The impacts of hyper and hypothyroidism on body composition, i.e. the relative quantity and quality of bone, adipose tissue and muscle, have traditionally been attributed uniquely to abnormal levels of free thyroid hormones. The presence of biologically active TSH receptors in bone, fat and muscle, raises the possibility that both thyroid hormones and TSH contribute to the changes in body composition associated with thyroid disease. This review evaluates the evidence for this in terms of the in vitro experimental approaches applied, data from in vivo sources (i.e. mouse models) and patient-based studies.