According to the American Heritage Medical Dictionary, euthyroidism refers to either the physiological state one has if her thyroid hormone serum levels are normal or it is the regular, healthy functioning of the thyroid gland. Euthyroidism is in contrast with abnormal thyroid function in the form of hyperthyroidism or hypothyroidism.
Hyperthyroidism refers to the condition of having an overactive thyroid, while hypothyroidism refers to having an underactive thyroid. The American Thyroid Association states that euthyroid sick syndrome describes abnormal thyroid hormone concentrations that occur without thyroidal illness. Instead, non-thyroidal illnesses cause low concentrations of triiodothyronine, total thyroxine and thyroid stimulating hormones.