TSH Unit Converter

mIU/L, µIU/mL and mU/L are the same number for TSH. That is the answer; the rest of this page is the interpretation, which is where TSH is actually difficult.

Last updated 4 Sep 2026

TSH converter

mIU/L = µIU/mL

TSH

mIU/L, µIU/mL and mU/L are numerically identical — the value does not change.

Reference groupThe upper limit rises genuinely with age: population data put the 97.5th centile near 6 mIU/L in the eighth decade and higher again beyond it. Pregnancy needs trimester-specific ranges instead.

2.10mIU/LExample

TSH 2.1 mIU/L

Units and what they mean

µIU/mL = mIU/L × 1
mU/L = mIU/L × 1
All three units are numerically identical

× 1: one micro-international unit per millilitre is one milli-international unit per litre; the label changes and the number never does
no molar unit: TSH is a heterodimeric glycoprotein standardised by bioactivity in international units against a WHO reference preparation, so no pmol/L figure would be meaningful and none is offered
age: the upper limit of the reference interval rises with age; a TSH near 5 to 6 mIU/L in an eighty-year-old often sits inside that person's age-specific range
with free T4: the pair separates primary, subclinical and central thyroid disease; a TSH read on its own cannot

Worked example

TSH 2.1 mIU/L
2.1 mIU/L = 2.10 µIU/mL = 2.10 mU/L — all three labels carry the same number
No arithmetic is involved: a micro-international unit per millilitre is a milli-international unit per litre
Within the adult reference interval of 0.40–4.00 mIU/L

The three labels are one number

There is no conversion to perform between any of these. A TSH of 2.1 is 2.1 in all of them, and a report that changes label between visits has not changed the measurement.
Written as	Read as	Value
mIU/L	milli-international units per litre	2.10
µIU/mL	micro-international units per millilitre	2.10
mU/L	milliunits per litre	2.10
uIU/mL	the same as µIU/mL, written without the Greek letter	2.10

TSH read with free T4

The last row is why TSH alone is not a sufficient test in anyone with pituitary disease or suggestive symptoms: in central hypothyroidism the TSH is not raised, and a TSH-only strategy misses it.
TSH	Free T4	Interpretation	Next step
Raised	Low	Primary hypothyroidism	Thyroid peroxidase antibodies; start levothyroxine
Raised	Normal	Subclinical hypothyroidism	Repeat in two to three months with antibodies before treating
Suppressed	Raised	Thyrotoxicosis	Determine the cause before treating; free T3 if free T4 is normal
Suppressed	Normal	Subclinical hyperthyroidism	Repeat; consider levothyroxine over-replacement, and biotin or contrast interference
Low or normal	Low	Central hypothyroidism	Assess the rest of the pituitary; do not treat on TSH alone

Same number, three labels — and then the hard part

There is no conversion to do. TSH is reported as mIU/L, µIU/mL and mU/L, and all three are numerically identical: a micro-international unit per millilitre is a milli-international unit per litre, and a milliunit is a milli-international unit. A TSH of 2.1 mIU/L is 2.1 µIU/mL and 2.1 mU/L. The form uIU/mL, written without the Greek letter, is the same thing again. If a report changes label between visits, the measurement has not changed.

No molar unit is offered, and that is deliberate rather than an omission. TSH is a heterodimeric glycoprotein whose glycoforms differ in mass and in biological activity, so it is standardised by bioactivity in international units against a WHO reference preparation instead of by mass. A pmol/L figure would have no defensible single value. Traceability to that standard is also not the same as harmonisation: platforms still differ, so serial values are best compared within one laboratory.

The interpretation is where TSH is genuinely difficult, and the first trap is the reference interval. The commonly quoted adult range of roughly 0.4 to 4.0 mIU/L is not age-neutral. Population data show the upper limit rising steadily through adult life, approaching 6 mIU/L in the eighth decade and higher beyond it, so a mildly raised TSH in an older person may sit within that person’s own age-specific range. Pregnancy moves the interval in the other direction and needs trimester-specific ranges.

The second is that TSH must be read against free T4. A raised TSH with a low free T4 is primary hypothyroidism; with a normal free T4 it is subclinical, and a single value should be repeated after two to three months with thyroid peroxidase antibodies before levothyroxine is started, because a substantial proportion normalise. A low or normal TSH with a low free T4 is central hypothyroidism, which a TSH-only strategy misses entirely. Acute illness, recent glucocorticoids, dopamine, amiodarone and high-dose biotin all distort the result, so an abnormal TSH in an unwell patient should be repeated after recovery.

Frequently asked questions

Is mIU/L the same as µIU/mL for TSH?

Yes, exactly the same number. One micro-international unit per millilitre equals one milli-international unit per litre, and mU/L is the same again. A TSH of 2.1 mIU/L is 2.1 µIU/mL and 2.1 mU/L, with no calculation required.

Why is TSH not reported in pmol/L?

TSH is a heterodimeric glycoprotein whose glycoforms differ in mass and bioactivity, so there is no single meaningful molecular mass for reporting. It is standardised by bioactivity in international units against a WHO reference preparation, which is why molar units are never used and none is offered here.

Is a TSH of 5 mIU/L abnormal?

It depends on the patient’s age and free T4. In a young adult it is above the usual interval and, with a normal free T4, indicates subclinical hypothyroidism that should be repeated in two to three months with thyroid peroxidase antibodies. In someone in their eighties it may sit inside the age-specific range, because the upper limit rises genuinely with age.

Should a single abnormal TSH be acted on?

Usually not. TSH is pulsatile, has a diurnal rhythm, and is distorted by acute illness, glucocorticoids, dopamine, amiodarone and high-dose biotin supplements. An abnormal result in an unwell patient should be repeated after recovery, and an isolated subclinical abnormality repeated after two to three months.

Can a normal TSH exclude thyroid disease?

Not entirely. A normal TSH in a stable outpatient makes primary thyroid disease unlikely, but central hypothyroidism from pituitary or hypothalamic disease presents with a low free T4 and a TSH that is normal or only slightly low. Anyone with pituitary disease or strongly suggestive symptoms needs a free T4 as well.

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References

Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751.
Surks MI, Hollowell JG. Age-specific distribution of serum thyrotropin and antithyroid antibodies in the US population: implications for the prevalence of subclinical hypothyroidism. J Clin Endocrinol Metab. 2007;92(12):4575–4582.
Biondi B, Cappola AR, Cooper DS. Subclinical hypothyroidism: a review. JAMA. 2019;322(2):153–160.