Functional Thyroid Testing Cheat Sheet

Functional Thyroid Testing Cheat Sheet

Functional thyroid testing, and thyroid testing in general, can be confusing! It can be a challenge to know what should be tested to assess your thyroid function, as well as how to dive deeper if your labs all come back “normal.” This can be especially challenging if you are experiencing any of the wide variety of thyroid-related symptoms, such as constipation, fatigue, menstrual irregularities, joint pain, low mood feeling cold, the list goes on.

Without a clear understanding of how to assess your thyroid function, and how thyroid dysfunction can develop, you may be left wondering how you can get to the root of your thyroid-related symptoms. That ends today!


Thyroid 101

The thyroid gland is part of our endocrine system, an intricate communication network operated by hormones. The brain is very intimately involved in the endocrine system, where different hormone cascades are ultimately controlled. 

When we think of the brain’s role in our hormones, we often think of the HPA (hypothalamic-pituitary-adrenal) axis, which is our body’s main stress response pathway. We also have what’s called the HPT (hypothalamic-pituitary-thyroid) axis, where signals from the brain are sent to the thyroid gland. The first step in understanding the thyroid is by noting that role of the brain’s stimulation of the thyroid, which is measured by thyroid stimulating hormone, or TSH.


Functional Thyroid Testing

Thyroid testing as a whole has not been utilized for very long when we take into consideration the timeline of Western medicine. It wasn’t until the 1970s, when the Whickham study was conducted to determine the prevalence of thyroid disorders in the community, that thyroid testing was really put on the map. 

In this study they looked at the role of thyroid stimulating hormone (TSH) and saw that concentrations above 2 are associated with an increased risk of hypothyroidism. TSH quickly became the norm to assess thyroid function. Doctors also test TSH because thyroid medications are primarily going to affect this number. 

Where this falls short is the fact that TSH is not a thyroid hormone. Rather, it is a brain hormone produced by the pituitary gland, and just like its name suggests, is a stimulating hormone. Therefore, in order to truly assess thyroid function we have to look downstream and take a functional approach to thyroid testing.

We know that these tests may not all be easily accessible to you. You may choose to begin with the basics and go beyond just TSH by looking at Free T4, Free T3, and Reverse T3. One step beyond that is to assess for any thyroid autoimmunity. To take it even further we can take into consideration additional testing in order to get a truly comprehensive assessment of thyroid health.


Where to Start: TSH and Thyroid Hormones

  • TSH
      • As mentioned previously TSH is not a thyroid hormone, but rather a pituitary hormone that tells us how loudly the brain is talking to the thyroid. TSH gives us good insight into the communication between the brain and the thyroid. It was also mentioned that levels above 2 mIU/L were found to increase your risk of hypothyroidism. For this reason, the functional reference range where we would like to see TSH is 0.5-2.5 mIU/L versus the range that you will see on conventional labs that goes up to 4.5 mIU/L. 
      • It can be confusing, but lower TSH levels correlate with higher thyroid function. Higher TSH levels correlate with lower thyroid function and lower thyroid hormone production. In order to properly identify this and really assess thyroid function, we have to look at Free T3 levels. 
  • Free T4
      • Free T4 is made by the thyroid gland after being stimulated by TSH. T4 is our inactive thyroid hormone, and has the possibility to go down one of two different pathways, either to T3 or Reverse T3. We like to see Free T4 above 1 nmol/L to reflect proper thyroid hormone production. Total T4 differs from Free T4 because Total T4 includes the hormones that are bound by binding proteins. Bound T4 cannot readily be converted to T3.
  • Free T3
      • Free T3 is our active thyroid hormone and the most important piece of the puzzle when it comes to thyroid function. We want most of our T4 to go down the route of T3, as this is what impacts our body systems and any potential symptoms. T3 regulates cellular function, the speed of digestion and metabolism, the menstrual cycle and fertility, hair and nail growth, and more!
      • Similar to T4, Free T3 is more informative than Total T3 in order to assess the amount of T3 that is available to be used by the tissues. There are situations in which we can have an excess of thyroid binding proteins, for example in the case of oral contraceptives or being in a high estrogen state. It’s ideal for Free T3 levels to be above 3 nmol/L.
      • With Free T3 levels is also where we will see the most impact on the dietary interventions that are implemented. 
  • Reverse T3
      • Reverse T3 is the second pathway that T4 can go down. Reverse T3 is the deactivated form of our thyroid hormones and is indicative of a stress response within the body. Reverse T3 can be elevated (above 15 ng/dL) in the case of nutritional deficiencies or underfueling, as the body’s way of protecting us and preserving nutrients. Note that because of this, Reverse T3 is not a good measure during times of acute stress because it will be elevated in response to that biological stress response. 


Assessing Autoimmunity

The next step in assessing thyroid health is looking into autoimmunity by testing for thyroid antibodies. In the case of autoimmune hypothyroidism, the immune system attacks and destroys thyroid hormones, leading to decreased levels of thyroid hormones. Autoimmune hyperthyroidism, on the other hand, involves the immune system attacking receptors on the thyroid gland to overstimulate thyroid hormone production. 

The development of autoimmunity usually occurs over time. Borderline elevated antibody levels still should be addressed to prevent further autoimmunity from developing. Another consideration is that if you already have an autoimmune condition, you may be more prone to developing another.

Anti-TPO Antibodies are most commonly tested, and can screen for both autoimmune hypothyroidism (Hashimoto’s thyroiditis) and autoimmune hyperthyroidism (Graves’ Disease). TPO (thyroid peroxidase) is an enzyme required for the building of our thyroid hormones. Anti-Tg Antibodies are more specific to Hashimoto’s and affect the thyroglobulin protein within thyroid cells. Anti-TR Antibodies are more specific to Graves’ Disease and affect the thyrotropin receptors to upregulate thyroid hormone production.


Comprehensive Assessment of Thyroid Health

Beyond TSH, thyroid hormones, and autoimmunity markers, we can look to different body systems to help determine the root cause of thyroid dysregulation. 

Sex hormone DHEA-S, when low, gives insight into chronically elevated cortisol levels, or chronic stress. With this information, we can turn to impactful nutrition and lifestyle strategies to help regulate the stress response.

70% of thyroid hormone conversion happens in the liver. If the liver is under an increased toxic burden from potential issues like mold illness, heavy metal toxicity, or endocrine disrupting chemicals, the portion of thyroid hormone conversion that happens there can be slowed down. With a test like the DUTCH Test, we can assess both phase 1 and phase 2 of liver metabolism to see if the liver needs additional support.

With 20% of thyroid hormone conversion occurring via gut microbes, it can also be valuable to assess the status of the gut microbiome. If we don’t have sufficient beneficial bacteria, this has the potential to slow down that T4 to T3 conversion.

Mineral deficiencies can both cause hypothyroidism and also block the body’s ability to convert T4 to T3. Iron, calcium, magnesium, potassium, selenium, and zinc are all crucial minerals for proper thyroid function. These minerals can be tested through a Hair Tissue Mineral Analysis (HTMA) as well as through red blood cell testing. Iodine, another key thyroid mineral, is most accurately tested via urinary iodine excretion. Too much or too little iodine can cause hypothyroidism, so it’s important to ensure iodine levels are balanced.


How Nutrition Can Influence Thyroid Hormone Levels

Alterations in thyroid function often point to underlying stressors. Those may be nutrition stressors, like long term carbohydrate restriction, nervous system stressors, biological stressors, such as gut dysbiosis or toxin exposure, and more. We have to go beyond thyroid testing alone to identify the root causes as to why those labs may be out of range. 

Carbohydrates are an essential micronutrient for T4 to T3 conversion, as well as suppression of cortisol. We can see low Free T3 levels as a result of low carbohydrate diets and dietary restriction. 

Minerals such as magnesium, selenium, and zinc also all have a direct impact on the conversion of T4 to T3. And as previously mentioned, too much or too little iodine, which is essential for thyroid hormone production, can cause hypothyroidism. Also to note, is that iron deficiency can cause an elevation of TSH, reflecting slower thyroid function. 

Due to the nature of the hormone cascade, the effects of thyroid dysregulation do not stop at the thyroid. We have to consider other downstream effects of hypothyroidism especially, particularly as it relates to blood sugar balance. Hypothyroidism effectively slows metabolism and can affect the nervous system, potentially leading to elevated cortisol levels. When cortisol is elevated, glucose is released from tissues in the form of glycogen and enters our bloodstream. Chronically elevated blood glucose can lead to insulin resistance.

Another nutrition consideration is how our dietary patterns influence our circadian rhythm, and how our circadian rhythm influences our hormones. The HPT axis is regulated by our circadian clock, and TSH secretion has been shown to exhibit a clear daily rhythm. Supporting our circadian rhythms by way of morning sunlight exposure and eating within the first hour of waking can positively impact thyroid dysregulation. 

Finally, inflammation can play a role in elevated TSH and low Free T3 levels, as the body tries to protect against the stressors that are causing that inflammation. Anti-inflammatory nutrients, such as omega-3 fatty acids (found in salmon, mackerel, sardines, chia and flax seeds), antioxidants (found in cold-pressed extra virgin olive oil, leafy green vegetables, dark colored berries), as well as ginger and turmeric, can all help to lower inflammation levels.


Get the Testing You Need!

Hopefully now you have a better understanding of how to assess your thyroid function, as well as how to determine the best next steps if you are experiencing thyroid-related symptoms. We know that many doctors will not routinely test markers other than TSH. 

Now you know how to level up your functional thyroid testing and advocate for the testing you need. Once we dive deeper into advanced testing to better understand the root causes of thyroid dysfunction, we can leverage nutrition and lifestyle strategies to make significant improvements. This is why we recommend working with a practitioner 1:1 to provide the support and personalized guidance to get your thyroid thriving.


Written by Romana Brennan, MS, RD

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