For most adults, a normal thyroid-stimulating hormone (TSH) level falls between 0.27 and 4.2 mIU/L. But TSH alone doesn’t tell the whole story. Your doctor may also check free T4, free T3, and sometimes thyroid antibodies to get a complete picture of how your thyroid is functioning.
What TSH Tells You
TSH is the most common screening test for thyroid function, but it doesn’t come directly from your thyroid. It’s a signal sent by your pituitary gland (a small gland at the base of your brain) telling your thyroid how much hormone to produce. When your thyroid is underperforming, your pituitary pumps out more TSH to compensate, pushing the number higher. When your thyroid is overactive, TSH drops because the pituitary backs off.
This inverse relationship is why a high TSH usually points to hypothyroidism (underactive thyroid) and a low TSH suggests hyperthyroidism (overactive thyroid). The standard adult reference range of 0.27 to 4.2 mIU/L covers ages 21 through 99, though where you fall within that range can matter depending on your age, pregnancy status, and overall health.
Free T4 and Free T3 Ranges
If your TSH comes back abnormal, or sometimes alongside TSH as a first step, your doctor will check your free T4 level. Free T4 measures the amount of thyroxine circulating in your blood that’s available for your body to use. For adults over 18, the normal range is 0.9 to 1.7 ng/dL. Children and adolescents have slightly wider ranges, with kids under 5 falling between 0.8 and 2.8 ng/dL.
Free T3 (triiodothyronine) is the more active thyroid hormone, and your body converts most of its T3 from T4. T3 testing is less routine but can be useful when hyperthyroidism is suspected or when symptoms persist despite normal TSH and T4 results. Your lab report will include a reference range specific to the assay used, so always compare your number to the range printed on your results rather than a generic number found online.
What Subclinical Thyroid Disease Looks Like
Sometimes your TSH is slightly high but your free T4 and T3 remain within the normal range. This is called subclinical hypothyroidism. You may have no symptoms at all, or you might notice subtle signs like fatigue or mild weight gain. It doesn’t always require treatment, but it does warrant monitoring because a percentage of people with subclinical hypothyroidism progress to full hypothyroidism over time.
The reverse pattern, a low TSH with normal T4 and T3, is subclinical hyperthyroidism. Both conditions are common findings on routine bloodwork and are typically rechecked in 6 to 12 weeks before any treatment decisions are made, since a single abnormal reading can reflect a temporary fluctuation rather than a lasting problem.
How Age Changes What’s “Normal”
TSH naturally drifts upward as you get older. Data from a large national health survey shows that the percentage of people with TSH above 4.5 mIU/L rises to about 14% in those aged 85 and older. In people over 80, the upper limit of normal (at the 95th percentile) exceeds 6.0 mIU/L, and in those over 90, it can reach 8.0 mIU/L. This shift happens even in people with no detectable thyroid disease.
This matters because an older adult with a TSH of 5.5 may not actually have hypothyroidism. Applying the standard adult cutoff of 4.2 to someone in their 80s could lead to unnecessary treatment. While formal age-adjusted reference ranges haven’t been officially adopted yet, many endocrinologists take a patient’s age into account before starting therapy, particularly for mild elevations.
Thyroid Levels During Pregnancy
Pregnancy shifts thyroid physiology significantly. Your body produces more thyroid hormone to support fetal brain development, especially in the first trimester before the baby’s own thyroid starts working. 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.
These tighter ranges mean a TSH of 3.5, perfectly normal outside of pregnancy, could signal a problem during early pregnancy. Uncontrolled hypothyroidism during pregnancy is associated with increased miscarriage risk and developmental concerns, which is why many OB-GYNs screen TSH early and monitor it throughout.
Thyroid Antibodies and Autoimmune Risk
If your TSH is borderline or your doctor suspects autoimmune thyroid disease, you may be tested for thyroid peroxidase antibodies (TPO antibodies). A normal level is less than 5.6 IU/mL. Elevated TPO antibodies are the hallmark of Hashimoto’s disease, the most common cause of hypothyroidism in the United States.
Here’s where it gets interesting: you can test positive for TPO antibodies while your TSH is still completely normal. This means your immune system is attacking your thyroid, but your thyroid is keeping up with demand for now. Over years, though, many people with elevated antibodies eventually develop hypothyroidism. If you’re in this category, periodic TSH monitoring (typically once a year) helps catch the transition early.
The “Optimal” Range Debate
You may have come across claims that the “optimal” TSH is narrower than the standard lab range, often cited as somewhere between 1.0 and 2.5. This idea has roots in both functional medicine and some professional endocrinology discussions. Research from the American Thyroid Association found that TSH values in the 60th to 80th percentile of the normal range, paired with free T4 values in the 20th to 40th percentile, were associated with the lowest risk of death and heart disease.
That said, this doesn’t mean a TSH of 3.8 is dangerous. Population-level associations don’t translate neatly into individual treatment decisions. Most endocrinologists treat based on symptoms, trends over time, and the full hormone panel rather than chasing a single “perfect” number.
What Can Throw Off Your Results
TSH follows a circadian rhythm. It peaks between 11 p.m. and 5 a.m. and hits its lowest point around 5 p.m. The total swing over a 24-hour period can roughly double from nadir to peak. In practical terms, a blood draw at 8 a.m. will typically show a higher TSH than one taken from the same person at 3 p.m. on the same day. If you’re tracking TSH over time, try to get your blood drawn at a similar time each visit, ideally in the morning, so the numbers are comparable.
Biotin supplements are another common culprit. Doses of 5 mg or more, found in many hair, skin, and nail supplements, can directly interfere with the lab assay and produce falsely abnormal thyroid results. Standard multivitamins with up to 1 mg of biotin haven’t been shown to cause problems. If you take a high-dose biotin supplement (5 to 10 mg), stop it at least 8 hours before your blood draw. For people on very high therapeutic doses of 100 mg or more per day, a 72-hour washout is recommended.
Illness, stress, certain medications (particularly steroids and some heart rhythm drugs), and even skipping breakfast can also nudge TSH readings. A single out-of-range result is almost always retested before any diagnosis is made.