The "euthyroid sick syndrome", representing the changes of thyroid hormone metabolism in disease, embraces the following hormonal changes: 1) A decrease in serum T3 levels which is often accompanied by an increase in rT3 levels. Serum T4 levels remain normal. These changes are extremely frequent in many varied diseases. 2) In very severe disease, serum T4 levels may also decrease. The free T4 index is often decreased and the free T4 may either be normal or decreased. These changes reflect alterations of thyroid hormone metabolism and can be distinguished from primary hypothyroidism by a normal serum TSH level. 3) Occasionally there are transient increases of total and free T4. This disturbance of serum thyroid hormone levels is mostly due to drug interference with thyroid hormone metabolism (amiodarone etc.). Differentiation from hyperthyroidism is difficult. It is important to diagnose the "euthyroid sick syndrome" since this will avoid erroneous diagnosis and treatment or hypo- and hyperthyroidism.