Practical approach to the treatment of hypothyroidism.

Hypothyroidism most commonly is the result of primary thyroid failure due to autoimmune thyroiditis, but also may follow treated Graves' disease or may occur as transient hypothyroidism. Laboratory evaluation of thyroid function has been refined by the development of ultrasensitive thyroid-stimulating hormone determinations, allowing accurate diagnosis of primary hypothyroidism and assessment of treatment response. Determining whether therapy is appropriate for patients with subclinical hypothyroidism or euthyroid sick syndrome requires clinical judgment in addition to laboratory evaluation. Thyroid hormone replacement for hypothyroidism is best accomplished with levothyroxine. Factors that may affect serum thyroxine levels include thyroid binding globulin levels, drug interactions and variations in thyroxine absorption and clearance.