Laboratory measurements also serve as useful guides in the treatment of hypothyroid patients with levothyroxine sodium. The dosage and rapidity of treatment with levothyroxine depend on the clinical setting and are described elsewhere. The goal of therapy is to restore most patients to a euthyroid state and to normalize the serum T₄ and TSH concentrations. The recent availability of TSH assays that distinguish between normal TSH concentration and suppressed values such as occur in hyperthyroidism (see below) now enables the clinician to select a dose of levothyroxine that normalizes not only the clinical symptoms but also the serum T₄ and TSH concentrations. Monitoring the serum TSH level to ascertain that it remains above the lower limit of the normal range will prevent the potential risk of overtreatment with levothyroxine. In patients with hypothyroidism caused by hypothalamic-pituitary failure, alleviation of the clinical syndrome and restoration of serum T₄ to the normal range are the only criteria available for estimating the appropriate replacement dosage of levothyroxine.