TG and Alzheimer disease: Conversely, chronically high thyroid hormone levels have been associated with AD, probably through progressive acetylcholine depletion, thereby giving rise to cognitive problems linked with the cholinergic deficit.[7] A community-based study in the elderly showed a threefold increase in dementia and AD in patients with baseline subclinical hyperthyroidism.[8] AD, by itself, can cause an undersecretion of thyrotrophin releasing hormone (TRH) from the hypothalamus.