Thyroid hormone circulates in the blood as T4 and T3, where T3 is the active hormone which binds to the cells’ thyroid hormone receptors in the nucleus.
MCT8 deficiency prevents T3 to get into certain cells of the body such as the brain.
While a defective or absent MCT8 causes thyroid hormone deficiency (hypothyroid) in the brain, it also increases the circulating active hormone T3 by causing an enzyme (deiodinase) to remove one iodine from T4.
This high level of T3 in the blood causes those other cells of the body, which do not depend solely on MCT8 for transport, to have excess active hormone (thyrotoxic) which results in increase in metabolism resulting in high heart rates, difficulty to gain weight and irritability.
