7 Causes Behind Low Thyroid Function

7 Causes Behind Low Thyroid Function

This article is about the 7 causes behind low thyroid function, and also how your thyroid is the underlying cause of a wide range of health problems.

We look at addressing the underlying causes so that you can get your thyroid and health back on track.

Also Read: Thyroid Gland Disorders: Symptoms, Causes, and Treatment

Low thyroid function can affect every aspect of your health, this small gland about the size of a walnut in your neck helps to control energy production, metabolism, gut function, and mood, so it is a big deal when it is not working optimally.

With an estimated 60% of people with low thyroid function undiagnosed, this could be the cause of your health problems.

If your doctor has told you that your thyroid is “fine”, read on to see why this may not be the reality.

Symptoms of low thyroid function include:

  • Tiredness and fatigue
  • Weight gain and hard to lose weight
  • Difficulty focusing
  • Trouble sleeping
  • Depression
  • Joint and muscle pain
  • Frequent and heavy periods
  • Sensitivity to cold temperatures

These symptoms can be connected to other imbalances in the body like infections, adrenal dysfunction, poor diet, insomnia, neurotransmitter problems and sex hormone imbalances. So how do we know that your thyroid is the culprit which is causing you to feel so crappy?

Testing low thyroid function

When I mention the possibility of low thyroid function to clients I am often told that their thyroid has already been tested by their doctor, and have been told that everything is “ok” with their thyroid.

But the doctor is only checking TSH levels, which is the brain telling the thyroid gland to make more T4 thyroid hormone. The TSH only measures the messages from the brain, and does not actually measure your thyroid hormones. TSH is a good indicator of thyroid hormone but it does not give us the full picture, and many cases of low thyroid function can get missed.

It is important when testing thyroid function to measure TSH, Free T4, Free T3, Reverse T3 and the thyroid antibodies, at Planet Naturopath we can organize this testing for you as most doctors will not do it, as they are restricted by medicare and insurance.

Find out about thyroid testing here

Thyroid test example

Normal TSH and low T3

When inflammation is present which can be caused by gut dysfunction, insulin resistance, obesity and infections this will lead to T4 being converted to the inactive Reverse T3, rather than the metabolically active T3 hormone. This is known as low T3 syndrome, and the answer is not to provide more thyroid hormones, but to address the drivers of inflammation which we will discuss soon.

Usually when reverse T3 is high and T3 is low you will have normal TSH levels, this is why you can have all of the low thyroid symptoms but your TSH is normal.

Thyroid antibodies

The thyroid antibodies are rarely tested by doctors unless the other markers of thyroid function are out of balance, yet the antibodies can be the best predictor of future thyroid disease, as it is already “under attack” from your own immune system. Knowing this information can enable you to make preventative diet and lifestyle changes to help improve thyroid function.

High thyroid antibodies are used to diagnose Hashimoto’s Thyroiditis. In the late stages of Hashimoto’s thyroid medication maybe necessary, but if you identify it early you can often reverse the autoimmune process by addressing gut function and dietary triggers.

Why is your thyroid so important?

If having symptoms of low energy, low mood, and weight gain from having a low thyroid function was not bad enough, there are other chronic health conditions that a low thyroid function can cause.

Increased risk of cardiovascular disease

There are multiple ways that a low thyroid function affects your cardiovascular health and this includes

  • Increased triglycerides and small dense LDL particles (the atherogenic ones)
  • Diastolic hypertension, this increased blood pressure can lead to arterial damage
  • Endothelial dysfunction caused by high blood pressure and increased insulin resistance
  • Increased risk of blood clots from the oxidative damage


Low thyroid function can also be the underling cause Small Intestinal Bacterial Overgrowth (SIBO) and Irritable Bowel Syndrome (IBS) due to the reduced motility of the digestive tract. These conditions can cause constipation, diarrhea, bloating, gas and reflux which can be uncomfortable and even debilitating.

The inflammation that this causes leads to nutritional deficiencies, inflammation and oxidative stress which can further worsen your thyroid function so that it becomes a vicious cycle.


Low thyroid function is one of the causes of diabetes and the many health problems associated with this condition. With low thyroid function you are going to have a reduced metabolism, this will lead to weight gain, inflammation and further insulin and leptin resistance which will make it even harder to lose weight, even when you try and reduce calories.

7 Causes behind low thyroid function

The first step when your thyroid hormones are out of balance should not be to start a prescription of thyroid medication, but to find out why your thyroid hormones are not optimal. Some people do need to take thyroid medication but by addressing the underlying cause you are going to be able to make it work more effectively.

Here are the 7 areas that I would address to optimize your thyroid and health.

1. Nutritional Deficiencies

Identify any nutritional deficiencies with a comprehensive pathology test. This test should include the following nutrients which are important co-factors in T4 production and the conversion of T4 to the metabolically active T3.

  • Vitamin D
  • Vitamin B12
  • Folate
  • Selenium
  • Zinc
  • Iron Studies
  • Magnesium
  • Iodine loading test

2. Calorie restrictions

Going on a low calorie diet can help with weight loss in the short term, but it can often mean a down regulation in thyroid hormone which can lead to a weight loss plateau and even weight gain.

A low calorie diet specifically reduces T4 to T3 conversion and increase in reverse T3 and the body slows down metabolism to conserve energy. The TSH levels often stay in the normal range which can give make it look like your thyroid is still ok.

3. Low carbohydrate diet

A low carbohydrate diet can be a great way to lose weight and improve energy levels, but if you already have borderline low thyroid levels or a number of other stressors in your life then a low carb diet can be a disaster for your thyroid.

Similar to a low calorie die the low carb diet can affect the conversion of T4 to T3, this does not mean that you have to be on a high carb diet but you do need to keep your carbohydrates in the 100-150 grams a day range.

4. High estrogen levels

High estrogen decreases sodium-iodine symporter gene expression, this reduces iodine uptake by the thyroid leading to an increase in thyroid binding globulin. The increased thyroid binding globulin leads to reduced free T3 levels.

Testing estrogen levels, as well as how your body is metabolizing estrogen is an important first step to getting your estrogen levels in balance. The best way to test estrogen and estrogen metabolism is with the DUTCH hormone test.

5. Inflammation

The thyroid is particularly sensitive to inflammation and oxidative stress. High levels of C-reactive protein (CRP), Interleukin-6 (IL-6) and Tumor necrosis factor (TNF) decrease T3 levels and increase the metabolically inactive reverse T3.

The key is to identify the cause of the inflammation which may come from food intolerances, gut function or environmental toxins.

6. Gut dysfunction

A healthy digestion is the key to good health overall, more and more studies are associating your gut bacteria to mood disorders, skin problems, autoimmune conditions (including Hashimoto’s thyroiditis) and hormone imbalances.

The G.I Map test and Small Intestinal Bacteria Overgrowth (SIBO) test can help to identify the underlying cause of digestion problems. These are tests that I would recommend even if you do not have digestion symptoms, as you can be symptom free but still have a bacteria overgrowth, on not enough of the beneficial bacteria.

7. Environmental toxins

In the last three decades the average age of developing an autoimmune thyroid condition has reduced by 20 years, in this same time period our exposure to toxins is ever increasing.

Toxins can directly disrupt the pituitary from releasing TSH, as well as affect the T4 to T3 conversion which will lead to low thyroid function.

What Next?

If you suspect that you have low thyroid function the first step is to get a comprehensive thyroid test including TSH, T4, T3, reverse T3 and the thyroid antibodies. You can find out more about the extensive thyroid test here.

Once you get the results it is important to work with a functional medicine practitioner who will be able to guide you on the optimal ranges for your results, as the reference ranges are too broad. At Planet Naturopath we can help guide you with the interpretation of tests and a treatment plan.

If the test results show that your thyroid levels are not optimal, the next step is to find out why by investigating the 7 causes that I listed above, and for most people this should be done before you go on thyroid medication.

Schedule a consultation to help you work out the best test options for you.

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