I google thyroid and calorie restriction and came up with the info below. My doc told me that with 800 calories for 8 weeks, my thyroid would likely go down and it did. From high normal to low normal after 7 weeks. He said that once I am on the 5/2 plan it should re-establish itself. See below.
http://cathe.com/how-low-calorie-diets-affect-thyroid-function
Your thyroid gland actually produces much more T4 than it does T3, but T3 is the most active form of thyroid hormone. Once your thyroid makes T4, it’s converted to active T3 when it reaches tissues in your body. It’s T3 that binds to receptors on cells to regulate your metabolism.
So, how does dieting and calorie restriction throw things off? When you drop your calorie intake too low or place your body under stress in other ways, through injury or illness, less T4 is converted to T3. Instead more is made into something called reverse T3.
The problem with reverse T3 is it isn’t active. What it does do is bind to the same receptors that T3 does and makes it difficult for T3 to bind. With reverse T3 keeping active T3 out of the picture, you burn less fat and carbohydrates and your metabolism, along with everything else, slows down. If you’re trying to lose body fat, this makes it quite a bit harder to achieve your objective. This type of thyroid dysfunction can be challenging to pick up on standard thyroid function blood tests unless your doctor measures reverse T3 levels.
Thyroid Hormones 101 – from Holtorfmed
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.