T3: The forgotten thyroid hormone

by William Davis, M.D. Health Professional

Millions of people take thyroid medications like Synthroid
®, Levoxyl
®, or levothyroxine. These are all forms of the T4 thyroid hormone, i.e., a thyroid hormone molecule containing 4 iodine molecules. Maintaining normal thyroid hormone levels are important for control over metabolic rate and weight control, energy, mood, cholesterol, and many other aspects of health.

But I ask them: "Do you feel normal?"

7 times out of 10, they will say something like, "No, I don't feel normal. I'm always tired. I'm often depressed and I've had to take antidepressants. My legs swell. And I gain weight even when I exercise and eat well."

I then tell them about the T3 thyroid hormone, what I call the forgotten thyroid hormone because few people take it. Conventional thinking among many doctors is that, when given the T4 thyroid hormone, the body automatically converts T4 to the T3 thyroid hormone simply by removing one iodine molecule. This conversion must occur to achieve normal thyroid function, since T3 is the true active thyroid hormone, not T4.

This is a contentious issue among thyroid experts: Some say that T4-to-T3 conversion can be impaired and that T3 supplementation is necessary to fully correct thyroid status. Others argue that T3 is unnecessary. There are studies supporting both sides of the argument, with some studies showing improved mood and energy with T3 added to T4, while others fail to show any improvement.

My experience has been most consistent with the first side of the argument: When someone responds "No" to my question about whether they feel normal, I will ask them to consider adding T3 thyroid hormone to their T4. (This is done by either adding a T3 preparation, liothyronine or Cytomel
®, or by switching to combination preparations like Armour
® thyroid or Naturethroid
®.) With rare exceptions, within a week they feel energized, mood improves, excess weight starts to drop.

What does this have to do with your heart? There's no question that low thyroid hormone levels act as a potent risk factor for coronary heart disease. While we've known for years that people with congestive heart failure or are seriously ill have abnormally low T3 hormone levels, two studies have recently found that people with coronary heart disease also have low T3 levels. These two studies now raise the question of whether low T3 by itself could be associated with increased risk for heart disease.

The million dollar question: If people with coronary artery disease have low T3 thyroid hormone levels, does increasing T3 levels reduce future risk for heart disease? This question remains unsettled. However, having watched many people add T3 to T4 feel more energetic, exercise more, be happier (which is part of a heart healthy program), and lose weight, I don't think that it's a big leap to predict that adding T3 to T4 for most people enhances health substantially, heart and otherwise. It's worth asking your doctor about it.

William Davis, M.D.
Meet Our Writer
William Davis, M.D.

William R. Davis is a Milwaukee-based American cardiologist and author. He wrote for HealthCentral as a health professional for Heart Health and High Cholesterol.