What every woman with PCOS should know about their Thyroid

Your thyroid is an incredibly important gland in your body. It regulates so many functions including metabolization, temperature regulation, and energy. It is estimated that approximately 1 in 10 Canadians deal with thyroid-related issues.

When it comes to Polycystic Ovarian Syndrome, it is even more important to get your thyroid assessed. For one thing, many of the symptoms associated with hypothyroidism look very similar to PCOS. These symptoms associated with hypothyroidism include:

  • Fatigue
  • Weight gain
  • Intolerance to cold temperatures
  • Brain fog
  • Memory loss
  • Dry skin
  • Brittle nails
  • Constipation
  • Irregular cycles
  • Hair loss
  • Difficulty losing weight
  • Depression
  • Low libido
  • Increased risk of miscarriages

Since these symptoms look so similar to PCOS. I thought I would share with you one of the biggest connections we see with PCOS and thyroid health. Insulin resistance and thyroid function…Let’s learn more.

Insulin Resistance and Thyroid Function

Insulin resistance may impede thyroid function. How? One very important way.

But first, you should know the mechanism of the thyroid. In brief, Thyroid Stimulating Hormone (TSH) comes from the brain and it stimulates the thyroid to release T4. T4 gets converted into T3 in our body, which is known as the active thyroid hormone. The higher the T3 the better! Sometimes T4 can get converted into Reverse T3 – this is not what we want. Reverse T3 is the inactive form of the thyroid hormone. See the figure below.

Thyroid Pathway
T3 = active form of thyroid hormone rT3 = inactive form of the thyroid hormone


Okay, so now you have an idea of how the thyroid works, let’s take a look at how insulin resistance decreases T3 formation.

Studies have shown that diabetes, insulin resistance and metabolic syndrome can decrease the conversion of Free T4 to Free T3, which mentioned previously, is the active form of the thyroid hormone.

How does this happen??

Higher levels of insulin can turn Free T4 into Reverse T3 (rT3) which is the inactive form of the thyroid hormone.

This comes from the increased activity of the deiodinase-2 enzyme which increases the conversion of Free T4 to Reverse T3. It is important to note that higher deiodinase-2 activity can lower TSH, which can actually make your thyroid look like it is working well but it isn’t until we dig deeper to look at Free T4, Free T3 and Reverse T3 that the problems are uncovered.

Other causes of higher levels of Reverse T3 (inactive thyroid hormone) include stress and inflammation.

In Dr. Fiona’s book, 8 steps to reverse your PCOS she quotes two studies. The first study found that women with the lowest insulin resistance also had the lowest TSH values, typically under 2 miU/L (this is good!). Women whose TSH was high had greater insulin resistance (this is bad!). However, this was not related to weight. Women who had high TSH and high Free T3/T4 (aka subclinical hypothyroidism) had higher insulin resistance regardless of weight.

Another study found that women with PCOS whose TSH was greater than 2.5 miU/L also had a higher BMI, fasting insulin, and higher total testosterone compared to women with a TSH of less than 2.5 miU/L.

Optimal levels of Free T4 and Free T4 should be around the top end of the reference range.

Both the thyroid and insulin are involved when we are talking about glucose metabolism. Insulin resistance can disrupt thyroid function, and hypothyroidism or low thyroid function can disrupt insulin resistance. These two go hand in hand and it is important to be assessed for your thyroid if you have PCOS and vice versa.


Many of the signs and symptoms of PCOS can look like hypothyroidism. So it is important to get your thyroid assessed. High levels of insulin (aka insulin resistance) can prevent the formation of our active thyroid hormone known as T3.

Action Steps

  1. Get your thyroid assessed with blood work. A full thyroid panel includes TSH, Free T3, Free T4, Reverse T3 and Anti-Thyroperoxidase and Anti-Thyroglobulin. Learn more here.
  2. Get assessed for insulin resistance. Here are some common signs of insulin resistance
  3. To learn more about PCOS check out this section of the website. 
  4. Check out Dr. Fiona’s book – 8 Steps to Reverse Your PCOS
Looking for more support? Join Your PCOS Planner Here

McCulloch, F. (n.d.). 8 steps to reverse your PCOS.

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