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General overview

Synthroid is a brand name for the medication levothyroxine which is used to treat primary and secondary (post-surgical removal of the thyroid) hypothyroidism. It's prescribed to be used as a thyroid hormone supplement since the thyroid does not produce enough thyroxine or T4.

Other indications for levothyroxine include the treatment of an enlarged thyroid, also called a Goiter, as well as treatment of thyroid cancer.

Precautions when using levothyroxine

Patients should always let their prescribing doctor know if they are using any other medication or have any medical conditions which may affect the efficacy of the medication.

It's important to let your doctor know if you are using any calcium supplements or calcium containing medications, such as certain antacids, because these medications can interfere with the absorption of levothyroxine. The calcium supplements and medications don't have to be stopped, but rather just taken 4 hours either before or after the levothyroxine is taken.

Important medical conditions which need to be taken note of include the following:

  • Any recent heart attack.
  • Adrenal gland insufficiency.
  • Diabetes.
  • Osteoporosis.
  • Pituitary gland problems.
  • Heart rhythm issues such as atrial fibrillation.
  • Any history of previous or current blood vessel diseases.
  • Any clotting issues.
  • Any previous issues with thyroid pathologies such as thyrotoxicosis.

Levothyroxine should not be used to treat obesity or for the purposes of helping patients to lose weight. It should also not be used as primary management for infertility unless the infertility is a secondary cause of hypothyroidism. This medication can also cause increases in glucose levels, therefore patients with diabetes should closely monitor their glucose levels

Elderly women on levothyroxine should have their bone densities monitored yearly as this medication can cause some bone loss. Also, if female patient become pregnant, then they need to let their doctor know as they may need larger doses of the medication.


As with any medication, levothyroxine can cause some unwanted side-effects which may need medical attention and they can include the following:

  • Chest pains.
  • Difficulty breathing.
  • Severe fatigue.
  • Irregular heartbeat.
  • Fever.
  • Heat intolerance.
  • Increased pulse and blood pressure.
  • Menstrual changes.
  • Irritability.
  • Irregular breathing or shortness of breath.
  • Increased sweating.
  • Tremors.

Some side-effects are self-limiting and may end up resolving on their own. They include the following:

  • Abdominal or stomach cramps.
  • Sense of feeling unwell.
  • Hair loss.
  • Headaches.
  • Tremors.
  • Nausea and vomiting.
  • Changes in appetite.
  • Weight gain.
  • Weight loss.

Symptoms of overdose

Patients experiencing symptoms of overdose need to seek immediate medical attention.

These can include the following:

  • Confusion.
  • Disorientation.
  • Changes in consciousness.
  • Cold, clammy and pale skin.
  • Lightheadedness.
  • Weak or fast pulse.
  • Sudden loss of coordination.
  • Sudden headache. 
  • Sudden slurred speech.

These side-effects can occur if the dose of the levothyroxine is too high. The medication would then need to be reviewed and if the dosage is correct, then other causes need to be excluded.

Patients may sometimes have a deficiency of T3 rather than T4 and the levothyroxine was then not needed. T4 then needs to be stopped immediately and these patients need to be switched over to T3 supplements.


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