TSH levels are one of the most important markers of thyroid health. This guide explains what high, low, and normal TSH levels mean, what causes them to shift, and when you should get tested.
What Is a Normal TSH Level?
For most adults, a normal TSH level falls between 0.4 and 4.0 mIU/L. Some laboratories use a slightly wider reference range of 0.5 to 5.0 mIU/L. Your ideal range can also vary based on age, sex, and whether you are pregnant.
| Category | TSH Range (mIU/L) |
|---|---|
| Normal | 0.4 – 4.0 |
| Low / Hyperthyroidism | Below 0.4 |
| High / Hypothyroidism | Above 4.0 |
| Subclinical (borderline) | 0.1 – 0.3 or 4.1 – 10.0 |
High TSH Levels
When TSH is high, it usually means your thyroid is underactive and your pituitary gland is trying to stimulate it. Learn more about causes, symptoms, and treatments on our high TSH and high TSH symptoms pages.
Low TSH Levels
Low TSH typically indicates an overactive thyroid or excess thyroid medication. Read our complete guides on low TSH and low TSH symptoms.
Factors That Affect TSH Levels
- Time of day: TSH is highest in the early morning.
- Pregnancy: Normal ranges drop significantly during the first trimester.
- Medications: Biotin, steroids, and thyroid medication can skew results.
- Illness: Non-thyroidal illness can temporarily lower TSH.
- Age: TSH tends to rise slightly in older adults.
Get the Complete Thyroid Picture
If your TSH is borderline or you have symptoms, a full thyroid panel with Free T3, Free T4, and antibodies can provide the clarity you need.
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