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What Are Normal Thyroid Levels?

Updated February 21, 2019

The thyroid gland is located in the front of the neck. Its main job is to secrete hormones that help to regulate your metabolism. These hormones also play a role in the normal development of organs such as the heart and brain, as well as for normal reproductive functioning. Proper levels are also essential for bone and calcium metabolism. The hormones it secrets are thyroxine (or T4) and triiodothyronine )or T3). If the levels of these hormones are too high or too low, you may develop serious health problems.

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Your thyroid levels are checked by a simple blood test performed by your doctor. This test can tell your physician if your thyroid gland's hormone production is normal, overactive or underactive.

What Does the Test Detect?

In order for your thyroid gland to do its job, your hypothalamus must release thyrotropin-releasing hormone (TRH). TRH then triggers your pituitary gland to produce thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to make T3 and T4. To determine if your thyroid is functioning properly, the blood test checks your levels of TSH.

Normal Levels

In adults, normal TSH levels are 0.4 to 4.5 mIU/L. In infants, normal levels are 3 to 18 mIU/L. The abbreviation mlU/L stands for milli-international units per litre. Levels outside these ranges are classified as hypothyroidism or hyperthyroidism.


This condition occurs when there is an overproduction of thyroid hormones. Symptoms include muscle weakness, trembling, fatigue, rapid heartbeat, weight loss, sensitivity to temperature and vision problems.


This condition is diagnosed when there is underproduction of thyroid hormones. Symptoms include fatigue, weight gain, feeling cold and sluggish, dry skin and hair and constipation.

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About the Author

I hold a Master's degree in exercise physiology/health promotion. I am a certified fitness specialist through the American College of Spots Medicine and an IYT certified yoga teacher. I have over 25 years experience teaching classes to both general public and those with chronic illness. The above allows me to write directly to the reader based on personal experiences.

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