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What Is a Low Thyroid Number?

By Lisa Newcomb ; Updated July 27, 2017

A low thyroid number is when blood test results indicate your thyroid is not functioning properly. Low levels of the thyroid hormone in your blood will result in abnormal test results. There is treatment available for this condition.

The Thyroid Gland

The thyroid gland is located in your neck. It secretes thyroid hormones T3 and T4. These hormones help regulate your metabolism. The pituitary gland, located at the base of your brain, produces thyroid-stimulating hormones (TSH) that tell your thyroid gland to produce T3 and T4.

Normal Levels

Normal TSH levels are 0.4 to 4.0 mIU/L, according to MedlinePlus. Normal T3 levels are 100 to 200 ng/dL. Normal T4 levels are 4.5 to 11.2 mcg/dL.


Tests for thyroid function are done with blood draws. There is a test for the TSH hormone and one for the T3 and T4 hormones.

Low Levels

If your blood test shows low TSH levels, you may have a common condition called hyperthyroidism, which is when your thyroid gland produces too much T3 and T4. Low levels of T3 and/or T4, however, can indicate a condition called hypothyroidism.


Both hyperthyroidism and hypothyroidism can be treated with medications.

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