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Is Fasting Before A Thyroid Blood Test Necessary

Is Fasting Before A Thyroid Blood Test Necessary?


In the US, an estimated 20 million people are suffering from one form or another of thyroid disease. Of the said number, around 40% receive a diagnosis of thyroid disorders after running thyroid blood tests. Thousands of thyroid blood tests are performed in the US alone each year. The possibility of one developing thyroid disease increases with age; nearly every 1 in 4 adults above the age of 65 presents with some kind of thyroid dysfunction.

However, to be diagnosed with thyroid dysfunction, one should first get tested. Should I fast before a thyroid blood test is one of the common concerns that people have. In this article, we address concerns regarding whether you need to fast before a thyroid blood test, medications to avoid before a thyroid blood test, and debunk myths surrounding thyroid tests and pregnancy.

What is a Thyroid Blood Test?

A thyroid blood test measures levels of a specific hormone in our body, known as the thyroid hormone or thyroid-stimulating hormone (TSH). For this reason, the thyroid blood test is also commonly referred to as a TSH test or a thyrotropin test. The thyroid is a small gland situated near your throat and produces hormones, which regulate a person’s mood, the body’s usage of energy, body weight, body temperature, and muscle strength.

Contrary to common belief, TSH is made by the pituitary gland in the brain, and not in the thyroid gland in the neck. Low thyroid levels in your body stimulate the release of TSH from the pituitary gland. Similarly, high levels of thyroid in your body will send signals to your pituitary gland to produce less TSH. levels of thyroid-stimulating hormone in the body deviating from the normal range indicates a thyroid dysfunction.

When Do You Need A Thyroid Test?

When your body has too little or too much thyroid hormone in the blood, it shows obvious symptoms. In cases where the thyroid hormone in your blood is lesser than normal (hypothyroidism), you may observe one or more of the following symptoms:

  • Constipation
  • Decreased tolerance for low temperatures
  • Feeling lethargic or tired
  • Hair loss
  • Irregular menstrual cycle in women
  • Weight gain

Symptoms of other cases where you may develop hyperthyroidism, and increased blood level of thyroid hormone, include:

  • Anxiety
  • Bulging of eyes
  • Difficulty sleeping
  • Increased heart rate
  • Puffiness or soreness in different parts of the body
  • Tremors in hand
  • Weight loss

If you present with any of the above-mentioned symptoms, whether for hyperthyroidism or hypothyroidism, and suspect that you have thyroid dysfunction, we suggest getting a thyroid blood test. Read ahead to find out whether you need to fast for a thyroid blood test.

The Different Types of Thyroid Blood Tests

A TSH test or a blood thyroid test only states the amount of thyroid-stimulating hormone or thyroid hormone present in the blood and shows whether or not it lies in the normal range. A skilled medical practitioner diagnoses any underlying conditions that lead to abnormal results, if any.

Your doctor may advise you tests to find out values for either of the two or both hormones – thyroid and TSH. A low TSH level means your thyroid is making high levels of thyroid hormones, as seen in hyperthyroidism. Similarly, a high TSH level implies that your thyroid is making low levels of thyroid hormones, as seen in hypothyroidism. While thyroid blood tests often do suffice for the doctors to make a clear diagnosis of the thyroid problem, additional tests may have to be ordered to reach a proper diagnosis in some cases. These tests may include, but are not limited to:

  • T3 thyroid hormone tests
  • FT3: free T3 or free triiodothyronine
  • T4 thyroid hormone tests
  • FT4: free T4 or free thyroxin
  • Thyroid antibodies

To diagnose Hashimoto’s thyroiditis (hypothyroidism causing autoimmune disease) and Grave’s disease (hyperthyroidism causing autoimmune disease)

  • Thyroglobulin

To diagnose thyroiditis or thyroid inflammation

How Many Hours Prior Do You Need To Fast For A Thyroid Blood Test?

A common question regarding thyroid tests is something along the lines of do you need to fast for a thyroid blood test. According to doctors and medical practitioners, no special precautionary measures are to be taken before getting a thyroid blood test. In fact, in many cases, doctors object to fasting before a thyroid blood test. Research suggests that TSH test results during fasting, or early morning without breakfast, show different results compared to those obtained on a full stomach, in the afternoon. It is suggested to not fast before a thyroid blood test, as it provides misleading results of increased TSH levels in the body. 

However, if you are undergoing some health tests other than thyroid blood tests and fasting is required, you may consult your doctor and fast for 8-10 hours before getting the test done. Always remember that fasting for a blood test excludes drinking water.

Abnormal Results of a Thyroid Blood Test

Since the most frequent concern, “do you need to fast before a thyroid blood test” has been addressed, let us move on to the three types of results of a thyroid blood test. Your thyroid blood test either shows a healthy result (where the thyroid hormone is present in the normal range) or an abnormal result (where the thyroid hormone is higher or lower than the normal). In the latter results, an abnormal result could be due to thyroid malfunction indicating either of the two –hypothyroidism and hyperthyroidism

A case where a high concentration of thyroid hormone is present in the blood is known as hyperthyroidism. Conditions that lead to this alleviation of blood thyroid hormone may include:

  • Excessive iodine
  • Grave’s disease
  • Thyroiditis
  • Thyroid Nodules

On the other hand, a condition where a lower than normal concentration of thyroid concentration is present in the blood is called hypothyroidism. Conditions that can lead to a decreased value of thyroid hormone in blood include:

  • Iodine deficiency
  • Non-functioning thyroid gland
  • Thyroiditis
  • Postpartum thyroiditis
  • Hashimoto’s thyroiditis

Medications to Avoid Before a Thyroid Blood Test

Similar to questions such as do you need to fast before a thyroid blood test, many people are concerned about any medications that may negatively affect or interfere with your thyroid test results. And similar to the effects of fasting, there are some medicines known to show anomalies in the results of thyroid blood tests.

One of these is a common over-the-counter daily supplement taken by many; Vitamin B7 or Biotin. Biotin may alter and show high values of thyroid in blood when otherwise the results lie in the normal range. As suggested by doctors, biotin should be avoided at least 2 days before getting tested for thyroid dysfunctionality.


If you are showing signs or symptoms of thyroid dysfunction, it is best to get tested. If you are above the age of 50, you should get routine screenings, which include thyroid tests, regardless of whether you present with any symptoms. It is essential to understand that an abnormal reading of your TSH or thyroid hormone does not necessarily imply the presence of a thyroid disorder. A thyroid test is likely to be affected by numerous factors, such as fasting, as mentioned above.

The fact that you do not need to fast for a thyroid blood test and can get it done at any time of the day makes it more feasible and effortless.

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