Thyroid Hormones 101: What are they?

November 17, 2023Hertility

Your thyroid is the gland placed at the front of your throat, and whilst it might be shaped like a beautiful butterfly, if it’s not handling its workload correctly, it sure could sting like a bee.

Despite its cute aesthetic, your thyroid dictates a lot throughout your body, such as your heart rate, metabolism (how well and fast your body processes what you eat and drink), growth and development and temperature, through the production of thyroid hormones.

Sometimes the thyroid gland makes too much or too little of these hormones, which can cause a thyroid disorder.

What are the thyroid hormones?

The thyroid gland is controlled by the hypothalamus and the pituitary gland, located in the brain. 

The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH), which tells the thyroid how much hormone to produce.

The thyroid gland produces 2 main hormones that can impact fertility – Triiodothyronine (T3) and Tetraiodothyronine or Thyroxine (T4).

When T4 and T3 levels are low in the blood, the brain produces more TRH and TSH, which stimulates the thyroid to make more T4 and T3.

When T4 and T3 levels are low in the blood, the brain produces more TRH and TSH, which stimulates the thyroid to make more T4 and T3.

When T4 and T3 levels are too high in the blood, the brain produces less TRH and TSH, which reduces the amount of T4 and T3 produced by the thyroid.

This whole network is called the hypothalamic-pituitary-thyroid axis, which adapts to metabolic changes and the body’s needs.

What do these hormones do in our body?

Thyroid hormones play an essential role in many different functions of the body, including:

  • Regulating the rate of metabolism, the process that stores and breaks down nutrients to provide energy, thus affecting body weight
  • Regulating your heart rate
  • Regulating body temperature
  • Influencing the rate at which food moves through the digestive tract
  • Controlling muscle contraction
  • Controlling the rate at which dying cells are replaced
  • Thyroid hormone can affect your menstrual cycle, ovualtion and fertility

T4 is the primary form of thyroid hormone circulating in the blood. It is converted to T3 mainly in the liver to become active and bring about its effect.

Most thyroid hormones in the blood are bound by the protein Thyroxine-binding globulin (TBG) responsible for binding and transporting thyroid hormones to the necessary tissues. When the thyroid hormones are bound to TBG, they are inactive.

Free thyroxine (FT4) and free triiodothyronine (FT3) are the unbound, free and active forms of thyroid hormones that can enter tissues and bring about an effect in the body. 

  Did you know?

Hertility at-home hormone and fertility testing kits help give you insight into your FSH, FT4 and FT3 hormone levels along with other hormones to give you insight into your fertility and overall health.

Sometimes the thyroid gland makes too much or too little of these hormones, which can cause a thyroid disorder.

Thyroid disorders can cause:

  • Problems with your periods: The thyroid hormones help control the menstrual cycle. An imbalance in its levels can thus directly impact your periods, making them lighter, heavier or even irregular. In some extreme cases, it has even been linked to increased risks of early menopause.
  • Difficultly getting pregnant: Since the thyroid hormones impact the menstural cycle, it affects a really important process called ovulation which is important for pregnancy. If there is no ovulation, there is no egg for the sperm to fertilise and thus a pregnancy can not occur in that cycle. Irregular ovulation can also make timing intercourse difficult.
  • Problems during pregnancy: Thyroid problems during pregnancy can impact the health of the pregnact person and baby.

You can read more about how thyroid hormones impact fertility here.

Did you know?

1 in 8 women and gender diverse people assigned female at birth are estimated to develop thyroid problems during their lifetime.

The risk of thyroid-related issues increases during pregnancy and around menopause.

You can read more about hyperthyroidism which is a condition when the thyroid hormones are abnormally high here.

You can read more about hypothyroidism which is a condition where the thyroid hormones are abnormally low here.

If you are experiencing any symptoms of a hormonal imbalance or simply want to understand your hormone health, we’re here to get you the answers you deserve. The Hertility Health journey includes a tailored at-home hormone test, digital personalised results, access to highly-rated health experts and clinics, educational content and a community of care. We’re here to support you throughout your journey from the comfort of your couch.


Shahid MA, Ashraf MA, Sharma S. Physiology, Thyroid Hormone. [Updated 2021 May 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from:

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