Blood test results are an important tool in healing Hashimoto's, but are reliable only if you prepare properly for them and interpret them correctly. 

Blood tests are the mainstay of health and wellness as they can provide an objective measure of the state of our health and can be an invaluable diagnostic tool. And because it is an objective measure, it’s routinely used to check if any changes that we’ve made to our nutrition, medication, supplements, and lifestyle are having a significant effect on our health. However, there are a number of reasons why blood tests can show erroneous results. Some of these reasons concern what you do before having the blood test and some in the interpretation of the tests.

Here are 5 ways blood test results can show erroneous results...

Before the Test

1. Not Preparing for the Blood Test.  

There are a number of activities prior to the blood draw that can affect or invalidate the test results. The most important include the following:

  • not being hydrated. Not drinking enough fluid can cause a decrease in plasma volume which has the effect of increasing the concentration of red blood cells and other constituents of the blood. It’s best to drink water the night before the blood test and avoid alcohol (which can increase levels of uric acid, lactate, and triglycerides). Remember, you want your results to provide insight into your health status, not how much you drank the night before.
  • not fasting. An overnight fast is recommended for an analysis of plasma glucose, lipids, iron, iron-binding capacity, vit B12, folate, insulin, and gastrin.
  • exercising before the test. Testing done immediately after strenuous exercise may show elevated liver enzymes, lactate, phosphorous, creatinine, uric acid, and others.

2. Not taking the blood draw at the correct time

The levels of many of the things that we test for fluctuate significantly over the course of a day – this is the circadian rhythm of our body. This means that in order to compare test results, they need to be taken at the same time for every subsequent test. The table below shows some of the normal daily changes for common tests.  Note the daily change for TSH which is one of the most widely used tests for thyroid disorders and is used to adjust medication levels.  Cortisol is another increasingly common test that shows wide fluctuations throughout the day.


Time of Peak Concentration

Daily Percent Change


1 pm



11 am



6 pm


Total Bilirubin

7 am



10 am



2 am



7:30 am



3 am



8 am



8 am


If you want to compare current and previous test results, make sure that you have the tests done at the same time of day.  If not, you won't be able to differentiate between change due to any health intervention and normal daily variation.

3. Taking Supplements that Interfere with Analyses

While the potential effect of other drugs on laboratory tests is enormous (and your doctor should be able to advise you on this), commonly used supplements can also have significant effects. One of the most important is Biotin (vit B7).

Biotin use can interfere in the tests for anemia, malignancies, autoimmune and infectious diseases and cardiac disease. There have even been reports of wrongful diagnoses of Graves’ disease due to biotin-containing supplements prior to testing.

Biotin does not interfere with laboratory tests at levels naturally found in foods and most over-the-counter multivitamins.  However, vitamins with a therapeutic dose (generally more than a daily consumption of 30 micrograms) will have a significant effect. In  order to avoid interference with test results, it’s recommended to stop consuming biotin 3 days prior to testing.

Getting a reliable and useful result from your blood tests requires taking some time to adequately prepare.

Errors in Interpretation

4. Not using the Best Reference Ranges

You might be surprised to know that the reference ranges commonly used in lab reports are not based on optimal health. In fact, they are a statistically-derived numerical ranges obtained by testing a number of individuals assumed to be healthy (i.e. showing no symptoms).  These reference ranges are usually established by obtaining an average value plus and minus two standard deviations (SD), which follows a normal distribution. However, there is a difference between being asymptomatic and enjoying good or optimal health.  Using these reference ranges can obscure potentially developing health issues.

Optimal ranges are available. These ranges have been studied as far back as the 1980’s starting with the work of Dr. Harry Eidenier. Information obtained from the blood tests was compared to the history intake forms, physical examinations findings, symptom analysis, urinalysis, hair mineral analysis, stool analysis, and other diagnostic criteria. Over 10,000 patients were analyzed and optimal ranges were calculated. Most functional medicine practitioners use optimal ranges.

5. Not Tracking Your Results over Time

It’s always a good practice to get a copy of your lab results and track any changes over time. Unfortunately, many medical practitioners only check to see if values are outside of the stated reference ranges. However, in many cases the absolute value of the result may not be as important as whether it is changing over time. Wouldn’t you want to know if a situation was becoming serious before it became a problem – this is prevention in action.

Many labs already provide results in a convenient graph form. If yours doesn’t, it only takes a few minutes to pop the values onto a spreadsheet.

You will find this a valuable tool for you and your medical practitioner to evaluate the efficacy of any nutritional/lifestyle changes or treatments that you have undertaken.


Blood tests are can be a useful diagnostic and tracking tool but only if care is taken in both preparation for the test and in the interpretation of the tests.  If you have any questions, you can contact me here.

Book a free 20-min consultation here:
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About the Author Sharon Walt

Dr Sharon is a certified Functional Medicine Health Coach who helps men and women with autoimmune disorders, such as Hashimoto's Thyroiditis, regain their health and start living life to the utmost again.