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What is the downside to taking levothyroxine?

Levothyroxine is a synthetic version of thyroxine, a hormone normally produced by the thyroid gland. It is used to treat hypothyroidism, a condition where the thyroid does not produce enough thyroid hormone. Levothyroxine replaces the natural hormone thyroxine, restoring normal levels and relieving symptoms of hypothyroidism.

While levothyroxine is an effective treatment for hypothyroidism, there are some potential downsides to taking this medication long-term that patients should be aware of. In this article, we’ll explore the downsides and risks associated with levothyroxine therapy.

Common Side Effects

Like most medications, levothyroxine can cause side effects in some patients. The most common side effects are usually minor but can impact quality of life. Potential side effects of levothyroxine include:

  • Headaches
  • Insomnia or difficulty sleeping
  • Appetite changes or weight fluctuations
  • Hair loss
  • Nervousness or anxiety
  • Fatigue
  • Muscle cramps or weakness
  • Changes in menstrual cycle

These side effects are usually temporary and often improve as the body adjusts to the medication. However, if side effects persist or are bothersome, patients should discuss options with their doctor, such as adjusting the dosage.

Risk of Overmedication

One potential downside is the risk of overmedication with levothyroxine. Taking too high of a dose can lead to symptoms of hyperthyroidism, which is when thyroid hormone levels are too high.

Signs of overmedication to look out for include:

  • Rapid or irregular heartbeat
  • Nervousness, anxiety, irritability
  • Difficulty sleeping
  • Weight loss
  • Diarrhea
  • Muscle weakness
  • Shortness of breath
  • Heat intolerance or sweating

Overmedication usually occurs when the levothyroxine dose is too high. It’s important for patients taking this medication to have thyroid hormone levels checked regularly through blood tests to ensure proper dosing. Doctors may need to adjust the dosage over time to keep levels in the normal range.

Increased Risk of Heart Problems

Some research indicates that there could be an increased risk of heart-related issues in patients taking higher doses of levothyroxine long-term.

Specifically, there is some evidence linking high doses of thyroid hormone replacement to increased risk of atrial fibrillation. Atrial fibrillation is an irregular heart rhythm that can lead to palpitations, shortness of breath, and complications like blood clots or stroke.

Studies have found mixed results in terms of the link between levothyroxine and atrial fibrillation:

  • One large study found levothyroxine use was associated with a 31% increased risk of atrial fibrillation in older adults.
  • Another study found a 3-fold increased risk of atrial fibrillation in levothyroxine users.
  • However, other analyses have not found a definitive link between levothyroxine therapy and atrial fibrillation.

More research is still needed, but current evidence suggests those on higher doses may need to be monitored more closely.

Interactions with Other Medications

Levothyroxine can potentially interact with many different medications. The absorption and effectiveness of levothyroxine can be affected when taken at the same time as certain other drugs.

Some examples of medications that may interact with levothyroxine include:

  • Calcium and iron supplements
  • Antacids
  • Cholesterol drugs
  • Seizure medications
  • Some antidepressants
  • Diabetes medications
  • Blood thinners
  • Blood pressure medications

These types of drugs can decrease the absorption of levothyroxine if taken too closely together. To prevent interactions, thyroid hormone medications should be taken on an empty stomach 30-60 minutes before food and other medications.

Patients starting any new medication should always check for potential thyroid drug interactions to ensure thyroid levels remain stable.

Increased Bone Loss

Thyroid hormones play an important role in bone health and remodeling. Although levothyroxine is used to return thyroid hormone to normal levels, there is some evidence that excessive amounts can increase the risk of bone loss and fractures.

Studies on the effects of levothyroxine on bone health have shown mixed results:

  • Some studies have found an increased rate of bone loss and fracture risk in women taking suppressive doses of levothyroxine after thyroid cancer treatment.
  • Other analyses have found no increase in fracture risk for patients on thyroid hormone replacement doses.
  • One study showed minimal bone density loss in premenopausal women on long-term levothyroxine treatment.

Current evidence overall does not indicate an increased fracture risk for patients taking replacement doses of levothyroxine. However, patients with risk factors for osteoporosis, like postmenopausal women, may need their bone health monitored more closely while on therapy.

Can Cause Thyroid Swelling

In some cases, levothyroxine treatment can cause swelling or enlargement of the thyroid gland, a condition known as goiter. This is not common with thyroid hormone replacement therapy but tends to occur more often when the medication is used to treat thyroid cancer.

Goiter may be more likely in people who take levothyroxine and:

  • Have residual thyroid tissue after surgery
  • Take too low of a replacement dose
  • Have underlying thyroid disease like Hashimoto’s thyroiditis

Thyroid swelling from levothyroxine use is usually mild. However, any significant neck swelling, tenderness, or difficulty swallowing should be evaluated by a doctor.

Risks if Overmedicated During Pregnancy

It’s important for women who become pregnant while taking levothyroxine to have their thyroid hormone levels monitored closely before and during pregnancy. Too much thyroid hormone could be dangerous for fetal development.

Potential risks of levothyroxine overmedication during pregnancy can include:

  • Premature birth
  • Low birth weight
  • Stillbirth
  • Developmental or cognitive problems for the child after birth

Thyroid requirements often change significantly during pregnancy, so frequent testing and levothyroxine dose adjustments are needed to keep levels optimized. Working closely with both an endocrinologist and obstetrician ensures safe regulation of thyroid hormones.

Long-Term Dependence on Medication

For most people with hypothyroidism, lifelong treatment with levothyroxine will be necessary. The thyroid typically does not regain normal function, so replacing thyroid hormone through medication is ongoing.

The long-term reliance on medication to manage hypothyroidism could be seen as a downside by some patients. Important points about levothyroxine dependence include:

  • Missing doses can quickly cause a relapse of hypothyroidism symptoms.
  • Thyroid hormone levels need to be regularly monitored through blood work.
  • The medication regimen requires discipline to take pills daily, avoid interactions with other drugs/supplements, and fast before blood tests.
  • The dose often needs periodic adjustment based on lab results and changing needs.

While burdensome for some, most patients are able to adapt well to long-term levothyroxine therapy. Working closely with an endocrinologist and thyroid care team can help optimize treatment over a lifetime.

Risk of Allergic Reactions

As with all medications, there is a small risk of allergic reaction in some patients taking levothyroxine. Signs of allergic reaction can include:

  • Rash or hives
  • Itching
  • Swelling of the face, lips, tongue, or throat
  • Difficulty breathing
  • Dizziness or fainting

Allergic reactions to levothyroxine are rare when the medication is taken as prescribed. However, patients who experience any concerning symptoms while on this medication should seek medical care immediately.

In the case of a severe reaction, doctors may recommend switching to a different thyroid hormone replacement, such as liothyronine or desiccated thyroid extract.

Masking Hyperthyroidism

In some cases, levothyroxine can mask the signs of hyperthyroidism if not dosed properly. Hyperthyroidism results when the thyroid produces too much thyroid hormone on its own.

Levothyroxine doses that are too high relative to a patient’s needs can overwhelm the body and create thyroid hormone excess. This can essentially disguise hyperthyroidism by overriding the symptoms.

Masking of hyperthyroidism is most likely in patients who:

  • Have thyroid nodules or Graves’ disease
  • Are undertreated for hypothyroidism
  • Have changing thyroid requirements

Careful monitoring of thyroid function is important in these patients to avoid missing a diagnosis of hyperthyroidism.

Increased Pressure Behind the Eyes

In rare instances, levothyroxine therapy can cause increased pressure around the eyes, leading to symptoms like eye pain, redness, bulging of the eyes, and vision changes.

This risk may be greater in patients who:

  • Have severe hypothyroidism
  • Are significantly overmedicated with levothyroxine
  • Have underlying eye issues like glaucoma or cataracts
  • Are elderly

Symptoms involving pressure behind the eyes warrant urgent evaluation, since permanent vision damage can occur if untreated. Prompt ophthalmological assessment is needed.


In summary, levothyroxine is typically an effective and well-tolerated treatment for hypothyroidism. However, there are some potential adverse effects patients should be aware of. The most common issues involve side effects, interactions with other medications, overmedication, and long-term dependence on the drug.

To minimize risks, patients taking levothyroxine should follow their prescribed dosage closely, report any side effects, undergo regular thyroid function tests, and notify all healthcare providers about their medication regimen. While levothyroxine therapy is generally safe under the care of a thyroid specialist, appropriate monitoring and follow-up helps detect any problems early.