A diagram of the thyroid gland with a text bubble saying what is thyroid hormone conversion?

Normal Thyroid Physiology

When we refer to ‘thyroid hormones’ we are talking about T3 and T4. They are the ones the thyroid gland secretes. T4 is more stable in the blood and better for transport around the body; T3 is more biologically active.

Of the thyroid hormones produced, 85-90% is T4 and 10-15% is T3. Most of these hormones are bound to carrier proteins.

a thryroid gland with T3 and T4 hormones being released

The Need for Thyroid Hormone Conversion

As mentioned above, the thyroid gland makes more T4 than T3, because T4 is more durable and resilient, allowing it to travel through the blood stream.

The hormone T3 is more biologically active – meaning it’s the one that actually carries out the actions of the thyroid. For a review of the functions of the thyroid, click here.

Once T4 travels to the cells, they need to convert to T3. Within our cells, special enzymes called deiodinases will convert the inactive T4 into T3 [8]. This is where things can get confusing, and for some people the process doesn’t happen properly.

Where Things Can Go Wrong

There are many potential causes of thyroid dysfunction, from inadequate thyroid hormone production, to improper thyroid hormone uptake, to faulty conversion issues. The focus of this article is on conversion issues.

When looking at lab work, TSH and free T4 can be optimal, but if conversion is an issue, it’s only T3 that would show dysfunction on testing. When testing for thyroid hormone conversion issues, if T3 isn't tested, conversion issues would be completely missed on blood work.

Where the conversion occurs

Conversion occurs throughout the body, via special enzymes called deiodinases. These deiodinase enzymes are present in the liver, kidney, adipose fat, CNS, placenta and hemangiomas [9]. Some are present inside cells; others are outside the cells. This goes to show how limited blood tests can be. The blood levels of T3 and T4 don’t really tell us how much active T3 will be present in the cell – where it will impart its action.

Deiodinases are activated and deactivated by a number of factors such as cold exposure, over feeding, catecholamines, bile acids, and tissue injury [9], in addition to presence of thyroid hormone.

For more information on how to increase and decrease conversion, please click here.


Thyroid hormones are produced by the thyroid gland, and in order for optimal function, hormone conversion must occur. There are many factors which contribute to hormone conversion. If blood work doesn’t seem to tell the whole picture, make sure comprehensive testing is being done.

To learn more about this please visit your naturopath.



Interested in learning more?

Read on in our series of articles on Thyroid Health!

About the Author - Dr. Johann de Chickera

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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.

To book in please call us at (519) 442-2206 or click here.

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