endocrine and metabolic disorders
Hypothyroidism
Treatment of Hypothyroidism
, adjusted until TSH levels are in midnormal range ( Treatment reference ) is preferred; the usual maintenance dose is 75 to 150 mcg orally once a day, depending on age, body mass index, and absorption (for pediatric treatment, see Treatment regimens ). The starting dose in young or middle-aged patients who are otherwise healthy can be 100 mcg or 1.7 mcg/kg orally once a day. In patients with heart disease, therapy is begun with low doses, usually 25 mcg once a day. The dose is adjusted every 6 weeks until maintenance dose is achieved. The maintenance dose may need to be increased in pregnant women. Dose may also need to be increased if drugs that decrease T4 absorption or increase its metabolic clearance are administered concomitantly. The dose used should be the lowest that restores serum TSH levels to the midnormal range (though this criterion cannot be used in patients with secondary hypothyroidism). In secondary hypothyroidism the dose oflevothyroxineshould achieve a free T4 level in the midnormal range. Liothyronine(L-triiodothyronine) should not be used alone for long-term replacement because of its short half-life and the large peaks in serum T3 levels it produces. The administration of standard replacement amounts (25 to 37.5 mcg twice a day) results in rapidly increasing serum T3 to between 300 and 1000 ng/dL (4.62 to 15.4 nmol/L) within 4 hours due to its almost complete absorption; these levels return to normal by 24 hours. Additionally, patients receivingliothyronineare chemically hyperthyroid for at least several hours a day, potentially increasing cardiac risks. Similar patterns of serum T3 changes occur when mixtures of T3 and T4 are taken orally, although peak T3 is lower because less T3 is given. Replacement regimens with synthetic T4 preparations reflect a different pattern in serum T3 response. Increases in serum T3 occur gradually, and normal levels are maintained when adequate doses of T4 are given. Desiccated animal thyroid preparations contain variable amounts of T3 and T4 and should not be prescribed unless the patient is already taking the preparation and has normal serum TSH. In patients with secondary hypothyroidism,levothyroxineshould not be given until there is evidence of adequatecortisolsecretion (orcortisoltherapy is given), becauselevothyroxinecould precipitate adrenal crisis. Myxedema coma is treated as follows: T4 given IV Corticosteroids Supportive care as needed Conversion to oral T4 when patient is stable Patients require a large initial dose of T4 (300 to 500 mcg IV) or T3 (25 to 50 mcg IV). The intravenous maintenance dose of T4 is 75 to 100 mcg once a day and of T3, 10 to 20 mcg twice a day until T4 can be given orally. Corticosteroids are also given because the possibility of central hypothyroidism usually cannot be initially ruled out. The patient should not be rewarmed rapidly, which may precipitate hypotension or arrhythmias. Hypoxemia is common, so PaO2 should be monitored. If ventilation is compromised, immediate mechanical ventilatory assistance is required. The precipitating factor should be rapidly and appropriately treated and fluid replacement given carefully, because hypothyroid patients do not excrete water appropriately. Finally, all drugs should be given cautiously because they are metabolized more slowly than in healthy people. 1.Jonklaas J, Bianco AC, Bauer AJ, et al: Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Thyroid24(12): 1670–1751, 2014. doi: 10.1089/thy.2014.0028 