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Synthroid Side Effects

Generic name: levothyroxine

Medically reviewed by Drugs.com. Last updated on Mar 11, 2024.

Note: This document provides detailed information about Synthroid Side Effects associated with levothyroxine. Some dosage forms listed on this page may not apply specifically to the brand name Synthroid.

Applies to levothyroxine: oral capsule liquid filled, oral solution, oral tablet.

Other dosage forms:

Important warnings This medicine can cause some serious health issues

Oral route (capsule; tablet; solution)

Thyroid hormones, including levothyroxine, should not be used either alone or with other therapeutic agents for the treatment of obesity or weight loss.

In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction.

Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects.

Serious side effects of Synthroid

Along with its needed effects, levothyroxine (the active ingredient contained in Synthroid) may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking levothyroxine:

Less common side effects

  • chest pain, discomfort, or tightness
  • decreased urine output
  • difficult or labored breathing
  • difficulty with swallowing
  • dilated neck veins
  • extreme fatigue
  • fainting
  • fast, slow, irregular, pounding, or racing heartbeat or pulse
  • fever
  • heat intolerance
  • hives or welts, skin itching, rash, or redness
  • irregular breathing
  • irritability
  • menstrual changes
  • nausea
  • pain or discomfort in the arms, jaw, back, or neck
  • sweating
  • swelling of the eyes, face, lips, throat, or tongue
  • tremors

Rare side effects

  • blurred or double vision
  • dizziness
  • eye pain
  • lack or slowing of normal growth in children
  • limp or walk favoring one leg
  • pain in the hip or knee
  • seizures
  • severe headache

Get emergency help immediately if any of the following symptoms of overdose occur while taking levothyroxine:

Symptoms of overdose

Other side effects of Synthroid

Some side effects of levothyroxine may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

Less common side effects

  • change in appetite
  • crying
  • diarrhea
  • false or unusual sense of well-being
  • fear or nervousness
  • feeling not well or unhappy
  • feeling of discomfort
  • feeling of warmth
  • feeling things are not real
  • feelings of suspicion and distrust
  • hair loss
  • headache
  • increased appetite
  • mental depression
  • muscle weakness
  • quick to react or overreact emotionally
  • rapidly changing moods
  • redness of the face, neck, arms, and occasionally, upper chest
  • restlessness
  • stomach cramps
  • trouble getting pregnant
  • trouble sitting still
  • unusual tiredness or weakness
  • vomiting
  • weight gain or loss

Managing side effects (general information)

For healthcare professionals

Applies to levothyroxine: compounding powder, injectable powder for injection, intravenous powder for injection, intravenous solution, oral capsule, oral solution, oral tablet.

General adverse events

The more commonly reported adverse events have included those of hyperthyroidism due to therapeutic overdose including arrhythmias, myocardial infarction, dyspnea, muscle spasm, headache, nervousness, irritability, insomnia, tremors, muscle weakness, increased appetite, weight loss, diarrhea, heat intolerance, menstrual irregularities, and skin rash.

Cardiovascular

Cardiac function was evaluated in 20 patients requiring TSH suppression for either thyroid goiter or following thyroidectomy and radioactive iodine therapy for thyroid cancer and in 20 age and sex-matched controls. TSH suppression was associated with an increased incidence of premature ventricular beats, an increased left ventricular mass index, and enhanced left ventricular systolic function. The clinical significance of these changes remains to be determined.

Overtreatment with this drug may cause an increase in heart rate, cardiac wall thickness, and cardiac contractility and may precipitate angina or arrhythmias, particularly in patients with cardiovascular disease and in elderly patients.[Ref]

Endocrine

Nervous system

Dermatologic

Musculoskeletal

A study evaluated the effect of long-term thyroid hormone therapy on bone mineral density in 196 women (mean age, 74.4 years) compared to a control group comprised of 795 women (mean age, 72.1 years). The mean daily thyroxine dose was 1.99 mcg/kg (range, 0.3 to 6.6 mcg/kg) with a mean duration of therapy of 20.4 years (range, less than 1 to 68 years). Women taking daily doses of 1.6 mcg/kg or more had significantly lower bone mineral density levels at the ultradistal radius, midshaft radius, hip, and lumbar spine compared to controls. However, estrogen use appeared to negate the adverse effects of thyroid hormone on bone mineral density.

Higher rates of femur fractures have been found in males (p=0.008) prescribed long-term thyroid hormone therapy as compared to controls in a case-control analysis of 23,183 patients, from the United Kingdom General Practice Research Database, prescribed thyroid hormone.

Overtreatment may result in craniosynostosis in infants and premature closure of the epiphyses in children with resultant compromised adult height.[Ref]

Gastrointestinal

Genitourinary

Hypersensitivity

Hypersensitivity reactions have occurred; however, it has been attributed to the inactive ingredients. These reactions have included urticaria, pruritus, skin rash, flushing, angioedema, various GI symptoms (abdominal pain, nausea, vomiting and diarrhea), fever, arthralgia, serum sickness and wheezing. Hypersensitivity to levothyroxine itself is not known to occur.[Ref]

Metabolic

Immunologic

Other

Psychiatric

References

1. Petersen K, Bengtason C, Lapidus L, et al. (1990) "Morbidity, mortality, and quality of life for patients treated with levothyroxine." Arch Intern Med, 150, p. 2077-81

2. Leese GP, Jung RT, Guthrie C, Waugh N, Browning MC (1992) "Morbidity in patients on L-thyroxine: a comparison of those with a normal TSH to those with a suppressed TSH." Clin Endocrinol (Oxf), 37, p. 500-3

3. (2002) "Product Information. Synthroid (levothyroxine)." Abbott Pharmaceutical

4. Sheppard MC, Holder R, Franklyn JA (2002) "Levothyroxine treatment and occurrence of fracture of the hip." Arch Intern Med, 162, p. 338-43

5. Cerner Multum, Inc. "UK Summary of Product Characteristics."

6. Pharmaceutical Society of Australia (2006) APPGuide online. Australian prescription products guide online. http://www.appco.com.au/appguide/default.asp

Frequently asked questions

Further information

Synthroid side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.

Medical Disclaimer