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What Are the Causes of Low TSH?
It's difficult for many patients to understand how low TSH levels are connected to the unpleasant physical signs and symptoms they've described to their doctor 1. The amount of thyroid-stimulating hormone, or TSH, present in your bloodstream has a direct link to thyroid function--and your overall metabolism 12. Many underlying medical conditions can cause low TSH levels, so it's helpful to first understand the nature of TSH and why the TSH test is a practical diagnostic tool for your physician 1.
The Importance of TSH
Your thyroid gland, a small butterfly-shaped organ at the front of your neck, has but one duty: to convert your iodine intake into the thyroid hormone that regulates all aspects of your body's metabolism 2. However, the pituitary gland in your brain plays a critical role in thyroid gland functioning, too 2. Whenever the thyroid hormone is running low, the pituitary gland releases TSH to stimulate the thyroid gland into producing more 12. Once everything is in balance, the pituitary gland stops producing TSH 1.
Low TSH Levels: What They Can Mean
Low TSH can indicate that there's an abundance of thyroid hormone being produced by the thyroid gland, so the pituitary gland sees no need to release TSH 12. However, to confirm this, a test called an FT4--this measures the amount of thyroxine, one of the hormones that make up the thyroid hormone--is often run concomitantly with a TSH test 12. Low TSH and high FT4 levels are usually seen in people with a condition called hyperthyroidism, the end result of an overactive thyroid gland 123. Low TSH levels can also indicate that the pituitary gland itself is damaged; however, this is very rare 1.
About Hyperthyroidism
Hyperthyroidism refers to a physical condition with distinct signs and symptoms 3. You may note excessive sweating, sensitivity to heat, sudden weight loss, tremors in the hands, inability to fall and remain asleep, anxiety, and nervousness. Women with hyperthyroidism may note a light or absent menstrual period 3. However, hyperthyroidism isn't caused by one medical condition alone 3. Narrowing down the list of possible medical conditions that cause low TSH--and hyperthyroidism--often requires additional testing 13.
Other Tests
Often, a thyroid antibody test--another blood test--is conducted to determine if your condition is caused by an autoimmune disorder such as Graves' disease, a frequent cause of hyperthyroidism 3. A radioactive iodine uptake test or thyroid scan is also helpful in determining how well the thyroid is functioning, as well as allowing your physician to visualize the characteristics of the thyroid 12. Sometimes hyperthyroidism is caused by thyroid nodules, which don't have a distinct autoimmune component 3. A biopsy may be in order to make sure that the nodules are benign and not cancerous.
TSH & Ongoing Treatment
Once the cause of your thyroid disorder is identified, TSH testing continues to be an excellent way to monitor the success of treatment 12. For example, patients who take antithyroid medications or use other therapies to treat Graves' disease are required to have regular TSH testing to make sure that their treatment is working effectively. To read more about TSH testing and its role in treating your thyroid condition, see the Resources link 12.