The standard range
These bands come from the clinical practice guidelines for hypothyroidism in adults, cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Your own laboratory's interval may differ slightly, and where it prints one, that is the one your result was flagged against.
| TSH | Interpretation |
|---|---|
| Below 0.45 | Below the reference interval; possible overactive thyroid |
| 0.45 to 4.12 | Reference interval, if your laboratory prints none |
| 4.12 to 10 | Subclinical zone with a normal T4; an individualised decision |
| 10 and above | Treatment generally considered |
Why the upper limit is not one number
The most useful piece of evidence here is a 2007 analysis of the third National Health and Nutrition Examination Survey by Surks and Hollowell, which looked at TSH distributions by age decade in people without thyroid disease. What it found is that the whole distribution shifts upward with age, not that more old people have thyroid disease.
| Age | 97.5th centile | Proportion with TSH above 2.5 |
|---|---|---|
| 20 to 29 | 3.56 | 10.6 percent |
| 80 and over | 7.49 | 40 percent |
In the 80-and-over group, 14.5 percent had a TSH above 4.5 mIU/L despite having no thyroid disease. The authors also checked whether this was simply undetected autoimmune thyroiditis hiding in the sample, and found the opposite: among those with a TSH above 4.5, the proportion carrying antithyroid antibodies fell with age, from 67.4 percent at 40 to 49 down to 40.5 percent in the 80-plus group. The frequency distribution curve of the oldest group was displaced toward higher TSH whether or not antibodies were present.
The 2.5 argument, and where it landed
In the early 2000s there was a serious proposal to lower the TSH upper limit from about 4.5 to 2.5 mIU/L, on the grounds that most people without thyroid disease sit below 2.5, and that people between 2.5 and 4.5 progress to overt hypothyroidism more often. That debate has largely settled in favor of leaving the upper limit at roughly 4.0 to 4.5, which is why the guideline figure above is 4.12 rather than 2.5.
It is worth knowing that this argument happened, because a TSH of 3 is sometimes still described online as abnormal on the strength of the losing side of it. Under the current guideline interval, 3 is inside the reference range.
Our TSH reference page lists each band with its source and effective date, and the lab result lookup will place a specific number against them.
Frequently asked questions
What is the normal TSH range?
0.45 to 4.12 mIU/L is the interval given in the AACE and ATA hypothyroidism guideline for use when a laboratory prints none. Individual laboratories publish their own intervals and yours may differ slightly; your result was flagged against your laboratory's range, not this one.
Is a TSH of 5 bad?
It falls in the subclinical zone, between 4.12 and 10 mIU/L, where treatment is an individualised decision rather than automatic if T4 is normal. Age matters here: in people without thyroid disease the 97.5th centile is 7.49 mIU/L at 80 and over, and 70 percent of older patients with a TSH above 4.5 sit within their own age group's range.
Does TSH increase with age?
Yes. The NHANES III analysis found the whole TSH distribution shifts upward with age in people without thyroid disease, with the 97.5th centile rising from 3.56 mIU/L at ages 20 to 29 to 7.49 at 80 and over. The proportion above 2.5 mIU/L rose from 10.6 percent to 40 percent across the same span.
Should the TSH upper limit be 2.5?
That was proposed in the early 2000s and the debate has largely settled against it. The upper limit has remained at roughly 4.0 to 4.5 mIU/L, and the guideline interval used here is 0.45 to 4.12. A TSH of 3 is inside the reference range.
At what TSH is treatment started?
The guideline treats 10 mIU/L and above as the level at which treatment is generally considered. Between 4.12 and 10 with a normal T4 the decision is individualised, weighing symptoms, antibodies, age and other factors with a clinician.
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When to talk with a clinician
A single TSH outside the range is not a diagnosis; TSH moves with acute illness, time of day and other medicines, so a repeat with free T4 is the usual next step. If you are over 70 and have been told you have subclinical hypothyroidism on the strength of a TSH between 4.5 and 7, the age-specific distribution above is a reasonable thing to raise, alongside whether antibodies were checked.
