Levothyroxine
Known brand names include Synthroid, Levoxyl, Unithroid, Euthyrox, and Tirosint.
Synthetic thyroid hormone (T4), taken once a day on an empty stomach to replace what an underactive thyroid no longer makes. It is usually taken for life.
Known brand names include Synthroid, Levoxyl, Unithroid, Euthyrox, and Tirosint.
Synthetic thyroid hormone (T4), taken once a day on an empty stomach to replace what an underactive thyroid no longer makes. It is usually taken for life.
Levothyroxine is a man-made copy of T4, the main hormone the thyroid makes, and it acts just like the body's own T4. Tissues throughout the body, mainly the liver, turn it into T3, which does most of the hormone's work. T3 and T4 enter the nucleus of cells, attach to receptors on DNA, and switch on genes that make new proteins.
The body clears it slowly, removing about half of it every 6 to 7 days. Its full effect may take 4 to 6 weeks to show, and people may not notice their symptoms improving for several weeks. Foods such as soybeans and fiber, and many other drugs, can lower how much is absorbed from the gut. Too much can speed up the heart and make it work harder, causing chest pain or an irregular heartbeat, especially in older people and those with heart disease.
Levothyroxine replaces thyroid hormone when the thyroid gland does not make enough, a condition called hypothyroidism. Without enough of this hormone, people can feel tired and cold, gain weight, become constipated, and have dry skin, slow speech, and low mood. Taken correctly, it reverses these symptoms by keeping hormone levels normal. It works whether the shortage is present from birth or develops later, and whether the problem starts in the thyroid itself or in the brain areas that control it.
It is used by adults, children, and newborns, and it is taken by mouth. It is also used along with surgery and radioactive iodine for some thyroid cancers, to keep thyroid-stimulating hormone, a signal these cancers depend on, low. It controls an underactive thyroid but does not cure it. It should not be used for weight loss, and the label rules out using it to shrink harmless thyroid nodules or goiter in people who get enough iodine.
Taken at the right dose, it should cause few problems. Too much brings signs of an overactive thyroid: feeling hot, sweating, weight loss, shakiness, trouble sleeping, anxiety, diarrhea, and a fast or pounding heartbeat. The label treats heart rhythm problems, chest pain, and heart failure from too high a dose as dangerous, along with bone loss, a sudden adrenal crisis in people with untreated adrenal problems, and worse blood sugar control in diabetes.
A clinician may order these while someone is taking it.
This is the boxed warning, the warning the FDA label prints first.
Not for obesity or weight loss. Replacement doses do not reduce weight in people with normal thyroid function, and larger doses can cause serious or life-threatening toxicity, especially with sympathomimetic appetite suppressants.
Sources: [1]