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Levothyroxine (T4) is a thyroid hormone that is commonly used to treat an underactive thyroid gland (hypothyroidism). It is sold under different brands, such as Synthroid, Unithroid and Levothroid. Our body needs enough thyroid hormones to function properly. Without levothyroxine, we may experience weight gain, muscle aches, lack of energy, dry skin, hair loss, sensitivity to cold temperatures, slow speech, constipation, and poor growth in children.

Levothyroxine sodium (Synthroid) is usually prescribed as an oral medication to be taken daily to replace your deficient hormone. Doctors usually prescribe a low dose at the beginning of treatment, and he/she may gradually increase this dose, depending on your body's response to treatment. It may take more than a few weeks before your symptoms improve. However, it should be taken regularly as directed and should not be discontinued, even when you feel better. In addition, you must consult your doctor if you wish to change brands or adjust your dosage.

Tell your doctor immediately if you experience side effects such as:

  • weight loss

  • tremors

  • nervousness

  • headaches

  • stomach cramps

  • diarrhea

  • nausea

  • vomiting

  • fever

  • excessive sweating

  • hot flashes

  • irritability

  • insomnia

  • increased appetite

  • changes in your menstrual cycle

  • increased sensitivity to heat

  • hair loss

Seek immediate help if signs of an allergic reaction occur, including hives, swelling of the face, tongue, lips, or throat, and difficulty breathing. You should also call your doctor if you experience chest pains or unusually rapid heartbeats.

Synthroid and Weight Gain

Weight loss is an expected side effect associated with increasing doses of Synthroid and other thyroid hormones. However, some patients have reported weight gain associated with the intake of Synthroid. If you are taking these hormones and are experiencing weight gain, it is possible that:

  • You are just beginning to take the drug and its full effects have not kicked in.

  • You are not taking the right dosage. Your dosage is usually based on your age, weight, laboratory test results, medical condition, and your individual response to treatment. Consult your doctor for reevaluation of you response to treatment.

  • You are taking other medications that reduce the absorption of the hormones. These drugs include cholestyramine, colesevelam, colestipol, antacids, Simethicone, sucralfate, iron, sodium calcium carbonate, polystyrene sulfonate, and orlistat. You must tell your doctor if you are currently taking any medication or dietary supplements, since any of these might affect the way your body is responding to treatment. You may have to take these medications hours apart so as not to affect the absorption of drugs.

  • You are consuming more calories than your body burns calories. Synthroid does not cause weight loss at normal doses.

  • You are not taking it as directed, and you often miss doses. This leads to lack of therapeutic effect, means that your symptoms are not improving.

Warning: Synthroid is a thyroid hormone replacement drug that must be used as directed.

It must not be used as a weight-loss drug, especially in obese individuals who have normal thyroid function. It must not be used with amphetamines to induce weight loss.

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