There is a lot of confusion around thyroid function and fertility, and what should be measured when couples are trying to conceive to make sure that the thyroid is optimized.

Before we dive into those details, let's talk a bit about the thyroid and why we're even having this conversation!

The thyroid gland is a butterfly-shaped gland that lives in your neck, just above your "adam's apple".  It is controlled by the brain (like all of our glands) and has a BIG function in the body!  The thyroid is responsible to set metabolism, controls body temperature (through metabolism) and the impacts of thyroid hormone in the body touch nearly every cell type and influence its function, INCLUDING the reproductive system!

Some common symptoms of under-functioning thyroid (called hypothyroidism) include weight gain, fatigue, sluggish bowels, hair loss, depression, skin and hair dryness, and many more. 

Symptoms of over-function (called hyperthyroidism) are essentially the opposite- diarrhea, weight loss, insomnia, anxiety, hair loss, fatigue, and more.

When it comes to fertility, hypothyroidism and hyperthyroidism are implicated in infertility, which is why almost all doctors screen for these using the most commonly run thyroid test, TSH (Thyroid stimulating hormone).  

TSH is actually a hormone produced by the pituitary gland (in the brain) that controls the thyroid gland.  It can feel a bit confusing to understand, because interpretation feels backwards- when TSH is high it means you are hypothyroid, and when it is low it means you are hyperthyroid.  You can think about TSH as a "parent" and the thyroid gland as a child- if the child is listening and doing what is asked of them, the parent (TSH) keeps their voice down to a minimum.  If the child (the thyroid gland) isn't "listening" to the parent (the brain making TSH), the parent starts to raise their voice (make more TSH).  

This is a critical first thing to evaluate when it comes to thyroid function.

In fact, thyroid function is so critical to fertility that we use a different (tighter) range of normal- this is well-established in literature and is not debated, though it is MISSED often by doctors who are not as experienced specifically in fertility and endocrinology.

TSH is considered normal by most labs between the values of 0.5-5.0 mIU/L.  

When you're trying to conceive, women need a tighter range- between 0.5- 2.5 mIU/L.  Anything above 2.5 is correlated with worse fertility outcomes.

But TSH isn't the entire story!  As it only looks at the brain's signal, it's possible to miss the whole picture when that is all that is measured. 

In our practice, we routinely measure the following:

TSH: explained above- the brain's master control of thyroid function

T3/T4 (measured as free T3/free T4): These are the main hormones made by the thyroid gland.  The active form (T3) and the inactive form (T4), which gets converted by the body into active. (actually, about 20% of this happens in the gut!!). Sometimes, people do not convert their hormones well, and can be a hidden form of hypothyroidism.

Thyroid antibodies:  There are a few antibodies we measure for the thyroid- anti-thyroglobulin and anti-thyroid peroxidase. It is estimated that 75-80% of thyroid dysfunction is autoimmune in nature, and you can also have antibodies present EVEN WHEN TSH IS NORMAL!  This is an important point because TSH is often used as a screening tool, and antibodies are only run as a follow up in cases where TSH is abnormal (called "reflex testing").  But this approach isn't appropriate for women trying to conceive, as data is clear that the presence of thyroid autoantibodies, even when TSH is normal, is independently correlated with infertility. 

I see these 3 thyroid complexities slip through the cracks OFTEN:

1) The wrong reference range being used- women are told their thyroid is "normal" when it's >2.5 and they're trying to conceive

2) TSH is normal but actual thyroid hormones are off, and this hasn't been checked

3) Not testing thyroid antibodies because TSH is normal.

The thyroid plays a critical role in fertility- it's important for women to have ALL of the information to ensure that they (and their doctor) are looking at the complete picture!