What are the side effects of Synthroid (levothyroxine)?

Medical Advisory BoardAll articles are reviewed for accuracy by our Medical Advisory Board
DS

Dainyal Siddiqui, MD

Internal Medicine

MJ

Manuel Jimenez, MD

Cardiology

FP

Franz Pierre-Louis, MD

Infectious Disease

Article Review Status
Submitted
Under Review
Approved

Last updated: March 28, 2025 • View editorial policy

From the Guidelines

Synthroid (levothyroxine) can cause significant side effects, particularly when doses are too high, including adverse effects on bone mineral density and the cardiovascular system, such as angina, arrhythmia, and increased cardiac wall thickness 1.

Common Side Effects

  • Headache
  • Insomnia
  • Nervousness
  • Irritability
  • Tremors
  • Increased sweating
  • Heart palpitations
  • Changes in appetite or weight Some patients may experience hair loss, particularly during the first few months of treatment. Allergic reactions are rare but can include rash, itching, or swelling.

Serious Side Effects

Serious side effects that require immediate medical attention include chest pain, rapid or irregular heartbeat, shortness of breath, or signs of excessive thyroid hormone (thyrotoxicosis) such as fever, heat intolerance, and extreme nervousness.

Management of Side Effects

Side effects often occur when the dose is too high, which is why regular thyroid function tests are important to ensure proper dosing. Starting with a lower dose and gradually increasing it can help minimize side effects, especially in older adults or those with heart conditions. Most side effects improve as your body adjusts to the medication or when the dosage is adjusted appropriately. Never stop taking Synthroid suddenly without medical supervision, as this can lead to worsening hypothyroid symptoms.

Long-term Risks

Long-term overdosing with levothyroxine could increase the risk for osteoporosis, fractures, abnormal cardiac output, or ventricular hypertrophy 1. Therefore, careful monitoring and dose adjustment are crucial to minimize these risks.

From the FDA Drug Label

Adverse reactions associated with levothyroxine sodium therapy are primarily those of hyperthyroidism due to therapeutic overdosage 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.

The side effects of Synthroid (levothyroxine) include:

  • General symptoms: fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating
  • Central nervous system symptoms: headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia
  • Musculoskeletal symptoms: tremors, muscle weakness, muscle spasm
  • Cardiovascular symptoms: palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest
  • Respiratory symptoms: dyspnea
  • Gastrointestinal symptoms: diarrhea, vomiting, abdominal cramps, elevations in liver function tests
  • Dermatologic symptoms: hair loss, flushing, rash
  • Endocrine symptoms: decreased bone mineral density
  • Reproductive symptoms: menstrual irregularities, impaired fertility
  • Rare reactions: seizures, pseudotumor cerebri, slipped capital femoral epiphysis, craniosynostosis, premature closure of the epiphyses
  • Hypersensitivity reactions: urticaria, pruritus, skin rash, flushing, angioedema, gastrointestinal symptoms, fever, arthralgia, serum sickness, wheezing [2] [3]

References

Related Questions

What is the cause, diagnostic workup, and treatment of pronounced ecchymosis (bruising) after minor trauma in a 36-year-old female with a history of easy bruising, now experiencing exacerbated ecchymosis over the past 2 months?
Can auditory hallucinations be related to nonspecific white matter hypodensities, particularly in the left parietal region, in the absence of other neurological symptoms or abnormal laboratory findings, with computed tomography (CT) of the head showing these findings?
What is the assessment for obesity?
When is a Magnetic Resonance Imaging (MRI) of the breast indicated?
What is the best course of action for a 17-year-old patient with chronic lower back pain (LBP) and radiating pain to the legs and feet, who has tried Naproxen (Naproxen) and Acetaminophen (Acetaminophen)/Ibuprofen (Ibuprofen) with limited relief, and has undergone normal Doppler and Electromyography (EMG) tests?
Is low-dose Quetiapine (Seroquel) a treatment option for insomnia in a 16-year-old female with a possible mood disorder?

Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

Ask a new Question. Chat with the Guidelines