Thyroid Thursday Episode 17: The 3 D’s of Thyroid Physiology
Hi, Dr Eric Balcavage, and we are back for another edition of Thyroid Thursday. Today we are going to talk about the 3 D’s in thyroid physiology, and no it’s not the grade your doctor got in endocrinology class.
The three D’s refer to the Deiodinase enzymes 1, 2 and 3 (also called D1, D2, D3). These are the enzymes that convert T4 into T3 and reverse T3. So, D1 and D2 are the enzymes that convert T4 into T3 primarily. D3 converts T4 into reverse T3 (and T3 to T2). If you’ve been following any of these videos, you know that rT3 is not the form of thyroid hormone we want because it is an unusable, inactive form of thyroid hormone, yet it can clog up the receptors and block active T3 function.
So, why are these guys important?
Well, type 1 converts inactive T4 to T3. It is found in every cell in the body. It is found in the pituitary gland, but in lower concentrations. Mostly it is found in all the other cells of the body. D2 is found primarily in large concentrations in the pituitary gland and D3 is found in all the cells in the body except the pituitary gland.
So, why is all of this important?
D1, D2 and D3 act differently under similar stressors and conditions. So, any type of chronic inflammatory or stressful condition will down regulate D1, which means that you’re going to have a decreased conversion of T4 to T3. So, any time you’re stressed out; if you have insulin resistance, diabetes, chronic inflammation, or if you go on a calorie restriction diet you will decrease your D1 activity.
So, what does that mean? That means essentially that your thyroid hormone physiology reduces, that you become hypothyroid in those types of chronic health states. Most people who have thyroid issues have some type of chronic health condition. People who are struggling to lose weight and think they have a thyroid issue, what do they do? They decrease their food intake; they go on a diet. What does that do to thyroid physiology at the cellular level and our overall metabolism? It totally slows it down. It is one of the worst things you can do to increase your metabolism, is to go on a calorie restricted diet for any length or period of time.
So, what about D2? What is so important about D2? Well, D2 is primary found in the pituitary gland. There is very little D1 in the pituitary gland. D2 is the primary enzyme in the pituitary gland. The pituitary gland will be spared when there is stress in the body, T3 levels will be spared in the pituitary gland and the body will really fight to keep thyroid hormone levels appropriate in the pituitary cells at the detriment to all the other cells in the body.
When there is that chronic stress situation, insulin resistance, diabetes, obesity, dietary issues, or any other type of chronic immune or inflammatory condition, even depression, D1 enzyme physiology is going to be down regulated, which means all the cells in the body except the pituitary gland are going to have decreased thyroid function. The pituitary gland, however, will be able to maintain its conversion of T4 to T3. Why would that be important?
As we discussed on one of the previous episodes of Thyroid Thursday, TSH and T4, which are the tests that most medical doctors run to evaluate thyroid hormone status are not good indicators of cellular thyroid status. They really only give us an indication of pituitary thyroid status.
In stressful situations, if the body is shutting down cellular thyroid conversion of T4 to T3 in all the cells except the pituitary and its actually upregulating the conversion of T4 to T3, then TSH and T4 would not be valid markers of what’s going on anywhere in the body except the pituitary gland, and that does not give us the state of all the other cells in the body. So, not good.
What is D3? Well, D3 is the enzyme that converts T4 to rT3. Remember, we said that rT3 is a non-usable form of thyroid hormone, it can’t be converted back to anything once you convert T4 to rT3. D3 is upregulated in all the cells when there’s an inflammatory processes, reduced cellular energy, like insulin resistance, diabetes, obesity and depression. So, when we have these chronic health issues going on and it impacts our body’s ability to make energy at a cellular level, D1 conversion is down regulated and D3 conversion is upregulated. So, we’ll make a lot more reverse T3, essentially wasted thyroid hormone.
What is unique about D3 is it is not found in the pituitary gland. So, you will not see problems with thyroid hormone over-conversion to rT3 in the pituitary gland because that enzyme doesn’t exist there. So, these enzymes are really important to understand when we take a look at somebody who has thyroid symptoms, because they could have totally normal TSH and T4 values, but have all the symptoms of hypothyroidism and it’s all related to how these three enzymes work.
Unfortunately, most docs probably don’t know, what these enzymes are, that there is a difference between the function and location of the three types of enzymes, and their importance.
So, hopefully that helps you. I look forward to giving you some more information on the next Thyroid Thursday edition. If there are any questions regarding this video or anything regarding thyroid health and thyroid physiology, you can always put a comment on the comments section below the video or call the office, 610-558-8920, and we’ll be more than happy to answer your questions. Alright, take care.
