TSH (Thyroid) Calculator

Enter your TSH (thyroid stimulating hormone) level to check which category it falls into.

TSH Calculator

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TSH (mIU/L)

Category

Normal

Reference ranges can vary slightly by lab and by age. This is informational only, not a diagnosis - discuss your full thyroid panel (including free T4) with a doctor.

The classification

Low (Hyperthyroid direction): under 0.4 mIU/L Normal: 0.4-4.0 mIU/L High - Subclinical Hypothyroid: 4.1-10.0 mIU/L High - Overt Hypothyroid: over 10.0 mIU/L

Example

A TSH of 2.0 mIU/L falls squarely in the Normal range (0.4-4.0).

Step-by-step guide

1. Get a TSH blood test from your doctor or lab - this is usually the first test ordered to screen thyroid function.
2. Enter your result in mIU/L (the standard unit on most US lab reports).
3. Read your category - remember that TSH moves in the opposite direction from thyroid hormone activity: high TSH means an underactive thyroid, low TSH means an overactive one.

Why TSH moves opposite to thyroid activity

TSH is produced by the pituitary gland, not the thyroid itself - it's the signal that tells the thyroid how much hormone to make. When the thyroid is underactive (hypothyroidism) and not producing enough hormone, the pituitary responds by releasing more TSH to try to stimulate it, so TSH goes up. When the thyroid is overactive (hyperthyroidism) and producing too much hormone, the pituitary senses the excess and dials TSH down. This is why a high TSH points toward an underactive thyroid, and a low TSH points toward an overactive one - the opposite of what the "high equals more activity" instinct might suggest.

Common mistakes

Assuming high TSH means an overactive thyroid - it's the reverse. High TSH indicates the thyroid isn't producing enough hormone (hypothyroidism), not too much.

Interpreting TSH alone without free T4 - a full thyroid panel usually includes both, since free T4 helps confirm whether an abnormal TSH reflects an actual thyroid hormone problem or a temporary fluctuation.

Frequently asked questions

Why does high TSH mean an underactive thyroid, not overactive?

TSH is released by the pituitary gland to stimulate the thyroid. When the thyroid isn't producing enough hormone, the pituitary releases more TSH trying to push it to work harder - so a high TSH signals the thyroid is underperforming, not overperforming.

What is "subclinical hypothyroidism"?

It's a mild elevation in TSH (roughly 4-10 mIU/L) while thyroid hormone levels (free T4) are still technically normal. It can be an early sign of developing hypothyroidism, and whether it needs treatment often depends on symptoms and other individual factors.

Do TSH reference ranges vary between labs?

Yes, somewhat - most labs use approximately 0.4-4.0 mIU/L, but some use slightly different cutoffs based on their specific reference population and testing method. Always compare your result to the reference range printed on your own lab report.

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