The heart is very sensitive to even small changes in the levels of thyroid hormones. The heart is a muscle and contains receptors for thyroid hormone, hence heart muscle growth and cardiac function may be influenced by too much or too little thyroid hormone. There is increasing evidence that even small changes in levels of thyroid hormone may be associated with measurable changes in how well the heart functions. For an overview of the literature, see Thyroid hormone and heart failure. Curr Heart Fail Rep. 2006 Sep;3(3):114-9 and Effects of subclinical thyroid dysfunction on the heart. Ann Intern Med. 2002 Dec 3;137(11):904-14.