From the Guidelines
Levothyroxine therapy can cause several side effects, including adverse effects on bone mineral density and the cardiovascular system, such as angina, arrhythmia, and increased cardiac wall thickness, as noted in the product label 1.
Common Side Effects
- Headache
- Insomnia
- Nervousness
- Irritability
- Tremors
- Increased sweating
- Heat intolerance These symptoms often indicate that the dose may be too high, resulting in a state of hyperthyroidism.
Less Common Side Effects
- Digestive issues such as diarrhea, vomiting, and abdominal cramps
- Hair loss, particularly during the first few months of treatment
- Allergic reactions, though rare, may manifest as rash, itching, or swelling
Serious Side Effects
- Chest pain
- Rapid or irregular heartbeat
- Shortness of breath
- Severe headache Elderly patients are particularly sensitive to levothyroxine and may experience cardiac symptoms even at therapeutic doses, as the previous review found that one quarter of patients who received levothyroxine were inadvertently maintained on doses high enough to make TSH levels undetectable, which could increase risk for osteoporosis, fractures, abnormal cardiac output, or ventricular hypertrophy 1.
Important Considerations
- Levothyroxine should be taken on an empty stomach, typically 30-60 minutes before breakfast, to ensure optimal absorption.
- Certain medications, supplements, and foods can interfere with levothyroxine absorption, including calcium supplements, iron products, antacids, and high-fiber foods.
- Regular monitoring of thyroid function through blood tests is essential to maintain appropriate dosing and minimize side effects, as five trials published since the previous review assessed the harms of treatment of hypothyroidism with levothyroxine and generally reported no indication of harms or no or minimal withdrawals from treatment due to adverse effects 1. It is crucial for patients to never adjust their dose without medical supervision, as both under-replacement and over-replacement can cause health problems 1.
From the FDA Drug Label
6 ADVERSE REACTIONS Adverse reactions associated with levothyroxine sodium therapy are primarily those of hyperthyroidism due to therapeutic overdosage [see Warnings and Precautions (5), Overdosage Error! Hyperlink reference not valid.] They include the following:
• General:fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating • Central nervous system:headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia • Musculoskeletal:tremors, muscle weakness, muscle spasm • Cardiovascular:palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest • Respiratory:dyspnea • Gastrointestinal:diarrhea, vomiting, abdominal cramps, elevations in liver function tests • Dermatologic:hair loss, flushing, rash • Endocrine:decreased bone mineral density • Reproductive:menstrual irregularities, impaired fertility Seizures have been reported rarely with the institution of levothyroxine therapy Adverse Reactions in Pediatric Patients Pseudotumor cerebri and slipped capital femoral epiphysis have been reported in pediatric patients receiving levothyroxine therapy Overtreatment may result in craniosynostosis in infants who have not undergone complete closure of the fontanelles, and in premature closure of the epiphyses in pediatric patients still experiencing growth with resultant compromised adult height. Hypersensitivity Reactions Hypersensitivity reactions to inactive ingredients have occurred in patients treated with thyroid hormone products These include urticaria, pruritus, skin rash, flushing, angioedema, various gastrointestinal symptoms (abdominal pain, nausea, vomiting and diarrhea), fever, arthralgia, serum sickness, and wheezing. Hypersensitivity to levothyroxine itself is not known to occur.
The side effects of levothyroxine include:
- General: fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating
- Central nervous system: headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia
- Musculoskeletal: tremors, muscle weakness, muscle spasm
- Cardiovascular: palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest
- Respiratory: dyspnea
- Gastrointestinal: diarrhea, vomiting, abdominal cramps, elevations in liver function tests
- Dermatologic: hair loss, flushing, rash
- Endocrine: decreased bone mineral density
- Reproductive: menstrual irregularities, impaired fertility Additionally, seizures, pseudotumor cerebri, and slipped capital femoral epiphysis have been reported in some patients, and hypersensitivity reactions to inactive ingredients can occur 2.
From the Research
Side Effects of Levothyroxine
The side effects of levothyroxine are not explicitly stated in the provided studies, but we can infer some potential issues related to its use.
- Hypothyroid patients on levothyroxine (LT4) may experience persistent symptoms, such as impaired cognition and tiredness, despite normalization of serum thyrotropin levels 3.
- Some patients may have poorly controlled hypothyroidism, also known as refractory hypothyroidism, which can be due to various factors, including optimization of ingestion routines and alternative formulations and routes of administration of LT4 4.
- The use of levothyroxine/levotriiodothyronine combination therapy versus levothyroxine monotherapy has been debated, with some studies suggesting that the combination therapy may not be more effective than monotherapy in improving clinical outcomes 5, 6.
- However, some patients may prefer combination therapy, and it may be beneficial for those with specific genetic and/or immunologic makeup 3, 6.
Potential Harms of Levothyroxine
The potential harms of levothyroxine are not extensively discussed in the provided studies, but we can mention the following: