Synthroid

Medically Reviewed by Harshi Dhingra, M.D.

Last Reviewed: Jun 24, 2022

Warnings


What are the risks of taking Synthroid (levothyroxine)?


Synthroid can cause serious health problems, especially in certain high-risk groups.

Some of the risks of Synthroid therapy include:
  • Not for weight control: Thyroid hormones, including Synthroid, should not be used alone or in combination with other medications to treat obesity or for weight loss. In euthyroid individuals (people with normal thyroid hormone levels), the usual doses of Synthroid are ineffective for weight reduction. Larger doses can lead to toxicity and potentially life-threatening health complications.
  • Heart problems: Large doses of Synthroid can cause heart problems. This risk is greater in older individuals (age 65+) and those with existing heart disease or those taking stimulant medications. You should call your healthcare provider or get medical attention immediately if you have chest pain, rapid or irregular heartbeat, or shortness of breath. Your doctor may ask you to stop taking Synthroid for a short time and then restart the medication at a lower dose.
  • Worsening of diabetes: Synthroid can interact with medications used to control diabetes and worsen blood sugar control. Be sure to check your blood glucose levels as directed by your healthcare provider. Contact your doctor if your blood sugar levels are abnormal. You might need an increased dose of your anti-diabetes medications.
  • Weakening of bones: Taking Synthroid can make the bones weaker over time, especially in postmenopausal women. The risk is higher when Synthroid is taken at larger doses or for an extended period. Keep all your appointments with your healthcare providers. They will monitor your thyroid levels and make sure you take the least amount of Synthroid needed to keep your thyroid levels in the normal range. Taking a lower dose of Synthroid can lower the risk of weak bones.  
  • Adrenal crisis: A small number of people can have low thyroid hormones and low adrenal hormones. In such individuals, replacing thyroid hormone with a drug like Synthroid can cause the adrenal hormones to drop further, leading to an adrenal crisis condition, which requires immediate medical attention. Symptoms of adrenal crisis may include high fever, dehydration, dizziness, stomachache, nausea, and vomiting. Adrenal crisis can be prevented by taking glucocorticoids. Your doctor will check adrenal hormone levels before prescribing Synthroid to see if you are at risk.

Be sure to tell your doctor about all the medications you are taking or plan to take


What should I tell my doctor before starting Synthroid (levothyroxine)?

Tell your doctor if you are allergic to levothyroxine, thyroid supplements, any other medications, or any of the active or inactive ingredients in Synthroid tablets and capsules. Your pharmacy can provide you with a list of ingredients.

Give your doctor a complete list of your prescription and over-the-counter medications, including vitamins, nutritional supplements, and herbal products. This can help avoid serious drug interactions. 

Tell your doctor about all your medical conditions. In particular, mention if you have adrenal insufficiency (a condition in which the adrenal glands do not produce enough adrenal hormones). If you have this condition, your doctor may decide Synthroid is not right for you. 

 Also, tell your doctor if you have ever had diabetes, atherosclerosis (hardening of the arteries), anemia, bleeding problems, osteoporosis (weak or brittle bones), pituitary gland problems, porphyria, kidney disease, liver disease, or heart disease. Also, inform your doctor if you have recently undergone radiation therapy. 

Women who are pregnant, could be pregnant, plan to become pregnant, or are breastfeeding should tell their doctor before starting Synthroid treatment. 

What precautions should I take while on Synthroid (levothyroxine)?

Keep all your healthcare appointments while on Synthroid treatment. Your doctor will check thyroid levels and make dose adjustments. They will also monitor you carefully for Synthroid side effects.

Certain foods and beverages can affect how Synthroid works, particularly foods containing walnuts, soybeans, dietary fiber, grapefruit, and drinking grapefruit juice. Talk to your doctor before eating or drinking these foods and beverages.

 If you take ferrous sulfate (iron supplement) or calcium carbonate (Tums), be sure to take them at least 4 hours away (before or after) your Synthroid dose. Likewise, if you take certain cholesterol-lowering medications like colesevelam (Welchol), cholestyramine (Prevalite), colestipol (Colestid), or certain other medications like sevelamer (Renvela, Renagel) or sodium polystyrene sulfonate (Kayexalate), take these medicines at least 4 hours after Synthroid.

If you become pregnant while taking Synthroid, call your doctor. Your dose of Synthroid may need to be increased.

If you have surgery, including dental surgery, while on Synthroid treatment, tell the doctor or dentist that you are taking this medication.

What are Synthroid (levothyroxine) interactions?

Taking Synthroid with certain other medications can affect how well this medicine works. Drug interactions can also increase the risk of serious adverse effects. Your physician may choose a different medication for hypothyroidism, change the dose or frequency of your medicines, and/or monitor you carefully for side effects if there are known interactions between Synthroid and any of your other medicines. 

There can be drug interactions between Synthroid and the following medications:


  • heart medications like amiodarone (Nexterone, Pacerone)
  • androgens like testosterone (Androderm) and nandrolone 
  • certain antacids that contain aluminum or magnesium (Mylanta, Maalox, others) 
  • anticoagulants or blood thinners like such as heparin or warfarin (Coumadin, Jantoven)
  • beta-blockers like propranolol (Inderal, Innopran), metoprolol (Lopressor), or timolol 
  • cancer medications like asparaginase, fluorouracil, and mitotane (Lysodren), antiepileptic drugs such as carbamazepine (Tegretol), phenobarbital, phenytoin (Dilantin, Phenytek)
  • sucralfate (Carafate)



  • clofibrate (Atromid)
  • corticosteroids like dexamethasone
  • cough and cold medications 
  • weight loss medications such as orlistat (Alli, Xenical)
  • digoxin (Lanoxin)
  • hormone replacement medications containing estrogen 
  • hormonal contraceptives (birth control pills, patches, implants, rings, or injections)
  • tamoxifen (Soltamox)
  • tyrosine kinase inhibitors such as imatinib (Gleevac) or cabozantinib (Cometriq)
  • furosemide (Lasix)
  • insulin or other medications to treat diabetes
  • maprotiline
  • mefenamic acid (Ponstel)
  • methadone (Methadose)
  • niacin
  • proton pump inhibitors such as lansoprazole (Prevacid), esomeprazole (Nexium), and omeprazole (Prilosec)
  • simethicone (Phazyme, Gas X)
  • rifampin (Rifater, Rifamate, Rifadin)
  • antidepressants like sertraline (Zoloft)
  • tricyclic antidepressants such as amitriptyline (Elavil)


This is not a complete list of Synthroid drug interactions. Many other medications may also interact with Synthroid. Be sure to tell your doctor about all the medications you are taking or plan to take, even if they do not appear on this list.