Levothyroxine is a prescription medicine that replaces thyroxine (T4), a hormone normally made by the thyroid gland. It is mainly used to treat hypothyroidism, when the body does not make enough thyroid hormone. It may also be used with surgery and radioactive iodine for certain thyroid cancers. By restoring or adjusting hormone levels, levothyroxine helps the body’s systems work properly; the dose is chosen and monitored by a clinician.
What conditions does levothyroxine treat?
Levothyroxine is a synthetic form of thyroxine, the main hormone produced by the thyroid gland. It is most commonly prescribed when the body cannot produce enough thyroid hormone. According to MedlinePlus, the medicine replaces this missing hormone and helps address problems caused by an underactive thyroid.
1. Underactive thyroid (hypothyroidism)
Hypothyroidism occurs when the thyroid gland does not make enough hormone to meet the body's needs. These hormones help regulate how the body uses energy and support normal functioning of the heart, muscles, brain, and digestive system. Without enough thyroid hormone, several body processes may slow down.
Levothyroxine restores the hormone supply, helping thyroid hormone levels return toward the appropriate range. It can help improve symptoms associated with hypothyroidism, such as:
- Persistent tiredness or low energy.
- Increased sensitivity to cold.
- Constipation.
- Dry skin or thinning hair.
- Unexplained weight gain.
- Low mood or difficulty concentrating.
- Heavy or irregular menstrual periods.
These experiences can have causes other than hypothyroidism, so they are not enough on their own to establish that thyroid hormone treatment is needed. A healthcare professional considers symptoms alongside medical history and thyroid blood test results.
2. Thyroid hormone replacement after thyroid surgery
Some people need levothyroxine after surgery to remove part or all of the thyroid gland. When the entire gland is removed, the body can no longer produce its usual supply of thyroid hormone, making lifelong replacement treatment necessary. After partial removal, the remaining thyroid tissue may produce enough hormone, so the need for medication depends on thyroid function.
The Mayo Clinic explains that people who have their entire thyroid removed need daily thyroid hormone medicine to replace the gland's normal function.
3. Certain types of thyroid cancer
Levothyroxine may also be prescribed as part of treatment for certain differentiated thyroid cancers, usually alongside surgery and radioactive iodine when appropriate. In these cases, it can be used to suppress thyroid-stimulating hormone (TSH), a hormone that signals thyroid cells to grow. The intended degree of TSH suppression depends on the person's cancer characteristics and treatment plan.
The American Thyroid Association describes this approach as suppression therapy, which is used in selected people with thyroid cancer to help reduce the risk of recurrence or progression. Not everyone with thyroid cancer needs the same degree of hormone suppression.
How does levothyroxine work in the body?
The thyroid gland normally releases hormones that help control energy use, body temperature, heart rate, digestion, and other important functions. Levothyroxine supplies synthetic T4, which acts like the thyroxine naturally made by the body. The body can convert some T4 into triiodothyronine (T3), another active thyroid hormone.
When levothyroxine is prescribed for hypothyroidism, the goal is to replace the missing hormone without providing too much. The correct dose supports normal body function while avoiding the effects of hormone deficiency or excess. The medicine controls an ongoing hormone deficiency but does not necessarily cure the underlying thyroid condition.
Who may need levothyroxine?
Levothyroxine can be prescribed to adults, children, and infants when a medical condition causes thyroid hormone levels to be too low. The reason for treatment and the appropriate dose depend on the person's age, thyroid function, medical history, and other health conditions.
Common situations in which a clinician may prescribe levothyroxine include:
- Hashimoto's disease: An autoimmune condition that can damage the thyroid and reduce hormone production.
- Congenital hypothyroidism: An underactive thyroid present from birth, which requires appropriate treatment to support normal growth and development.
- Thyroid surgery or radiation: Treatment may be needed when the thyroid can no longer produce enough hormone.
- Pituitary or hypothalamic disorders: Problems affecting the body's hormonal control system can contribute to inadequate thyroid hormone production.
- Selected thyroid cancers: Hormone treatment may be used to replace thyroid hormone and, when indicated, suppress TSH.
Older adults and people with heart disease may need particularly cautious dose selection. People who are pregnant or planning pregnancy should also tell their healthcare professional, because thyroid hormone requirements and monitoring may change during pregnancy.
How should levothyroxine be taken?
Levothyroxine is usually taken once a day as a tablet or oral liquid. The most important point is to take it consistently and follow the instructions for the specific product prescribed. According to the NHS, tablets are generally taken on an empty stomach, 30 to 60 minutes before the first meal of the day.
For many people, a practical routine involves taking the medicine with water after waking up and waiting before eating breakfast or drinking coffee. Food, caffeine, and certain medicines can affect how much levothyroxine the body absorbs. If a morning routine is difficult, ask a healthcare professional whether a consistent bedtime schedule would be suitable.
Which medicines and supplements can affect levothyroxine?
Some medicines and supplements can interfere with levothyroxine absorption or change the amount needed to maintain appropriate thyroid hormone levels. The Cleveland Clinic identifies calcium supplements, iron supplements, certain antacids, and several other medicines as potential interactions.
Calcium and iron supplements are generally separated from levothyroxine by at least four hours. Other medicines may need different timing or additional monitoring. Tell your prescriber and pharmacist about all prescription medicines, over-the-counter products, vitamins, and herbal supplements you use. Do not change the timing of essential medicines without checking the relevant instructions.
How long does levothyroxine take to work?
Levothyroxine does not always relieve symptoms immediately. Some people begin to notice improvement within a few weeks, while others need more time. The response depends on the starting hormone levels, the underlying condition, and whether the dose is appropriate.
Feeling better is not the only way to judge whether treatment is working. Healthcare professionals use thyroid blood tests, especially TSH and sometimes free T4, to assess the response and decide whether a dose adjustment is necessary. Follow-up testing is particularly important after starting treatment or changing the dose.
Can levothyroxine cause side effects?
When the dose is appropriate, levothyroxine is generally well tolerated. Many unwanted effects occur when the body receives more thyroid hormone than it needs. Possible signs of excess hormone include:
- A racing, pounding, or irregular heartbeat.
- Shakiness or trembling.
- Feeling unusually anxious or restless.
- Excessive sweating or increased sensitivity to heat.
- Difficulty sleeping.
- Unexplained changes in weight.
Contact your healthcare professional if these symptoms develop, persist, or become troublesome. Chest pain, fainting, or a fast or irregular heartbeat accompanied by significant symptoms requires urgent medical assessment. Do not reduce, increase, or stop levothyroxine on your own, even if you think the dose is too high.
Is levothyroxine used for weight loss?
No. Levothyroxine is not a weight-loss medicine and should not be used to lose weight in people whose thyroid function is normal. Although untreated hypothyroidism can contribute to weight gain, taking extra thyroid hormone does not provide a safe or appropriate weight-loss strategy for someone who does not need hormone replacement.
The official prescribing information on DailyMed warns that thyroid hormones should not be used to treat obesity or promote weight loss. Taking excessive amounts can cause serious, potentially life-threatening effects, including dangerous heart problems.
Does levothyroxine need to be taken for life?
Many people with permanent hypothyroidism need levothyroxine for the rest of their lives. This is common when the thyroid gland has been removed or when an ongoing condition prevents it from producing enough hormone. However, treatment duration depends on the cause of the hormone deficiency and whether thyroid function can recover.
Continue the medicine for as long as it is prescribed. If your medical circumstances change, your healthcare professional can review your thyroid function and determine whether the dose or treatment plan needs to change.
How do doctors monitor levothyroxine treatment?
Monitoring helps ensure that the prescribed dose provides enough thyroid hormone without causing excess. Healthcare professionals generally use TSH blood tests to assess treatment for primary hypothyroidism and may also measure free T4 when appropriate. The timing of follow-up tests depends on whether treatment has just started, the dose has changed, or thyroid function is already stable.
Age, pregnancy, changes in other medicines, significant changes in health, and certain medical conditions can affect treatment requirements. Tell your clinician if your symptoms change or if you start a new medicine or supplement. If your pharmacy changes the manufacturer or formulation, ask whether additional monitoring is appropriate.