Levothyroxine Side Effects: Brand names Synthyroid, Levothroid and others. Doses are available in 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 250 mcg, 300 mcg. Levothyroxine is used for replacement of thyroid hormone in patients who are hypothyroid, meaning they produce too little thyroid hormone. Most patients who are hypothyroid have an autoimmune disorder, although surgical removal and congenital hypothyroidism are relatively common. Women are affected by hypothyroidism more commonly than men. Thyroid hormone plays a role in metabolism and levothyroxine side effects are largely due to taking a dose of levothyroxine that leads to serum levels higher than the body would naturally produce in the euthyroid state.

Black Box Warning: Levothyroxine is not to be used for weight loss by inducing a hyperthyroid state pharmacologically. Doses that make patients euthyroid are ineffective for weight loss and doses that make a patient hyperthyroid may lead to serious or life threatening conditions. Levothyroxine is also not to be used in combination with sympathomimetic amines to induce weight loss as this may also lead to life threatening conditions.

Common Levothyroxine Side Effects: In ideal dosing where the person using levothyroxine is euthyroid, levothyroxine side effects are very uncommon. That stated levothyroxine side effects are fairly common because it is not unusual to become mildly to moderately hyperthyroid from taking a levothyroxine dose that is higher than needed. Levothyroxine side effects in this situation that are common include cardiac effects including tachycardia and palpitations. Psychologic symptoms manifesting themselves as anxiety including tremors, nervousness, and insomnia, as well as gastrointestinal symptoms like diarrhea, nausea, anorexia, abdominal cramps, and weight loss are common. Sweating, headache, hair loss, menstrual irregularities and heat intolerance are also often reported.

Serious Levothyroxine Side Effects: Again serious levothyroxine side effects are largely due to intentional or inadvertent dosing that make the individual hyperthyroid. Starting thyroid replacement too quickly in elderly patients or those at risk for coronary disease is to be avoided. Seizures, cardiac arrhythmias, congestive heart failure, angina pectoris, and high blood pressure can be seen. In children and neonates pseudotumor cerebri, craniosynostosis (premature closure of the skull bone separations), and premature closure of bone growth plates can be noted.

Levothyroxine in Pregnancy and Lactation: Pregnancy category A, essentially proven safe, and felt to be safe to use during lactation.

Cautions and Drug Interactions: Caution is urged in using levothyroxine in the treatment of elderly patients and those with risks for coronary artery disease. In patients with coexisting adrenal insufficiency treatment of the adrenal insufficiency is critical to avoid adrenal crisis. Treatment in the face of acute MI is of concern. Drug interactions are many, and are largely with drugs where cardiac arrhythmias are associated and where the additional drug may either interfere with absorption or metabolism of the levothyroxine.

Interesting Facts: Levothyroxine is a synthetically produced human thyroid hormone. It is generally felt to be superior in consistency of dose and efficacy to USP thyroid which is porcine thyroid hormone. Many naturopathic physicians and others promote the use of Armour Thyroid or USP Thyroid as a superior “natural” alternative. It seems odd that use of another species thyroid glands dried out, ground up and standardized is felt to be more natural than taking synthetically produced human levothyroxine.

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