What Does a Low TSH Mean?

Thyroid stimulating hormone (TSH) is secreted by the pituitary gland. The function of TSH is to stimulate the thyroid to produce more thyroid hormone.

Low TSH

A low TSH level means too much thyroid hormone is circulating in your blood.

How It Happens

When your thyroid gland is overactive and producing too much thyroid hormone (hyperthyroidism), your pituitary gland decreases the amount of TSH to slow down production of thyroid hormone, creating a low TSH level.

Causes

According to the University of Michigan Health System, possible causes of hyperthyroidism are taking too much thyroid replacement hormone, an infected or inflamed thyroid gland, a thyroid gland that has grown too large or a tumor in the thyroid gland that is producing extra thyroid hormone.

Additional Testing

If your TSH level is low, your doctor may do additional testing to find the cause.

Considerations

According to the Alt.Support.Thyroid Newsgroup, testing TSH doesn't always provide an accurate indicator of thyroid health. Because TSH is a pituitary hormone, if the pituitary gland or the hypothalamus (which controls the pituitary) has a problem, TSH could test normal, but your thyroid hormones (T4 and T3) could be too low or too high.

References

• Thyroid-Stimulating Hormone (TSH) Test
• The TSH Story