Overt hypothyroidism is known to be associated with increased serum creatine kinase (CK) levels. However, there is little information on CK levels in subclinical hypothyroidism. The aim of the study was to assess the relationship between CK levels and thyroid function in overt and subclinical hypothyroidism. Thyroid function tests (thyrotropin [TSH], free thyroxine [FT4], free triiodothyronine [FT3]) and the serum levels of CK were obtained from 23 patients admitted to a general hospital for illnesses other than thyroid or muscular diseases, myocardial ischemia, or brain damage. Overt hypothyroidism, based on thyroid function tests, was present in 10 patients, whereas hypothyroidism could be classified as subclinical in the other 13. A positive correlation was observed between CK and thyrotropin, and to a lesser extent between CK and thyroid hormones. Moreover, the correlation between CK and TSH and between CK and FT4 was detectable in subclinical hypothyroidism. Our data suggest that even in subclinical hypothyroidism there is some degree of dysfunction in skeletal muscle metabolism.