to 4.2 mIU/L. A result below 0.27 suggests your thyroid may be overactive (hyperthyroidism), while a result above 4.2 points toward an underactive thyroid (hypothyroidism). Keep in mind that lab reference ranges vary slightly from one facility to another, so always check the specific range printed on your lab report rather than comparing your number to a generic cutoff.
If your TSH is elevated but your actual thyroid hormone levels (T4 and T3) come back normal, the diagnosis is subclinical hypothyroidism. This means your brain is working harder to stimulate the thyroid, but the gland is still producing enough hormone for now. Whether this warrants treatment depends on how high the TSH is, your symptoms, and other risk factors like thyroid antibodies.
Why the “Ideal” Number Changes With Age
TSH naturally drifts upward as you get older. A reading of 5.0 in a 30-year-old woman is worth investigating, but the same number in a 75-year-old may be perfectly normal. The American Thyroid Association recommends raising the target range to 4 to 6 mIU/L for women between 70 and 80. The French Endocrine Society goes further, proposing a simple formula for patients over 60: divide your age by 10 to get your personal upper limit. Under that approach, a healthy 80-year-old’s TSH could run as high as 8.0 mIU/L without needing treatment.
This matters because treating mildly elevated TSH in older women can do more harm than good. Pushing TSH too low with medication increases the risk of bone loss and heart rhythm problems, risks that outweigh the benefits when the “high” number is actually age-appropriate.
Target TSH During Pregnancy
Pregnancy changes the equation significantly. Your baby depends entirely on your thyroid hormone supply during the first trimester, before its own thyroid gland starts working. Low thyroid hormone levels during this window have been linked to miscarriage, preterm delivery, and problems with the baby’s brain development.
The Endocrine Society recommends keeping TSH between 0.2 and 2.5 mIU/L during the first trimester, and between 0.3 and 3.0 mIU/L for the second and third trimesters. For women already taking thyroid medication, the American Thyroid Association advises maintaining TSH below 2.5 mIU/L throughout pregnancy. For women not on medication before becoming pregnant, starting treatment is generally recommended if TSH rises above 4.0, especially when thyroid antibodies are present.
TSH Targets When Trying to Conceive
If you’re planning a pregnancy, your target TSH is tighter than the general population range. Most guidelines suggest keeping TSH below 2.5 mIU/L before conception. This gives you a buffer, since TSH tends to rise in early pregnancy as the body’s demand for thyroid hormone increases.
Women with Hashimoto’s thyroiditis (the most common cause of hypothyroidism) need an even lower pre-pregnancy target. Research suggests these women should aim for a TSH that’s 30 to 50 percent below the 2.5 cutoff, roughly 1.2 to 1.7 mIU/L, to maintain normal thyroid levels through those critical early weeks. If you have Hashimoto’s and are planning a pregnancy, getting your TSH checked and adjusted beforehand can make a real difference.
What Can Throw Off Your Results
TSH follows a daily rhythm that can shift your results significantly. Levels peak overnight between 11 p.m. and 5 a.m., then gradually fall throughout the day, hitting their lowest point in the late afternoon and evening. The total swing over 24 hours can be roughly double, meaning a morning blood draw might show a TSH of 3.0 while an afternoon draw from the same person on the same day could come back closer to 1.5. Most labs draw blood between 7 and 10 a.m., but there’s no universal requirement. If you’re tracking your 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 far above what you’d get from food), can interfere with the lab assay and produce falsely low or falsely high TSH readings. The American Thyroid Association recommends stopping biotin for at least two days before any thyroid blood test. If you’re taking a multivitamin or beauty supplement, check the label for biotin content before your next draw.
What Your Number Actually Means
TSH is an indirect measure. It tells you how hard your pituitary gland is working to stimulate the thyroid, not how much thyroid hormone is actually circulating. A “normal” TSH doesn’t always mean your thyroid function is perfect, and an out-of-range result doesn’t always mean you need medication. Context matters: your symptoms, your age, whether you’re pregnant, and whether you have thyroid antibodies all factor into the decision.
If your TSH comes back outside the expected range, the next step is usually a repeat test along with free T4 (and sometimes T3 and thyroid antibody levels) to get the full picture. A single elevated or suppressed TSH on its own isn’t a diagnosis. Two consistent results, combined with your clinical picture, are what guide treatment decisions.