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January 27, 2022 | Blood Tests

What is a TSH Blood Test?

Woman getting thyroid checked by healthcare provider for thyroid testing.

The thyroid is a small butterfly-shaped organ in the neck that’s a part of our endocrine system. Despite its small size, it plays a central role in maintaining energy levels, homeostasis and overall health. The thyroid gland’s main function is the production of hormones which help regulate your body’s temperature, metabolism and heart rate. These hormones are delivered by the bloodstream to fulfill roles throughout the body. Below, we explain the top things you should know about thyroid testing to monitor thyroid gland function, what different tests measure, and how to interpret test results.

What Healthy Thyroid Gland Function Looks Like

The production of thyroid hormones is regulated via a negative feedback loop working with the hypothalamus and the pituitary gland. The process looks something like this:

  • The pituitary gland produces a hormone called TSH, or thyroid-stimulating hormone. This hormone triggers the thyroid to produce several hormones, including one called thyroxine (T4). Thyroid testing can help reveal if you have normal TSH levels.
  • The level of T4 in the blood is detected by the hypothalamus which sends a signal to the pituitary gland to produce TSH. If T4 levels are low the hypothalamus sends a signal to the pituitary gland to produce TSH.  If T4 levels are high or normal the hypothalamus does not send the signal and the pituitary gland does not release TSH. As a result, low or high production of either TSH or T4 can lead to hormonal imbalance.

Thyroid Testing with the TSH Blood Test

Because the bloodstream plays such a critical role in the transportation of these hormones, thyroid testing with blood tests are a useful tool to monitor thyroid gland function. A blood test to measure thyroid function can help your healthcare provider determine whether you have normal TSH levels.

What Does High TSH Mean?

High TSH in the bloodstream indicates low thyroid hormone production by the thyroid. This condition is called hypothyroidism. If your TSH levels are high, your healthcare provider may recommend medication to help restore normal TSH levels.

The most common symptoms of hypothyroidism are:

  • Sensitivity to cold
  • Weight gain
  • Dry skin
  • Hair loss
  • Hoarse voice
  • Fatigue or weakness
  • Memory loss
  • Irregular or abnormal menstrual periods

What Does Low TSH Mean?

If thyroid testing determines that you have low TSH, it means that your thyroid is producing too much thyroid hormone; a condition known as hyperthyroidism. If your healthcare provider determines you have an overactive thyroid, they may recommend medication to restrict thyroid hormone production by the thyroid to help you return to normal TSH levels.

The most common symptoms of hyperthyroidism are:

  • Trouble sleeping
  • Irritability
  • Sensitivity to heat
  • Muscle weakness
  • Unexplained weight loss

Do I Need to Fast Before a TSH Test?

Fasting isn’t necessary before thyroid testing. However, if your healthcare provider has ordered additional tests from the same blood sample, they may ask you to stick to a clear diet for 8 to 12 hours. Always follow the recommendations of your healthcare provider to ensure accurate test results.

Next Steps for Thyroid Testing

If you have a lab order from your healthcare provider for thyroid blood tests, Scarlet® can help. A Scarlet Health Professional will visit you at your home or workplace, complete the blood draw, and deliver the sample to BioReference Laboratories for testing in order to determine whether or not you have normal TSH levels.


Thyroid Testing Sources:

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