In order to study the relationships between serum enzymes and the degree of hypothyroidism, 114 patients with primary hypothyroidism aged from 7 to 65 years were investigated. Forty one percent of patients exhibited normal levels of serum enzymes, while 59% had high levels either alone or in combination. The frequency of enzyme elevation was as follows: creatine kinase: 37%, aspartate aminotransferase: 35%, alanine aminotransferase: 29%, amylase: 15%, alkaline phosphatase: 3%. No significant correlation between thyroid stimulating hormone and serum enzyme levels was observed. This was due to highly variable release of enzymes from cells resulting presumably from individual metabolic set-point. Replacement therapy with thyroxine resulted in remarkable lowering of creatine kinase not only from high level to normal as early as 3 weeks even before normalization of thyroid stimulating hormone, but also from high normal to low normal level. The elevation of amylase and its response to thyroxine is being reported for the first time.