gloved hand holds a vial of FT4 blood sample

Free Thyroxine (FT4) Hormone: Purpose, Healthy Ranges & More

At first glance, your hormone test results might appear to be a veritable alphabet and numbers soup. And if you’re trying to conceive, you’ll want to pay special attention to your FT4 levels. Here’s why.

The human body is truly fascinating, being capable of incredible physical and intellectual feats. It’s also highly complex. New discoveries about the human body are made, even till today. For example, a 2016 study in Europe found that pregnancy leads to long-lasting changes in a woman’s brain structure, suggesting that they’re adapting to motherhood [1].

This proved that ‘pregnancy brain’ is a real issue…or adaptation, depending on how you look at things. And speaking about adapting, did you know that our bodies contain over 50 hormones [2]? These chemical messengers help to regulate our bodies in almost every way imaginable, from our mood to sexual function.

For women trying to conceive, certain hormones act as indicators for their fertility levels as well. Among these molecules responsible would be FT4. It might not be as recognisable as oestrogen and progesterone, but it’s still a very important hormone nonetheless. Here’s why.


What is FT4?

First things first, let’s dive deeper into what FT4 is. This hormone, bound T4, and triiodothyronine (T3) are all produced by the thyroid, a gland that’s located in your neck [3]. If your doctor wishes to know how well your thyroid is functioning, they’ll perform a test that measures your FT4 levels specifically.

However, all the hormones produced by the thyroid are responsible for regulating your body’s metabolism [4]. And in infants, these very hormones are crucial for developing their brains [5]. Unfortunately, as with every other hormone in our body, it is all about balance and we can’t have too much or too little FT4 circulating through it.

Therefore, when your doctor requests you to perform an FT4 test, they’re also trying to suss out whether you have thyroid disease or any other conditions afflicting this gland. Furthermore, an FT4 test may be conducted as part of a thyroid panel, a number of tests specifically for the thyroid, as its name suggests [6].


How does FT4 act as an indicator for your fertility levels?

Given that FT4 is responsible for regulating your body’s metabolism and brain development in infants, how does it indicate a woman’s fertility levels? Enter hyperthyroidism and hypothyroidism. These are conditions that result from excessively high and low FT4 levels respectively. More importantly, they are linked to abnormal menstrual cycles [7].

And when your menstrual cycle is disrupted or stopped entirely, getting pregnant naturally can be next to impossible. Therefore, your FT4 levels could be an indicator of your reproductive health. Having said that, should you receive an unfavourable FT4 test result, it’s not the end of the world because thyroid dysfunction can be managed through appropriate treatments, but more on that later [8].


How does FT4 differ from TSH?

TSH refers to Thyroid-Stimulating Hormone, which is produced in the pituitary gland. This gland is located in your brain and this particular hormone communicates to your thyroid how much thyroid hormone it needs to produce [9]. With just these facts on hand, it’s clear how different FT4 and TSH are already.

Firstly, FT4 and TSH have different names, obviously. Secondly, FT4 is produced in the thyroid whereas the pituitary gland makes TSH. These hormones have different responsibilities as well, with FT4 regulating your metabolism and TSH stimulating your thyroid [10]. However, there is a key similarity between these 2 hormones.

Both FT4 and TSH are indicators of how well your thyroid is functioning and whether you’re suffering from any thyroid disorders or conditions. Excessively high and low FT4 levels result in hyperthyroidism and hypothyroidism respectively. On the other hand, excessively high and low TSH levels indicate hypothyroidism and hyperthyroidism respectively [11].


How do you test for FT4?

doctor draws blood from man's arm

Fortunately, FT4 tests are quick and painless. A vial of blood will be collected from you at the hospital or clinic before being sent to a laboratory [12]. All you need to do after that is wait for said hospital or clinic to contact you with your results. Often, a follow-up consultation will be held if your FT4 levels are too high or low.

For women who have a packed schedule, you’ll be glad to know that at-home hormone tests for FT4 are becoming increasingly common. One such test would be twoplus Fertility’s Hormone Test Kit. Not only does it test for FT4, but this hormone test will also measure the levels of other key fertility hormones, such as Thyroid-Stimulating Hormone (TSH), prolactin, Anti-Mullerian Hormone (AMH), and Follicle-Stimulating Hormone (FSH).

Whether you’re taking your FT4 test at home or in a professional setting, there’s not much that you need to do to prepare for it. However, there are certain medications and supplements out there that might skew your test results [13]. Therefore, it’s always best to let your doctor know what you’re taking before the test is carried out.


What do high FT4 levels mean?

For adults, a normal FT4 level ranges between 0.7ng/dL and 1.9ng/dL [14]*. If your FT4 levels are above this acceptable range, it means that your thyroid is overactive. In other words, you’re suffering from hyperthyroidism. Unfortunately, there are a wide range of symptoms associated with this condition, which doesn’t make it easy for doctors to diagnose [15].

Some examples include losing weight at an usually fast pace even though your diet and physical activity levels haven’t changed. Additionally, you might discover that your heart is beating erratically or too quickly even when you aren’t exercising. For women, your menstrual cycle may be disrupted or stopped entirely [16].

Abnormally high FT4 levels may also be the result of Graves’ disease, Plummer’s disease, or thyroiditis. With regards to the first 2 ailments, they result in the thyroid producing too much T4 (a combination of bound T4 and FT4). As for thyroiditis, it causes thyroid hormone leakages into your bloodstream, hence the high levels on your test [17].


What do low FT4 levels mean?

Remember, a normal FT4 range is between 0.7ng/dL and 1.9ng/dL*. Should your FT4 levels dip below this range, it indicates that your thyroid isn’t producing enough of this hormone. In this case, you’re suffering from hypothyroidism.

The symptoms of hypothyroidism include irregular menstrual cycles or flows that are heavier than usual, stiff joints, muscle aches, and an impaired memory. Should hypothyroidism be left unchecked, it can lead to more severe health issues like obesity and heart disease [19]. Hypothyroidism can also be caused by thyroiditis, iodine deficiency, and hereditary conditions [20].

On the bright side, both hypothyroidism and hyperthyroidism can be managed through appropriate treatments. Here’s how.

*Depending on the laboratory, normal FT4 levels may vary due to different measurements and/or testing of different specimens.


How do you treat low and high FT4 levels?

As mentioned above, there are treatments available if you have unbalanced FT4 levels. However, whether you’re suffering from hyperthyroidism or hypothyroidism, these need to be carefully managed for the rest of your life. Taking your prescribed medication regularly and sticking to your scheduled consultations with your doctor will keep these conditions in check.


Hypothyroidism, low FT4 levels

For hypothyroidism, you’re simply supplementing the FT4 that your thyroid isn’t producing enough of. A commonly prescribed medication is levothyroxine and it evens your levels back out [21]. As you meet your doctor through the months and years, the amount of medication you need to take changes. That’s why it’s paramount for you to attend your consultations.


Hyperthyroidism, high FT4 levels

For hyperthyroidism, you’re stopping your thyroid from producing excess FT4. Thionamides are commonly prescribed and once your FT4 levels stabilise, your dosage may be reduced [22]. Alternatively, your doctor might recommend radioactive iodine treatment to destroy some cells in your thyroid. This is not painful, despite what you might be imagining right now.


In closing

twoplus Fertility Hormone Test Kit

FT4 may be a relatively obscure hormone to us, but it is no less important to our body than the other 50-odd chemical messengers we produce. And for women who are trying to conceive, it’s an important hormone that could provide insight to your reproductive health. If your FT4 levels exceed the normal range for adults, it’s time to set up an appointment with your doctor.

That being said, it goes to show how important taking a hormone test is when you’re trying to conceive. Even if your primary care physician or gynaecologist doesn’t offer or suggest it, there are plenty of hormone tests available online that you can purchase and use in the comfort of your home.

And while you’re waiting for your results, why not take the time to make sense of your other hormones?

Shop The twoplus Hormone Test Kit

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[2] United States Environmental Protection Agency, What Is The Endocrine System?,
[3] University Of Rochester Medical Center, Free T4,
[4] Cleveland Clinic, Thyroid Hormone,
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[6] U.S. Department Of Health And Human Services, Thyroid Tests,
[7] U.S. Department of Health and Human Services, Thyroid disease,
[8] Indu Verma, Renuka Sood, Sunil Juneja, Satinder Kaur, Prevalence Of Hypothyroidism In Infertile Women And Evaluation Of Response Of Treatment For Hypothyroidism On Infertility,
[9] U.S. Department Of Health And Human Services, TSH (Thyroid-Stimulating Hormone) Test,
[10] Remedy Health Media, LLC, How Your Thyroid Works,
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[15] Mayo Foundation For Medical Education And Research, Hyperthyroidism (Overactive Thyroid),
[16] Dotdash Media, Inc, How Thyroid Function Affects Menstruation,
[17] National Institute of Diabetes and Digestive and Kidney Diseases, Hypothyroidism (Underactive Thyroid),
[18] Mayo Foundation For Medical Education And Research, Hypothyroidism (Underactive Thyroid),
[19] GoodRx, Inc., Can Hypothyroidism Cause Obesity?,
[20] American Thyroid Association, Hypothyroidism (Underactive),
[21] Cleveland Clinic, Hypothyroidism,
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