What this marker measures
TSH is measured on a logarithmic scale, meaning a TSH of 0.5 vs. 1.0 is a much bigger change than 3.0 vs. 3.5. Lab reference ranges can vary; some experts advocate for narrower ranges (0.5–2.5 mIU/L) for optimal health.
High or low: what it can mean
High TSH
A high TSH suggests the pituitary is working harder than usual to stimulate the thyroid, which often means the thyroid is underactive (hypothyroidism). The TSH response amplifies small changes in thyroid hormone — small TSH changes can be significant.
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Low TSH
A low TSH suggests the pituitary is sending fewer signals to the thyroid, often because thyroid hormone levels are already elevated (hyperthyroidism). A suppressed TSH alongside symptoms warrants further investigation.
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Lifestyle considerations
- •TSH values can fluctuate slightly throughout the day; testing at the same time of day improves consistency
Frequently asked questions
What is TSH?
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TSH is produced by the pituitary gland and tells the thyroid gland how much thyroid hormone to make. It is the most sensitive and widely used test for thyroid function.
What does TSH measure?
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TSH is measured on a logarithmic scale, meaning a TSH of 0.5 vs. 1.0 is a much bigger change than 3.0 vs. 3.5. Lab reference ranges can vary; some experts advocate for narrower ranges (0.5–2.5 mIU/L) for optimal health.
What does high TSH mean?
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A high TSH suggests the pituitary is working harder than usual to stimulate the thyroid, which often means the thyroid is underactive (hypothyroidism). The TSH response amplifies small changes in thyroid hormone — small TSH changes can be significant.
What does low TSH mean?
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A low TSH suggests the pituitary is sending fewer signals to the thyroid, often because thyroid hormone levels are already elevated (hyperthyroidism). A suppressed TSH alongside symptoms warrants further investigation.
What supports healthy TSH levels?
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TSH values can fluctuate slightly throughout the day; testing at the same time of day improves consistency
Related biomarkers
Editorial note
LabTestly explains lab markers using plain-English educational content. Reference ranges may vary by laboratory and should be interpreted with a clinician.
Educational content · Editorially reviewed for clarity · Last updated
Common sources used for lab-test education
Not medical advice
This page is for general understanding only and does not replace medical advice from your clinician. Reference ranges can vary by laboratory, age, sex, pregnancy status, medications, and clinical context.