Right, Chris Kresser - see my citation from Tucker Goodrich below.
Source Dr. Dominic D’Agostino:
"When the body senses low thyroid hormone levels, the hypothalamus will release Thyrotropin Releasing Hormone (TRH) to stimulate the release of Thyroid Stimulating Hormone (TSH) from the pituitary gland. TSH binds to the thyroid gland, and stimulates the release of thyroxine (T4; inactive thyroid hormone); TSH is in control of circulating levels of T4…
“Carbohydrate-restriction and the ketogenic diet can cause a drop in active thyroid hormone (T3), when calories are restricted and even when calories are kept at maintenance. However, a drop in T3 does not automatically mean you have an under-active thyroid or that you are on the path to hypothyroidism. Hypothyroidism is characterized not only by a drop in T3 but a drop in resting energy expenditure/metabolic rate, abnormally high TSH levels (body’s way of asking for more T4, to make more T3), and abnormally low T4 levels…”
Source Stephen Phinney Virta Health
"… reduction in active thyroid hormone (called T3) has been taken as evidence that carbohydrate restriction impairs thyroid function. Some have opined that carb restriction should never be maintained below 100 grams per day in order to prevent this effect. Others advocate that people on a low-carbohydrate, high fat (LCHF) diet take intermittent ‘holidays’ from carb restriction to boost thyroid function back up to ‘normal.’
“An alternative explanation for these changes in thyroid hormones 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 ketogenic 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.”
Source Amber O’Hearn
“There is no evidence that we are aware of indicating that ketogenic diets cause hypothyroid, or negatively impact thyroid function. The fact that T₃ is lower in ketogenic dieters is probably part of the mechanism that protects lean mass when fat is being lost. Moreover, low T₃ may possibly even be an indicator of a life extending effect, an effect we have suggested elsewhere when examining the cortisol profile of ketogenic dieters.”
Source Tucker Goodrich
“… two of the top low-carb nutritional health researchers in the world — Dr. Stephen Phinney and Dr. Jeff Volek — say this phenomenon with low thyroid while on a low-carbohydrate diet promoted by people in the Paleo community like Kresser and Paul Jaminet is “a myth” and has not manifested itself in any of the research subjects in their numerous studies of people who are properly following a well-formulated low-carb diet with adequate calories over the past three decades.”
Source Sophie Laura Certified Nutrition Coach
“Decreasing carb intake dramatically, moderating protein and increasing fats increases tissue sensitivity to T3, and due to this, serum T3 levels go down all the while the psychological response to T3 remains normal. In this scenario, both the thyroid and the liver have far less work to do in order to maintain a normal thyroid physiological response – which is what we want!”
Source Thyroid Nutrition Resource Guide:
“There were only two studies showing negative thyroid outcomes in a ketogenic diet. Both focused on children and possible pre-existing thyroid conditions…”
Note: this is from a very long and detailed explanation of thyroid function in a ketogenic condition. Well worth the time to read and learn.
PS: In the Tucker Goodrich Source cited above he discusses several related studies/analyses of how thyroid function/hormones assist insulin to clear glucose. The relationship of insulin, TSH, T3 and T4 to glucose management is well worth the read.