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As the title suggest is it necessary to fast before thyroid test. Last week I have given thyroid test in my office. They conducted T3,T4 and TSH testing and my result came as thyronormalcy. I didnt fast before giving this test. My friend planned to take the test in a test lab outside (not from my office). The lab technician asked her not to drink water or eat any food before giving the test. I am confused now. Is it necessary to fast or not?

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According to a study published in 1994 entitled "Serum TSH variability in normal individuals: the influence of time of sample collection" by Scobbo et al., there were significant differences between late-morning non-fasting TSH and early-morning fasting TSH tests:

The late morning non-fasting TSH tests declined in 97 of 100 subjects by an average of 26.39% when compared to early morning, fasting, TSH test results. This lead to reclassification of 6% of patients from presumptive subclinical hypothyroidism to "normal." Since the time of day of phlebotomy or the fasting or non-fasting status of the patient, or both, can significantly affect serum TSH test results, the diagnosis of subclinical hypothyroidism should not be made only on a fasting TSH measurement.

A 2012 study in Endocrine Research by a Russian group looked at TSH levels throughout the day in women with hypothyroidism compared to another group taking L-thyroxine replacement therapy (treatment for hypothyroidism). They found the range of TSH variability between morning and afternoon blood draws reached 73% in the hypothyroid group, and 65% in the treated group, with higher levels observed in the morning. This confirms previous data showing significant circadian variability of TSH, so the time of the blood draw probably influences the results more than the intake of food. Scobbo's study did not correct for time of draw (early AM without food vs. late AM with food), so one can't even reasonably conclude from their data that the presence of food actually affected the results.

So, from the available data, if you test later in the day and you've had food, you are likely to have a lower TSH level than if you test earlier in the day with fasting. Any diagnosis of hypothyroidism shouldn't be made on the basis of a single test, anyway, so if your levels of TSH are higher than normal, your doctor will order one or more additional tests to confirm the diagnosis, along with taking into account any other symptoms that may be present, such as:

  • Feeling tired, weak, or depressed.
  • Dry skin and brittle nails.
  • Not being able to stand the cold.
  • Constipation.
  • Memory problems or having trouble thinking clearly.
  • Heavy or irregular menstrual periods

from WebMD

As is the case with with any type of medical intervention, you should always follow your doctor's or the lab's instructions when undergoing any type of test, particularly concerning fasting, the time that you should take your medication before the test, and the time that the test should be performed. If in doubt, please ask!

This post should not be construed as, nor is it intended to be medical advice. Always talk to your doctor or other healthcare professional about medical issues, including test results and symptoms.

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