Synthroid Side Effects

Generic Name: levothyroxine

Note: This page contains side effects data for the generic drug levothyroxine. It is possible that some of the dosage forms included below may not apply to the brand name Synthroid.

It is possible that some side effects of Synthroid may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

For the Consumer

Applies to levothyroxine: oral capsule liquid filled, oral tablet

As well as its needed effects, levothyroxine (the active ingredient contained in Synthroid) may cause unwanted side effects that require medical attention.

If any of the following side effects occur while taking levothyroxine, check with your doctor immediately:

Less common
  • Chest pain or discomfort
  • 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
  • increased blood pressure
  • increased pulse
  • irregular breathing
  • irritability
  • menstrual changes
  • nausea
  • pain or discomfort in the arms, jaw, back, or neck
  • shortness of breath
  • skin itching, rash, or redness
  • sweating
  • swelling of the eyes, face, lips, throat, or tongue
  • tightness in the chest
  • tremors
  • troubled breathing
Rare
  • 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

If any of the following symptoms of overdose occur while taking levothyroxine, get emergency help immediately:

Symptoms of overdose
  • Change in consciousness
  • cold, clammy skin
  • confusion
  • disorientation
  • fast or weak pulse
  • lightheadedness
  • loss of consciousness
  • sudden headache
  • sudden loss of coordination
  • sudden slurring of speech

Some levothyroxine side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned about them:

Less common
  • Abdominal or stomach cramps
  • change in appetite
  • crying
  • diarrhea
  • false or unusual sense of well-being
  • fear
  • 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
  • nervousness
  • quick to react or overreact emotionally
  • rapidly changing moods
  • redness of the face, neck, arms, and occasionally, upper chest
  • restlessness
  • trouble getting pregnant
  • trouble sitting still
  • unusual tiredness or weakness
  • vomiting
  • weight gain
  • weight loss

For Healthcare Professionals

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

General

Levothyroxine is usually well tolerated. Side effects associated with levothyroxine (the active ingredient contained in Synthroid) therapy typically resulted from therapeutic overdosage and included the signs and symptoms of hyperthyroidism. Weight loss, increased appetite, insomnia, anxiety, heat intolerance, diarrhea or increase in bowel frequency, palpitations, hypertension, tachycardia, angina, and menstrual irregularities may be reported. Given the long half-life of levothyroxine, such effects may not be present for several weeks after therapy initiation or dosage increases.

Cardiovascular

Cardiac function was evaluated in twenty patients requiring TSH suppression for either thyroid goiter or following thyroidectomy and radioactive iodine therapy for thyroid cancer and in twenty 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.

A 38-year-old female experienced with severe hypothyroidism experienced myxedema coma and cardiac ischemia coincident with levothyroxine (the active ingredient contained in Synthroid) therapy. After 3 months of levothyroxine therapy (initial dose: 12.5 mcg/d; maintenance dose: 125 mcg/d), all abnormal laboratory values associated with hypothyroidism returned to normal. However, three weeks after initiating treatment, the patient reported intermittent chest pains during the course of treatment, and a coronary artery angiogram revealed diffuse stenosis of all 3 branches. She underwent coronary artery bypass grafting, with subsequent improvement in coronary perfusion.

Cardiovascular side effects have included symptoms of palpitations, hypertension, tachycardia, and angina which may be exacerbated in patients with underlying cardiovascular disorders. Ischemic heart disease and significant effects on cardiac function including an increased incidence of premature ventricular beats, an increased left ventricular mass index, and enhanced left ventricular systolic function have been reported in clinical trials.

Endocrine

Endocrine side effects have included changes in symptom presentation for diabetes and adrenal cortical insufficiency.

Nervous system

Nervous system side effects have rarely included seizures during initiation of therapy.

Dermatologic

Dermatologic side effects including transient dermatologic effect and hair loss have been reported during the initial months of therapy.

Musculoskeletal

Musculoskeletal side effects have included an increase risk of osteoporosis. However, data from long-term studies are conflicting.

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.

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