Carbohydrates are necessary for the conversion of inactive thyroid hormone to active thyroid hormone, and if you’re on an extremely strict low carbohydrate diet, then you may actually be limiting this conversion. Your TSH is what tells your thyroid gland to “release more hormone,” so your TSH rises when your thyroid gland is underactive, or conversion of inactive to active thyroid hormone is inadequate. A high TSH means that the pituitary gland is releasing its hormone to try to get the thyroid to respond and produce more thyroid hormone. Because of inadequate carbohydrates, TSH will often elevate in a high-fat, low-carber – indicating potential for long-term thyroid and metabolic damage.
On the ketogenic diet, carbohydrates are restricted and so cannot provide for all the metabolic needs of the body. Instead, fatty acids are used as the major source of fuel. These are used through fatty-acid oxidation in the cell's mitochondria (the energy-producing parts of the cell). Humans can convert some amino acids into glucose by a process called gluconeogenesis, but cannot do this by using fatty acids. Since amino acids are needed to make proteins, which are essential for growth and repair of body tissues, these cannot be used only to produce glucose. This could pose a problem for the brain, since it is normally fuelled solely by glucose, and most fatty acids do not cross the blood–brain barrier. However, the liver can use long-chain fatty acids to synthesise the three ketone bodies β-hydroxybutyrate, acetoacetate and acetone. These ketone bodies enter the brain and partially substitute for blood glucose as a source of energy.