What is a normal TSH, and why does your lab's range differ?

A typical adult range is about 0.4–4.0 mIU/L. But "typical" is doing a lot of work in that sentence, and the number that matters for you is probably narrower.

Typical adult ranges

Source: Thyroid Function Tests, American Thyroid Association. These ranges are approximate and vary between laboratories.

Why two labs disagree about the same blood

A reference range is not a law of nature. Each laboratory establishes its own interval from its own assay and its own reference population, so the printed range moves from lab to lab — and can move again when a single lab changes equipment.

The practical consequence is worth stating plainly: compare your result to the range printed on that result, not to a range you read somewhere else. A value that looks marginally high against a stranger's range can sit comfortably inside your own lab's.

"Normal" and "your target" are not the same thing

The printed range describes a population. The number your doctor is steering you toward describes you — and for someone on levothyroxine it is often narrower than the full range, and sometimes deliberately offset from the middle of it.

Someone told to stay under 2.5 does not get useful information from an app that colours everything below 4.0 green. That is why Thyrowise asks for your band and draws the chart against that, instead of against a generic normal.

TSH moves slowly, and that is the point

After a levothyroxine dose change, TSH is usually rechecked about 6 to 12 weeks later, because that is roughly how long it takes to settle at the new dose.

Source: Hypothyroidism and ATA Professional Guidelines, American Thyroid Association.

This is why a single reading tells you so little on its own, and why a chart with the dose changes marked on it tells you so much: the question is never "what is my TSH", it is "what did my TSH do after the thing that changed".

Free versus total, and the tests that are not trend lines

• Free T4 and free T3 measure unbound hormone. A total counts bound and free together, so it shifts when binding proteins change — in pregnancy, for instance — while the free measurements more accurately reflect how the thyroid is working. (ATA, Thyroid Function Tests)
• TPO antibodies are the marker associated with Hashimoto's thyroiditis. They are usually measured once, to answer a question, rather than tracked over time. (ATA; NIDDK)
• Thyroglobulin antibodies are reported against each laboratory's own cut-off rather than one shared number. They help make the initial diagnosis, but following their level over time does not track how the thyroid is doing. (MedlinePlus)
• Reverse T3 is not supported for routine use in the care of hypothyroidism or hyperthyroidism, and most clinicians do not use it. (ATA, Clinical Thyroidology for the Public 12(1))

What Thyrowise does about it

You enter the target band your doctor actually gave you, and the TSH chart is drawn against that band with your dose changes marked on the same timeline. Every value you log keeps its own date, so the shape of the last two years is one screen rather than a folder of PDFs. Thyrowise stores results; it does not interpret them or tell you what your dose should be.

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