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Precautions and Warnings With Levothyroxine

It's important to review levothyroxine warnings and precautions with your healthcare provider before starting the medication. For example, tell your healthcare provider about existing medical conditions you have and any other medications you are currently taking. If you are taking levothyroxine, it is important to know that the medicine can make heart problems worse and may increase your risk of developing osteoporosis.

Levothyroxine: What Should I Tell My Healthcare Provider?

You should talk with your healthcare provider prior to taking levothyroxine sodium if you have:
Also, let your healthcare provider know if you are:
  • Pregnant or thinking of becoming pregnant
  • Breastfeeding.
Make sure to tell your healthcare provider about any other medicines you are taking, including prescription and non-prescription medicines, vitamins, and herbal supplements.

Specific Precautions and Warnings With Levothyroxine

Some of the warnings and precautions to be aware of prior to taking levothyroxine include the following:
  • Levothyroxine should not be used to treat goiters or thyroid nodules in people who have a low level of thyroid-stimulating hormone (TSH), as it could cause dangerous side effects. A low TSH level signals that thyroid hormone levels are already high, and adding levothyroxine could cause them to increase to a dangerous level. Even if the TSH level is normal, frequent monitoring is required to make sure problems do not develop.
  • The medication can make heart problems worse. If you have heart problems, your healthcare provider should start you at a low levothyroxine dosage and slowly increase it. If you notice that levothyroxine makes your heart symptoms worse, let your healthcare provider know right away.
  • Levothyroxine is a "narrow therapeutic index" medication. This means that for any given person, the range of levothyroxine doses that are both safe and effective is small. Even tiny changes in doses can cause significant problems (either making levothyroxine ineffective or causing serious side effects).

Your healthcare provider should carefully and slowly adjust your dose, and should monitor your progress often using blood tests. Even if you have been stable on levothyroxine for years, your healthcare provider should still do these blood tests regularly.  

  • People who do not have thyroid problems should never use levothyroxine to lose weight. A levothyroxine dosage high enough to cause weight loss in these people is often high enough to cause serious or even life-threatening problems.
  • Levothyroxine may increase the risk of osteoporosis, particularly if your levothyroxine dosage is too high. This is one of the reasons why it is a good idea for your healthcare provider to prescribe a dose of levothyroxine that is just enough to return your thyroid hormone levels to normal.
  • Sometimes, thyroid problems occur in conjunction with other health problems (such as anemia or diabetes). It is a good idea for your healthcare provider to give you a thorough physical exam when you are diagnosed with a thyroid problem.
  • It often takes levothyroxine several weeks to start working. Try not to get frustrated if you are not feeling better more quickly.
  • If you have diabetes, it is a good idea to check your blood sugar more often when starting or stopping levothyroxine (or when changing your dose), as the medication can affect your blood sugar levels.
  • Levothyroxine can potentially interact with many other medications (see Drug Interactions With Levothyroxine).
  • Levothyroxine is considered a pregnancy Category A medication. This means that it is safe for use during pregnancy (see Levothyroxine and Pregnancy).
  • Levothyroxine does pass through breast milk in humans, although it is not likely to cause problems. If you are breastfeeding or plan to start breastfeeding, discuss this with your healthcare provider prior to taking the drug (see Levothyroxine and Breastfeeding).
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