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Thyroid Hormone Production and Function, Simplified

A Practical Guide to the Thyroid Gland and How Its Hormones Affect Your Health

Key Takeaways:

  • Knowing what hormones are produced by the thyroid gland and in what amounts can help you be proactive about your thyroid health.
  • The main thyroid gland hormones are thyroxine (T4) and triiodothyronine (T3).
  • TSH (thyroid-stimulating hormone) is also considered a “thyroid hormone”, even though it’s produced by the pituitary, as it’s used for screening and monitoring thyroid health.
  • Thyroid hormone medication is usually necessary when BOTH T4 and TSH levels are outside normal limits.
  • You may be able to avoid medication if only your TSH is outside the normal limits (symptoms are your better guide).
  • Gut and thyroid symptoms can overlap so don’t rush to medicate “thyroid” symptoms that may actually originate from the gut.
  • Probiotics and an anti-inflammatory diet can help prevent minor thyroid issues like subclinical hypothyroidism from progressing further.

Knowing what hormones are produced by the thyroid gland and how these influence health can be really helpful to understanding a thyroid diagnosis. Even if you don’t have a thyroid condition right now, becoming savvy about thyroid health is good knowledge for the future if any symptoms start to crop up.

Why is this? Well, your thyroid gland controls all aspects of your metabolism and so is critical to your energy levels, cognitive function, and general well-being. If you understand the basics of thyroid hormones and how they function, it’s handy knowledge if things do go wrong.

At the clinic, we’ve helped many patients with mismanaged or misdiagnosed thyroid conditions. Often, the patient’s doctor has been too black-and-white in how they have interpreted “low” thyroid hormone levels. Other times the patient has unrecognized issues, such as gut disturbances, which are impacting on their thyroid symptoms.

In this article let’s break down the basics of thyroid hormones, how to interpret your thyroid test results, and the wider factors that influence thyroid health.

Thyroxine (T4) and Triiodothyronine (T3)

The thyroid gland is a butterfly-shaped gland in the neck that produces the hormones that control your body’s metabolism. 

The two metabolism-influencing hormones your thyroid gland produces are thyroxine (T4) and triiodothyronine (T3) [1]

Approximately 20% of triiodothyronine is made directly by the thyroid gland and secreted into the bloodstream. Around 80% is made by converting T4 to T3 in organs including the liver and kidneys [2].

T3 is the more active form of the two hormones with respect to its impact on your cells and metabolism. The more T3 and T4 you produce, the higher your metabolic rate, i.e. the quicker your cells carry out various processes. To name a few, the effect of the thyroid hormones in adults is to:

  • Increase your heart rate
  • Burn fuel (calories) more quickly 
  • Activate the nervous system to ensure better concentration and faster reflexes
  • Speed up digestion
  • Raise body temperature

In children, the thyroid hormones also influence growth. Kids who don’t produce enough thyroid hormones can have slowed growth.

The thyroid gland also produces a third hormone, calcitonin, which we won’t be focusing on here because it’s not a conventional “thyroid” hormone in the sense of influencing metabolism. It’s a hormone that helps control calcium levels in the blood and is rarely involved in thyroid disease or symptoms.

The Thyroid Feedback Loop

The secretion of T3 and T4 is controlled via a loop involving two other hormones called thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH).

TRH is released from the hypothalamus when this part of the brain receives signals that levels of thyroid hormones are too low. TRH makes the pituitary gland release TSH, which in turn “tells” the thyroid gland to release more of the T3 and T4 hormones that stimulate our metabolism. 

Even though TSH is produced by the pituitary, it’s often loosely defined as a “thyroid hormone” for its role in detecting and monitoring thyroid issues.As your T3 and T4 levels get higher, your pituitary gland shuts off TSH production until your levels of T3 and T4 begin to fall again. This feedback loop constantly cycles to keep your thyroid hormones in check. However, the cycle can sometimes go awry, resulting in too little or too much thyroid hormone production, which affects your metabolic rate.


When the thyroid gland is underactive and produces insufficient levels of thyroid hormones this can result in hypothyroidism, which abnormally slows your metabolism. Typical symptoms include [3]

  • Fatigue, depression, and brain fog
  • Feeling cold
  • Weight gain
  • Dry skin
  • Constipation
  • Menstrual irregularities

When your circulating levels of thyroid hormone (particularly T3) are too high, this can result in hyperthyroidism (thyrotoxicosis), which abnormally speeds your metabolism. Typical symptoms include [3]

  • Weight loss
  • Increased appetite
  • Heart palpitations (arrhythmias)
  • Irregular menstrual cycle
  • Tiredness
  • Irritability
  • Hair thinning

The question is, what levels of thyroid hormone are too high or too low? With an underactive thyroid, in particular, the TSH and T4 levels used to guide a hypothyroidism diagnosis need to be interpreted carefully.

Making Sense of Thyroid Function Tests

As I mentioned, it’s not unusual for patients to be given a hypothyroidism diagnosis too early, which can set them down a road of taking thyroid hormones for a lifetime and potentially mask the true cause of their symptoms. Both overt and subclinical hypothyroidism tend to be overdiagnosed [4, 5, 6, 7].

Diagnosing Hypothyroidism 

In actual fact, diagnosing overt hypothyroidism (advanced enough to need thyroid hormone medication) is actually pretty easy.

Diagnosis requires a simple blood test that measures your levels of TSH and free T4 (T4 that is not attached to a protein in the blood). 

  • The healthy range of TSH is about 0.45−4.5 IU/mL [8]. 
  • The healthy range of free T4 is 0.82−1.76 ng/dL [9].

If your TSH is above the 4.5 IU/mL upper limit AND your free T4 is below the 0.82 ng/dL lower limit, this signifies overt hypothyroidism that requires treatment with thyroid hormones.

Diagnosing Subclinical Hypothyroidism

In contrast, if your test results show raised TSH but normal free T4, this suggests subclinical hypothyroidism. In other words, you may be at risk of developing an underactive thyroid in the future but have a fully functioning thyroid for now.

Some practitioners jump to treat subclinical hypothyroidism with thyroid hormones, but you will not usually need medication unless your TSH levels start to approach 8–9 IU/mL. 

I encourage patients to instead treat subclinical hypothyroidism by tackling the autoimmune and/or other inflammatory issues that often underlie the condition. This, together with taking steps to improve your gut health, can prevent subclinical hypothyroidism from developing into overt hypothyroidism.

TSH Levels Rise With Age

Sometimes a subclinical hypothyroidism diagnosis is given too hastily to older age groups. TSH levels naturally rise with age and the ranges above should be adjusted for those above the age of 50.

I discussed TSH levels with accomplished thyroid expert Dr. Antonio Bianco in a recent podcast. His insights were fascinating and the whole podcast is worth a listen if you have thyroid concerns. 

With regard to how TSH levels change with age, a useful rule of thumb from Dr. Bianco is that for each decade from 50 onwards, you can essentially go up one value of TSH. So,

  • If you are in your 50s, it’s okay to have TSH in the 5s. 
  • If you are in your 60s, it’s okay to have TSH in the 6s. 
  • In your 70s, it’s okay to have TSH in the 7s, and so on.

A 2021 study of over 2,500 older people found that the estimated prevalence of subclinical hypothyroidism dropped hugely (to 10% of the original estimate) when clinicians used an adjusted, age-appropriate TSH reference range instead of the standard range detailed above [10]. To be clear, TSH levels over 4.5 IU/mL are still indicative of overt hypothyroidism at any age when accompanied by low free T4. 

Diagnosing Hyperthyroidism 

An overactive thyroid is less common than an underactive one. However, it’s still estimated that around 0.5% of the US population have overt hyperthyroidism and another 0.7% have subclinical hyperthyroidism [11]. 

If you’ve been having symptoms of hyperthyroidism such as feeling wired, having palpitations, losing weight despite eating loads, and not being able to sleep, a blood test will shed some light.

A test for hyperthyroidism checks the same hormones (TSH and free T4) as the test for hypothyroidism. Free T3 may also be added to the test panel for hyperthyroidism as it can increase certainty when making an overactive diagnosis [12]:

  • Low TSH AND high free T4 and/or free T3 confirms a diagnosis of hyperthyroidism [12]
  • Low TSH but normal free T4 and free T3 suggests subclinical hyperthyroidism 

Again, when diagnosing subclinical hyperthyroidism your age matters. Given you expect TSH levels to rise with age, an unusually low level for a senior is probably more significant than the same in a younger person. 

It’s also important to know that biotin (a B vitamin) can lead to a falsely high T4 and T3 and low TSH result, leading to an incorrect hyperthyroidism diagnosis or a false assumption that you’re taking too much thyroid hormone [13, 14]. If you take high-strength B vitamins it’s a good idea to swear off them for a few days before a thyroid panel test.

Generally speaking though, hyperthyroidism is more clear-cut to diagnose than hypothyroidism as the symptoms are more distinct.

Autoimmune Conditions Lie Behind Most Thyroid Problems

Worldwide, the most common cause of hypothyroidism is low iodine in the diet, as the thyroid gland needs iodine to make thyroid hormones. However, low iodine isn’t the main cause in the USA.

Rather, the most common cause of both hypo- and hyperthyroidism is an autoimmune attack on the thyroid gland. Autoimmune conditions occur when the immune system is overactive and harms the body’s own tissues.

  • Hashimoto’s thyroiditis is an autoimmune condition that can lead to hypothyroidism [15
  • Graves’ disease is an autoimmune condition that can lead to hyperthyroidism [16]. 

Antibody tests can be helpful in identifying both types of thyroid autoimmunity:

  • If Graves’ disease is suspected, TSH-receptor antibodies (TRAbs) should be measured [17]. High TRAbs indicate you likely have Graves’ disease and are at risk for hyperthyroidism [18].
  • If Hashiomoto’s disease is suspected, a thyroid peroxidase (TPO) antibody test is advised. 

If you’ve had a TPO antibody test and wonder what the result mean, this rough guide will help you:

  • Anything over 35 IU/mL is generally considered positive for thyroid antibodies, which would indicate you likely do have an autoimmune issue (most likely Hashiomoto’s). 
  • TPO levels between 35 and 500 IU/mL indicate minimal risk of progression to hypothyroidism [19].
  • TPO levels over 500 IU/mL indicate a moderate risk of progression to hypothyroidism [20, 21].

The good news is that not all patients with elevated thyroid antibodies will develop hypothyroidism. In fact, the majority do not.

A large thyroid study that followed 5,783 people for 9.1 years [22], found that the majority (62.3%) who were positive for TPO antibodies actually continued to have healthy thyroid function.

That said, if you have symptoms of hypo- or hyperthyroidism, or lab results that suggest your risk is increased, it’s important to give your thyroid some attention. One of the most important ways to do this is by improving your gut health. 

More specifically, a healthier gut can help improve the underlying autoimmunity behind most thyroid problems. It can also ensure you get the nutrients needed for a healthy thyroid, as well as help tackle gut symptoms masquerading as thyroid symptoms.

Why Gut and Thyroid Health Are So Intertwined

The gut and thyroid aren’t physically close but researchers have discovered that there’s an important relationship between the two. You might have heard it referred to as the  ‘gut-thyroid connection’ or ‘gut-thyroid axis’.

The way this relationship works is that when our gut microbiome is out of balance it can create a leaky gut and body-wide inflammation that disrupts thyroid hormones and increases the risk of autoimmune conditions like Hashiomoto’s thyroiditis [23, 24, 25, 26, 27, 28, 29].

Suboptimal gut health can also interfere with the absorption of nutrients like iron, iodine, selenium, and zinc needed for thyroid function. It may cause decreased absorption of thyroid hormones, which is why healing your digestive system can help optimize any medication you may need.

Gut inflammation can also trigger brain fog, depression, and other cognitive issues independently. In fact,  if you have a gut issue and a thyroid issue it can be very difficult to tell which of the two is causing your symptoms.

In dealing with patients at the clinic we’ve noticed how similar gut and thyroid (particularly hypothyroid) symptoms are, and how they can masquerade as each other.

In fact, we saw so many patients in the clinic who believed their gut symptoms were thyroid symptoms that we published a case series and literature review to alert other functional and integrative practitioners to this connection.

The bottom line is that when you improve your gut health, your thyroid health is likely to improve. Improving gut health may mean that early signs of thyroid issues, like subclinical hypothyroidism, can correct themselves without the need for medication.

I have had patients who have been able to reduce or come off thyroid hormone medication after they successfully worked on gut health. In other cases, a gut-healing program has meant the patient can reduce their medication dosage.

What’s the Best Gut and Thyroid Friendly Diet?

Healing your gut and reducing the autoimmunity or other inflammation that underlies most thyroid conditions, calls for a diet that calms inflammation. The most effective way to reduce gut inflammation is to eat a whole food, unprocessed diet. Try to include high-quality proteins such as oily fish and lean red meat with a wealth of colorful fruits and vegetables.

However, it’s also important to investigate and be aware of any foods that are triggering inflammatory responses in your gut. This can be very individual to you. 

While many of my patients do really well on something like a Mediterranean diet or a slightly stricter Paleo diet (minimizing wheat and dairy), others continue to have gut symptoms. If this is the case for you, you’ll need to try a more targeted elimination diet to pinpoint the foods that are a problem for you.

I walk you through elimination diets in detail here, but some options are: 

  • The low-FODMAP diet eliminates and then slowly reintroduces carbs that can create bloating.
  • The autoimmune protocol diet (AIP) works to bring down thyroid autoantibodies in some people (though is usually a last resort option because it’s so restrictive).

Probiotics May Help Thyroid Issues

A well-formulated, multi-species probiotic supplement can work alongside a gut-healthy diet to help promote a healthier microbiota and ease gut and thyroid symptoms. 

Probiotics simultaneously address gut and thyroid issues in numerous ways, for example, by:

  • Reducing leaky gut syndrome, which is associated with more significant symptoms in Graves’ and other thyroid diseases [26, 30, 31, 32, 33].
  • Rectifying conditions like small intestine bacterial overgrowth (SIBO) and H. pylori [34, 35, 36] that are closely linked with thyroid autoimmunity and hypothyroidism [37, 38].
  • Reducing autoimmunity risk and symptoms. While we don’t have evidence of probiotic benefits for Hashimoto’s yet, other autoimmune diseases like type 1 diabetes, multiple sclerosis, and rheumatoid arthritis do respond well to probiotics [39, 40, 41]. 

One study found that joint probiotic and prebiotic use reduced the need for medication and increased energy levels in hypothyroidism, likely due to its positive benefits on the digestive microbiome [42]. 

When You Need Thyroid Hormones, Which Medication is Best?

Before signing off on the subject of thyroid hormones I can’t stress enough that if you need to take thyroid hormones (because you are having significant symptoms and/or both your TSH and thyroid hormone levels are out of whack), then go ahead and do it.

Correcting your thyroid levels when you’ve been hypothyroid is a game-changer that can make you quickly feel better.

T4 or Combination T3/T4 Therapy?

There are two hormone options that can be prescribed for hypothyroidism. The most common option, used by many conventional and integrative doctors, is a synthetic version of thyroxine (T4) hormone, called levothyroxine. The rationale for using T4 alone is that your body will then go ahead and convert an appropriate amount of T3 from this hormone, as needed.

However, some functional health practitioners prefer to use a desiccated thyroid extract (made from pigs’ thyroid glands), which contains a combination of T3 and T4. This more “natural” combo approach is based on a theory that the body may have a hard time converting from T4 to T3.

At the clinic, we extensively reviewed the evidence examining desiccated thyroid extract versus synthetic T4. Our finding after combing through several reviews and meta-analyses was that there is no reliable evidence that the combination of T4 and T3 supplied by a natural extract is any better than synthetic T4 alone at treating symptoms [43, 44]. 

In fact, there are potential problems with desiccated thyroid extracts because they are difficult to standardize and may have varying amounts of T3 and T4 from batch to batch. Excess T3 can also have side effects, including increased fatigue, worsened mood and sleep, and higher anxiety levels [45]. 

For this reason, I recommend hypothyroid patients start on T4 medication (levothyroxine) alone. If T4 monotherapy doesn’t resolve their symptoms, or if they feel unhappy with their results, we then discuss moving on to combination T3/T4 therapy.

Once you’ve found a thyroid medication regimen that suits you, you’ll need to keep on having your thyroid hormone levels tested. But it’s important to note that small fluctuations in hormone levels are not an immediate cause for concern. Making changes to medication shouldn’t be based on symptoms as much as it is on lab test results.

Thyroid Hormones Simplified

We’ve not only looked at what hormones are produced by the thyroid gland, but also when (and when not) to worry about higher or lower than “normal” levels of these hormones. As is always the case with lab values, it’s important to interpret your test results in conjunction with the symptoms you are experiencing.

It’s also a good idea to look at the issues that can underlie thyroid problems, in this case, gut imbalances.

If you’d like to work through thyroid or gut issues in more depth, I created a Thyroid Self-Management Course that you can work through. And if you’d like a more hands-on approach, you can reach out for a virtual one-to-one consultation with one of our highly experienced practitioners.

The Ruscio Institute has also developed a range of high-quality formulations to help our patients and audience. If you’re interested in learning more about these products, please click here. Note that there are many other options available, and we encourage you to research which products may be right for you.

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