This introductory article is one of many in-depth articles on thyroid function and focuses on normal thyroid anatomy and physiology. Learn more about hypothyroid, hyperthyroidism and testing by clicking the links.
Basic Physiology and Anatomy
The most important anatomical structures are shown in the image below:
The hypothalamus creates a hormone called thyrotropin-releasing hormone (TRH). Although we can measure this in the blood, it is not done routinely. It will be tested in cases where we think the hypothalamus is dysfunctional.
TRH signals the Anterior Pituitary Gland, which then releases Thyroid Stimulating Hormone (TSH). TSH is one of the most commonly run blood tests. Click here for an article on the pitfalls of testing Thyroid hormone alone.
TSH then stimulates the thyroid gland, which in turn releases the thyroid hormones (T3 and T4). It is T3 and T4 that perform the actions of the thyroid gland. Testing T3 and T4 in the blood is not always necessary, but for some people, these two tell a more complete picture of thyroid health.
After they’re produced, T3 and T4 enter the blood and can go to any cell throughout the body.
In reality, it’s a complex network of organs and hormones working together. It’s mainly T3 (and to a lesser extent T4) that ‘act’ on other tissues imparting the action of the thyroid gland. As shown in the image above, there are many steps involved in producing T3 and T4, and each step needs to work for optimal thyroid function.
The thyroid hormones (T3 and T4) are involved in :
- Heart Rate
- Central and Peripheral Nervous systems
- Metabolism (Body Weight, temperature)
- Muscle activity
- Menstrual cycles
- Cholesterol metabolism
......and much more
Some Important Terms
When you read about thyroid function, some terms are used frequently and I refer to them in many of my articles.
Euthyroid State: describes NORMALLY functioning thyroid activity.
Hypothyroid State: describes LOW functioning thyroid activity.
Hyperthyroid State: describes HIGH functioning thyroid activity.
Thyroid Function Tests: These are usually blood tests. They measure TSH, T3, T4 and reverse T3 (which will be discussed later). When assessing thyroid function, doctors will use Clinical Assessment (signs, symptoms, and physical exams) as well as Laboratory Thyroid Function Tests to make diagnoses and prescribe treatment.
Normal T3 and T4 Physiology
T3 and T4 get their names based on their composition. T3 is made up of a Tyrosine molecule and 3 iodines. T4 is made up of a Tyrosine molecule and 4 iodines.
In the normal, healthy thyroid (euthyroid state), the thyroid gland secretes about 85–90% T4 and 10–15% T3 .
T4 is Better for Transport and T3 is More Biologically Active
The thyroid gland releases more T4, which is actually a more stable, durable molecule than T3. T4 must travel through the circulatory system to get to the tissues. Being tough and durable is useful for the trip from one place to another.
Once it gets to the tissue, T4 needs to convert to T3. T3 is actually more biologically active – meaning it has a stronger bond and produces a stronger effect on the target cells . Click here to read more about this conversion process and how some people have conversion issues which may complicate their blood work.
Bound vs Free Thyroid Hormone
The diagrams above simplified the whole relationship between T3, T4 and TSH. In reality, when they circulate the blood, T3 and T4 are heavily (>99.5%) protein bound to thyroxine binding globulin (TBG), albumin and transthyretin (prealbumin) . These protein carriers serve one purpose – to take the thyroid hormones safely to the cells that need them.
When bound to protein, thyroid hormones are actually inactive. The remaining 0.5% of thyroid hormone is considered free. Free thyroid hormone is the only one able to act on tissue. That is to say only free hormone is biologically active. When the carrier protein with its bound thyroid hormone gets to the target cells, the thyroid hormone should be released, making it free and therefore biologically active.
Thyroid function is crucial for health. The main hormones discussed when looking at thyroid function are TSH, T3 and T4. The physiology seems quite simple, but as you learn more about thyroid function, you’ll see it can get quite complicated.
Interested in learning more?
Read on in our series of articles on Thyroid Health!
About the Author - Dr. Johann de Chickera
Dr. Johann is a fully licensed Naturopathic Doctor. His approach emphasizes the importance of living a healthy lifestyle and improving one’s health naturally. Dr. Johann obtained a Doctor of Naturopathy at the Canadian College of Naturopathic Medicine (CCNM). Education at CCNM is a vigorous four years, with a curriculum involving biomedical sciences, physical diagnosis, clinical nutrition, traditional Chinese medicine and acupuncture, botanical (herbal) medicine, physical medicine, homeopathy and lifestyle management.
While Dr. Johann has a general practice, he focuses on fertility, hormonal imbalances, gut health, and autoimmune disease.
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