Thyroid hormone plays an important role in metabolic function. While many patients are familiar with the thyroid stimulating hormone, or TSH, most are not aware that there are a number of thyroid hormones. TSH, created by the pituitary gland, tells the thyroid to make thyroxine. Thyroxine, or T4, is an inactive thyroid hormone that must be converted before it has an effect on the body. T4 can be converted to triiodothyronine (T3), the active hormone that has a metabolic effect on the body, or it can be converted to reverse T3, an inactive form of T3 that actually blocks the cell receptors for thyroid hormones, thus blocking the effect of T3. If a patient has too much RT3 in comparison to T3, that patient will be hypothyroid at the cellular level, with a reduced metabolic rate. Or, explained more simply, that patient will have difficulty losing weight and keeping weight off.

Studies Find Dieting Reduces Metabolism by as much as 25%

A study published in the American Journal of Physiology, Endocrinology and Metabolism found that a mere 25 days of calorie restriction resulted in a significant reduction in T4 to T3 conversion, with a 50 percent reduction in T3. And, as importantly, this study found that patients who experienced reduced T3 as a result of this calorie restriction actually saw a decrease in TSH, indicating an increase in thyroid hormone levels, when the opposite was actually true. The period of calorie restriction caused a clinically significant reduction of T3, which could potentially cause a person to be unable to lose weight or to regain weight already lost.

In a second study, published in the journal Metabolism, patients who had lost weight in the past had a significantly lower metabolism than other patients who were the same weight and had not gained or lost significant weight in the past year. The weight-loss patients had a metabolism that was 25 percent less that the comparable equal-weight person, with a metabolic rate that was more appropriate for someone who weighed 60 percent less than the weight-loss patient. Additionally, this reduction in metabolic rate was still present years later

A 25 percent reduction in metabolism is equivalent to an approximate deficit of 500 to 600 calories per day. Imagine reducing your caloric intake by 500 to 600 calories each day and not losing any weight. This is the effect that acute or chronic dieting can have on a person's metabolism. And, looking at the effect from a different angle, a person who is experiencing a 25 percent reduction in metabolism not only would have difficulty losing weight, but actually would have to reduce their calorie intake even further just to keep from gaining a pound of weight each week.

Standard Thyroid Labs Don't Tell the Story

Unless the physiological thyroid dysfunction caused by severe or yo-yo dieting is corrected, any attempts at diet and exercise are doomed to fail. Unfortunately, the dramatic reduction in T3 levels, combined with the increase in reverse T3, is not detected by the standard TSH blood test. Many patients will have a difficult time finding a doctor willing to treat their thyroid dysfunction due to doctor's over-reliance on the TSH test.

Proper Testing and Treatment for Success

In healthy individuals with a properly functioning metabolism, reverse T3 levels are typically below 50 pg/ml, and the ratio of Free T3 to Reverse T3 is greater than 1.8 if free T3 is measured in ng/dl, or 0.018 if free t3 is measured in pg/ml. The higher a patient's reverse T3 levels, the less effective T4 medications will be. For these patients, T4/T3 preparations such as natural desiccated thyroid are significantly better than T4-only preparations. And for those patients with higher RT3 levels, or whose FT3/RT3 ratio is less than indicated above, treatment with straight or time-released T3 can help reverse the metabolic dysfunction and put these patients on the road to achieving optimal body weight.

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