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Could it be your thyroid? Low Thyroid Function and Infertility

Updated: Nov 24, 2023

Low thyroid function or hypothyroidism (an underactive thyroid) is very common in females and can be an undetected contributor to fertility problems and recurring miscarriage. Hyperthyroidism (an overactive thyroid) can also be problematic but not as common so I am going to focus on hypothyroidism in this blog post.

I wrote this post primarily for my fertility followers; however, it can be helpful for anyone who is concerned about their thyroid.

I spent 10 years trying to get pregnant with baby number 2 and during this journey I often had a gut feeling that I was having issues with my thyroid, although it was hard to prove as the limited tests that my GP and fertility specialist were prepared to do always came back within normal range. This is a worldwide problem that affects many women as conventional thyroid testing unfortunately is unable to give a full picture of what is going on with the thyroid. As a result, many women live their lives with undetected subclinical hypothyroidism, wondering why they often don’t feel great, are constantly fatigued and just can’t lose weight despite a regular exercise programme and a healthy diet.


Tests via your GP

When your GP tests for your thyroid function they will mostly only test TSH (Thyroid Stimulating Hormone) and if there is an abnormality with TSH free T4 (the thyroid hormone thyroxine). TSH is a pituitary hormone that stimulates the thyroid gland to produce thyroid hormone when needed. TSH ideally needs to be between 1.0 and 2.0 for optimum function, anything above 2.5 indicates some level of hypothyroidism. The problem is many women have normal TSH levels and still have symptoms of low thyroid function. Unfortunately, in these cases testing just TSH highlights what the pituitary gland is doing and not the actual function of the thyroid. Testing just Free T4 (Thyroxine) as well isn’t really a reliable indicator of thyroid function as T4 is an inactive hormone that needs to be converted to T3 (triiodothyronine) to become active. So testing Free T3 the active thyroid hormone is going to give a better picture of thyroid function. Unfortunately, it is really hard to get a Free T3 test from your GP unless you push really hard for a full thyroid panel and it is not always easy or successful. This is why so many women have undiagnosed subclinical thyroid problems.

There are a number of factors that can impair the conversion of T4 to T3 in the liver. These are: ageing, stress, severe injury, calorie restriction and fasting, poor gut health, an imbalance of gut bacteria, chemical or toxic metal exposure, lack of antioxidants, increased free radical damage, selenium deficiency, high alcohol intake, liver or kidney disease, severe illness and surgery.

I pushed for full thyroid testing on a number of occasions and my GP said she would run the tests. When I asked for the results though on many of those occasions, I was frustrated to find that just the TSH and Free T4 was tested. You really need to be your own advocate and push really hard to get the tests done and ask for a copy of your results, don’t just accept over the phone that they are in the normal range.

The full thyroid panel test also includes thyroid antibodies TPO (Thyroid Peroxidase Antibodies), which is really important to test to see if an autoimmune issue such as Hashimoto’s Thyroiditis is affecting the function of the thyroid gland. Hashimoto’s is a common autoimmune condition in females that can contribute to fertility problems and the only way to identify this is to test for thyroid antibodies. This is a particularly important test to do if there is a family history of autoimmune conditions and Celiac Disease.

Reverse T3

A full thyroid panel will also test for reverse T3, which is an inactive form of T3 (triiodothyronine). In a normal healthy person, T4 is converted into both T3 and RT3 (Reverse T3) continually and the body is able to eliminate RT3 quickly. In some situations though the production of RT3 increases, which reduces levels of the active T3 in the body. This slows down metabolism and results in hypothyroid symptoms (this is known as thyroid resistance).

The key issue with high levels of RT3 is that it will bind to T3 receptor sites in the body, but is inactive so has no activity. So essentially high RT3 levels can give you symptoms of hypothyroidism even though your other thyroid hormones TSH and Free T4 (the ones usually tested by GPs) are in the normal range.

So what causes high levels of RT3?

The main causes are:

  • Stress – our stress hormone cortisol increases the conversion of T4 into Reverse T3

  • Chronic Illness

  • Low calorie diets

So if you do have a high RT3 test then your focus needs to be reducing your stress levels in any way you can and making sure you are eating well, not on a low calorie or too low carb diet.

Stress is not just being busy. If you have an infection, gut issue or food intolerance that is causing inflammation and uncomfortable symptoms, this is stressful for your body as well. These situations can weaken the immune system and can become a form of stress that impacts your adrenal and thyroid glands.

Alternative testing options

Fortunately, over the last few years, a number of alternative testing options have become available via private labs and testing companies in NZ and overseas. As a practitioner, I am able to access and support you with these so if you would like to explore alternative testing options and the costs involved, then please get in touch.

Checking your Basal Body Temperature

A simple test you can do at home to check your thyroid function is to take your basal body temperature upon waking for 5 days in a row (between day 5 and 14 of the menstrual cycle if you are female). A normal basal body temperature should be between 36.4 and 37.0 degrees Celsius. If it is consistently below 36.0 in the 35. 5 – 35.9 range then that could mean that you have an underactive thyroid as a lower body temperature indicates a slow metabolism.

What causes an underactive thyroid?

There are a number of factors that can cause an underactive thyroid so it is beneficial to work with a holistic practitioner to help to identify the root cause. Some of the most common causes are:

Stress and adrenal health - If you are suffering from chronic stress and your adrenal glands are depleted and you are feeling fatigued, this could be your body down regulating your thyroid function. When we are under stress, the body slows down the metabolism to conserve energy so non-essential organs such as the thyroid, reproductive organs and the digestive system are often down regulated. This was a huge issue for me. As a chronic asthma and eczema sufferer, I had spent 20 plus years using various steroid medications on and off. Long term steroid use depletes adrenal function so I had to work really hard to keep my adrenals in a healthy balance. As adrenal fatigue can cause the thyroid to down regulate, It is important to not just look at the thyroid in isolation but to look at supporting the whole glandular system. Treating the thyroid without looking at the adrenal glands can make symptoms worse.

Nutritional deficiencies -the key nutrients Iodine, tyrosine, zinc, selenium, and iron are all important for the production of thyroid hormones. More about these later.

Poor gut health – intestinal permeability (leaky gut), candida, and gluten intolerance. All these can cause inflammation, an over reactive immune system and autoimmunity. Autoimmune thyroid issues (Hashimoto’s, Graves) are linked to gluten intolerance.

Postpartum depletion and postpartum thyroiditis – this can be brought on by a previous pregnancy and postpartum period overwhelming the immune system and causing inflammation of the thyroid. Although this typically occurs within the first year of giving birth it can also start to occur many years later. This is common with secondary infertility and although I was never tested at the time, I believe I was suffering from postpartum thyroiditis after my son was born who is now 12. In hindsight it makes sense and I was displaying most of the symptoms such as low body temperature, weight gain, inability to lose weight after pregnancy and breastfeeding despite a healthy diet and regular exercise programme and fatigue.

What are the common symptoms of low thyroid function?

Some of the typical symptoms are fatigue, constipation, unexplained weight gain, hair loss, dry skin, brittle nails, poor concentration, enlarged thyroid gland (goitre), inability to lose weight, cold hands and feet, chills, feeling the cold. You may only have a few of these symptoms and still, have low thyroid function.

Supporting the thyroid with key nutrients

An underactive thyroid can often be the result of nutritional deficiencies such as iodine, tyrosine, selenium, zinc and iron which are essential for the function of the thyroid. To support thyroid function, manufacture thyroid hormones and covert T4 hormone to the active T3 hormone the following nutrients are needed on a daily basis:


Essential for the production of thyroid hormones (along with tyrosine)

150mcg a day is the recommended dose in the form of potassium iodide.

Good food sources: seafood, seaweeds such as kelp and bladderwrack, kombu, nori, iodised salt.


An amino acid that is essential for the manufacture of thyroid hormones (along with iodine)

500mg a day to support thyroid function

Good food sources: dairy foods, red meat, eggs, almonds, avocados, bananas.


Is an essential component of the enzyme that converts T4 to T3 the active thyroid hormone.

150 – 200 mcg a day is the recommended dose

Good food sources: Brazil nuts, salmon, chicken, brown rice, beef, walnuts


Involved in the production and conversion of thyroid hormones

15mg a day is the recommended dose

Good food sources: oysters, beef, chicken, dairy, cashews, almonds.


Iron is also very important in thyroid function as iron is involved with the production of TSH

24mg a day is the recommended daily dose.

Good food sources: beef, lamb, pork, liver (haem sources = higher absorption) beetroot, leafy greens such as spinach and kale, beans, lentils, tofu, molasses, dried fruit (non haem sources = lower absorption)

Eating vitamin C rich foods with iron rich foods helps with absorption. Citrus fruits, strawberries, broccoli, capsicum and potato are rich sources of vitamin c.

Additional support

It is staggering how many women are suffering from symptoms of low thyroid function despite being told that their blood tests are normal. If you suspect you have a thyroid problem as a starting point it is worth ensuring you are getting adequate levels of these four nutrients daily which will help your thyroid manufacture and convert hormones.

It is also important to avoid foods that can block the absorption of iodine by the iodine receptors in the thyroid gland. These are raw brassicas in excess (due to goitrogens) soy and unfiltered water (due to fluoride and chloride which can bind to iodine receptors)

Going gluten free is highly recommended if you are experiencing unexplained infertility and you suspect you may have a thyroid problem. This is because the autoimmune condition Hashimoto’s Thyroiditis is a common cause of an underactive thyroid has been linked in numerous studies to gluten consumption. You can download a free guide on Getting Started with Gluten Free.

Just a final note of caution. It is important not to supplement with high doses of iodine in an attempt to support your thyroid function unless under the care of a qualified practitioner. High doses may cause your thyroid to tip into a hyperthyroid state (overactive) which is not good for you. I learnt the hard way with this when I was a nutrition student and experimenting with different supplements as you do. At one point I felt like I was swinging between a hypo and hyper thyroid state. If you start to develop anxiety, have problems sleeping or develop heart palpitations this could be a sign you are taking too much iodine.

Please get in touch if I can be of further help. Why not take advantage of my FREE 15 Minute Discovery Session online via Zoom. This is a chance for us to have a quick chat about your current situation and health goals and how I may be able to help. You can book online here.

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