In acquired hypothyroidism, the usual starting dosage of levothyroxine is based on body surface area (100 mcg / m 2 orally once a day) or on age and weight as follows: For ages 1 to 3 years: 4 to 6 mcg / kg once a day For ages 3 to 10 years: 3 to 5 mcg / kg once a day For ages 10 to 16 years: 2 to 4 mcg / kg once a day For ages ≥ 17 years: 1.6 mcg / kg once a day For both forms of hypothyroidism, the dose is titrated to maintain serum T4 and TSH levels within the normal range for age.