A 46 year old woman has gained weight. She is sensitivity to cold. Her pulse regular at 50bpm and heart is enlarged. What is the single most likely underlying mechanism for this condition

The key is A. Autoimmune. [The most common cause of hypothyroidism in UK is autoimmune

hypothyroidism. In developing countries it is often nutritional (iodine deficiency).The diagnosis is

autoimmune hypothyroidism (in hypothyroidism heart is often gets enlarged)].