Low T3 thyroid hormone is one of the largest causes of Dementia, which is often misdiagnosed as Alzheimer’s Disease. T3 is the active form of thyroid.
Thryroid consists of T1, T2, T3, T4 and calcitonin. However, standard lab tests measure only TSH (Thyroid Stimulating Hormone) and T4 (the storage form of the thyroid). Even if these are abnormal, most thyroid supplements consist of T4. If your body can no longer convert T4 to T3 then your lab tests will be normal, yet you will still be low thyroid.
One of the best websites for a basic education on Thyroid is www.stopthethyroidmadness.com.