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Can your thyroid function properly in the absence of dietary carbs?
You see, in vitro, insulin has been shown to stimulate deiodination of T4 to T3 in hepatocytes (deiodination - the removal of iodine from a compound (as a thyroid hormone))
T4 is converted into T3, the active form of thyroid hormone, by two enzymes called deiodinases
Iodothyronine deiodinases are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones**
Keto & Thyroid Studies
As we know, starvation and very-low-calorie diets decrease the level of circulating thyroid hormones in the bloodstream
Low-carb and ketogenic diets have also been found to reduce levels of T3 in the bloodstream
Now, a 1976 study, published in The Journal of Clinical Endocrinology and Metabolism, looking at obese people showed that T3 levels dropped significantly during a carb-free 800-calorie diet but didn't change much on an 800-calorie diet providing at least 50 grams of carbs per day
Keto diets do lead to lower T3 levels, but RMR remains the same
Keto & Thyroid Sensitivity - Dr. Phinney
As mentioned, a reduction in active thyroid hormone (called T3) has been taken as evidence that carbohydrate restriction impairs thyroid function
However, Dr. Stephen Phinney has an alternative explanation as to why this occurs:
"When one is weight stable on a LCHF diet is that the body becomes more responsive to these hormones due to beneficial changes in cell structure and function when in nutritional ketosis - as a result, it can function normally at lower T3 levels
Put another way, a keto diet seems to result in improved thyroid hormone sensitivity (i.e., it takes less hormone to produce the same effect), which, if anything, puts less of a burden on thyroid hormone (T4) production in the thyroid gland and its conversion to T3 in the liver"
"Interpretation of the Data
The only viable interpretation of these data is that ketogenic diets markedly increase tissue sensitivity to T3, and thus serum T3 levels decline while the physiological response to T3 remains normal.
In this scenario, both the thyroid and the liver have to do far less “work” to maintain a normal thyroid physiologic response."
Dr. Phinney Studies
Study 1: https://www.ncbi.nlm.nih.gov/pubmed/7000826
Study 2: https://www.ncbi.nlm.nih.gov/pubmed/6865775
Study 3: https://www.ncbi.nlm.nih.gov/pubmed/6865776
In addition, a study published in Nutrition & Metabolism in which 28 diabetics were given a LCHF diet for four months, during which their mean TSH values did not change significantly (1.6 to 1.4 uU/L)