A different cause is a hyper functioning thyroid adenoma where the follicular cells start growing uncontrollably, forming a benign tumor that makes excess thyroid hormones.
The leading cause of that is a pituitary adenoma, which is a benign tumor of the pituitary gland- and this specific condition is called Cushing disease.
The exact reason for the development of this sort of benign tumor isn't known - but unlike a malignant tumor - the cells don't invade neighboring tissues or spread to other parts of the body.