Thyroid Hormones 101 – What Do Your Thyroid Levels Mean?-image

Thyroid Hormones 101 – What Do Your Thyroid Levels Mean?

Medically Reviewed by Hertility on March 28, 2024

Thyroid hormones are made in the thyroid gland and play a crucial role in regulating many different body functions. But what happens when these hormones get out of balance and what is a thyroid disorder? Read on to find out. 

Quick facts:

  • There are 3 thyroid hormones, all very important for overall health 
  • Thyroid hormones regulate metabolism, body temperature, growth and development
  • These hormones can become imbalanced resulting in thyroid disorders
  • This can in turn cause issues for your fertility and reproductive health 
  • It’s important to test your hormones if you think you have any symptoms of a thyroid disorder

What are thyroid hormones?

Thyroid hormones are incredibly important hormones for overall health. They play a crucial role in influencing many, many different bodily processes.  

There are three thyroid hormones—TSH, T4 and T3. Each has a different function in the body which we’ll explain in a second. 

TSH is made in the pituitary gland in the brain. TSH then stimulates the thyroid gland—a butterfly-shaped gland in your throat to produce T4 and T3.

What are the different types of thyroid hormones?

There are three main thyroid hormones:

  • Thyroid Stimulating Hormone (TSH): TSH is released by the pituitary gland, located below the brain and behind the sinus cavities, and stimulates the thyroid 
  • Thyroxine (T4): This is the primary form of thyroid hormone circulating in the blood.
  • Triiodothyronine (T3): Another active form of thyroid hormone that is produced both in the thyroid and by cells in the body by converting T4 to T3. 

What do thyroid hormones do in the body?

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

  • Regulating metabolism
  • Regulating heart rate
  • Regulating body temperature
  • Regulating the rate of digestion
  • Controlling muscle contraction
  • Controlling the rate at which dying cells are replaced
  • Regulating your menstrual cycle and ovulation.

The thyroid hormone feedback loop

Just like lots of hormones, our thyroid hormones are all linked—involved in what’s known as a feedback loop. Essentially they all depend on each other to work optimally. 

Here’s how the thyroid feedback loop works:

  • TSH is released from the pituitary gland in the brain.
  • TSH stimulates the thyroid gland—to produce T4.
  • T4 is then converted into T3.  
  • Both T4 and T3 circulate in the blood to bind to tissues and bring about the necessary bodily processes. 

Can thyroid hormones become imbalanced?

Yes. Thyroid imbalances, known as thyroid disorders, are really common and can disrupt many aspects of your health—bringing on a whole range of unwanted symptoms. 

1 in 8 women are estimated to develop thyroid problems during their lifetime. Thyroid disorders are caused by either an underactive thyroid (known as hypothyroidism) or an overactive thyroid (hyperthyroidism).

Let’s take a look at each in turn.

Hypothyroidism: an underactive thyroid

Hypothyroidism is when your thyroid isn’t producing enough T4 (and sometimes T3). This means you have an abnormally high level of TSH, compared to your T4 and/or T3 levels.

Because of the thyroid hormone feedback loop, when T3 and T4 are low our brains will overcompensate. This means you’ll produce more TSH to try and boost your T3 and T4 levels. This results in the classic low T3 and T4 levels paired with high TSH reading seen in hypothyroidism.

Symptoms of an underactive thyroid 

Hypothyroid symptoms include:

  • Fatigue
  • Weight gain
  • Heavy or irregular menstrual periods 
  • Fertility issues due to irregular periods and irregular ovulation 
  • Depression
  • Constipation
  • Trouble concentrating
  • Poor memory
  • Increased sensitivity to cold
  • Poor appetite
  • Slowed heart rate
  • Decreased sweating
  • Swelling of the face and neck
  • Thyroid pain
  • Joint and muscle pain
  • Dry skin or dry, thinning hair
  • Brittle hair and nails
  • Low libido (sex drive)

Having an underactive thyroid can also increase the risk of developing long-term health problems such as cardiovascular conditions, insulin resistance, nerve damage and a swollen enlarged thyroid (goitre). 

Causes of an underactive thyroid

Hypothyroidism can be caused by a range of factors including age, poorly balanced diets and pre-existing medical conditions like Polycystic Ovary Syndrome (PCOS).

Additionally, taking certain medications such as oestrogen-containing hormonal contraception and high doses of steroids can interfere with thyroid function. 

Treatments for an underactive thyroid

Hypothyroidism is typically treated with a medication called Levothyroxine that replaces T4. Cytomel, a T3 replacement, may also be required in specific cases.

Lifestyle changes can also improve your thyroid function. Eating iodine-rich foods like milk and dairy products and adding more white fish, eggs, beans, pulses or red meat into your diet can help.

Stress is a big factor in many hormone imbalances and conditions. So, like always, try to find a stress management method that works for you. If you smoke, consider quitting. Cutting down on your alcohol intake can help too.

Hyperthyroidism: An overactive thyroid

An overactive thyroid means you’re making too much T4 and T3. This triggers your brain to make less TSH—giving the characteristic low TSH paired with high T3 and T4.

Symptoms of hyperthyroidism:

  • Changes in mood, especially nervousness, irritability, restlessness, anxiety and depression
  • Irregular menstrual cycles, decreased menstrual flow (hypomenorrhea) and missed periods (amenorrhea), may cause fertility issues
  • Frequent bowel movements or diarrhoea
  • Heat intolerance and excessive sweating
  • Increased appetite
  • Unintentional weight loss, even if food intake is the same or increases
  • Hair thinning or hair loss
  • Difficulty sleeping and insomnia
  • Persistent tiredness and weakness, difficulty exercising
  • Thyroid pain
  • Neck pressure, breathlessness
  • Rapid heartbeat (tachycardia) — more than 100 beats a minute
  • Irregular heartbeat (arrhythmia)
  • Feeling of having a pounding heart (palpitations) 
  • Increased blood pressure
  • Muscle weakness
  • Twitching or trembling
  • Protruding eyes (exophthalmos)

Causes of an overactive thyroid

Much like an underactive thyroid, lifestyle factors like poor diet, smoking, alcohol and stress can cause an overactive thyroid. 

Genetics, autoimmune disorders like diabetes or Grave’s disease in addition to long-term problems with cardiovascular health and weight loss are associated with an overactive thyroid.

Excess iodine consumption has also been linked to an overactive thyroid. Be mindful of rich sources of iodine like kelp and seaweed and take extra care to check any supplements.

Treatments for an overactive thyroid

Prescription medications like carbimazole can be prescribed to decrease the function of your thyroid. Beta-blockers can also provide symptom relief from a racing heart or heart palpitations.

If needed, surgical intervention to partially remove your thyroid gland can also be an option.

Can a thyroid imbalance affect fertility?

Thyroid disorders can affect your fertility indirectly, because of the various issues they can cause to your metabolism and other hormones. One study demonstrated that 76% of women who fixed their thyroid were able to conceive between 6 weeks to 1 year afterwards. 

Thyroid disorders can indirectly cause:

  • Problems with your periods: Thyroid hormones help control the menstrual cycle. An imbalance in TSH can cause irregular, light or heavy periods. In some extreme cases, it has even been linked to increased risks of early menopause.
  • Difficulty getting pregnant: Since the thyroid hormones impact the menstrual 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 pregnant person and baby.

Why should I test my thyroid hormones?

If you suspect you have an issue with your thyroid or are experiencing any hormonal symptoms, testing your thyroid levels is the best place to start to help you understand whether you may be suffering from a thyroid disorder. 

Most thyroid function tests will not measure T3 unless you are known to have problems with your thyroid. This is because you are unlikely to have abnormal levels of T3 without having abnormal levels of TSH and/or T4.




Posts that Hertility has created for the Hertility Blog.

  • facebook
  • instagram
  • twitter