Thyroid Stimulating Hormone

Also written TSH

ThyroidMeasured in mIU/L

What it measures

The pituitary signal instructing the thyroid to produce hormone. It rises when the body senses insufficient thyroid hormone.

Why it matters

It is the most sensitive single screening test for thyroid dysfunction and the standard first-line thyroid test.

Where the claimed ranges actually sit

Measured in mIU/L. Each row is one claim about where your result should fall.

What a laboratory reportsWhat a seller markets as “optimal”

Laboratory reference range0.45–4.5 mIU/L

Laboratory reference intervals; population-derived

0.5–2.0 mIU/LC0.5–2 mIU/L

Functional medicine, hormone optimization clinics, and thyroid supplement sellers

›View as table

Claim	Range	Source
Laboratory reference range	0.45–4.5 mIU/L	Laboratory reference intervals; population-derived
0.5–2.0 mIU/L	0.5–2 mIU/L	Functional medicine, hormone optimization clinics, and thyroid supplement sellers

Where the sources disagree

This is the most commercially consequential disputed range in the registry. Moving the upper limit from 4.5 to 2.0 converts millions of healthy people into candidates for thyroid medication. The steelman is legitimate and taken seriously by endocrinologists. But the marker has large diurnal and seasonal variation, drifts upward normally with age, and a mildly elevated TSH with normal free T4 frequently resolves on its own. Timing matters: a morning draw and an afternoon draw can straddle the entire disputed zone.

The marketed ranges, examined

Each claim gets its strongest possible case before it gets its criticism. Several of these are substantially correct.

0.5–2.0 mIU/L

Promoted by functional medicine, hormone optimization clinics, and thyroid supplement sellers

CConfounded

The strongest case for it

The reference population used to set the upper bound historically included people with undiagnosed autoimmune thyroid disease, which does plausibly inflate the top of the range.

What it leaves out

TSH rises naturally with age, varies as much as 40% within a single person across a day, and peaks overnight. Narrowing the range to 2.0 reclassifies an enormous number of healthy people as having a treatable problem; the clinics promoting the narrower range typically sell the treatment.

Before you act on your result

Substantial diurnal variation; standardize draw time when tracking over time.
Rises normally with age. An elderly person's 4.0 is not equivalent to a 25-year-old's.
Never interpret in isolation from free T4 and thyroid antibodies.
Biotin supplements interfere with many thyroid immunoassays; stop them 48 hours before a draw.

Last reviewed July 28, 2026How we grade evidence and attribute claims →

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