What is Synthroid: Uses, Warnings & Interactions

Take your medicine as directed. 

Your doctor will tell you how much to take and how often.

Your dose may need to be changed several times to find what works best for you. 

You may have to take this medicine for four to eight weeks before your symptoms start to get better.

Take this medicine in the morning on an empty stomach, at least 30 to 60 minutes before eating breakfast.

Take your medicine at the same time every day.

If you are giving this medicine to a baby or child who cannot swallow the tablet whole, crush the tablet and mix it in one to two teaspoons (5 to 10 mL) of water. Give the mixture right away using a spoon or dropper. Do not mix the tablet with any other liquid except water. Do not store the mixture for future use.

Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. 

Keep all medicine out of the reach of children. Never share your medicine with anyone.

Do not stop taking this medicine without first checking with your doctor. You may need to take smaller and smaller doses before completely stopping the medicine.

Read and follow the patient instructions that come with this medicine. Talk to your doctor or pharmacist if you have any questions.

Take the missed dose as soon as possible.

Skip the missed dose if it is almost time for your next regular dose.

You should not use two doses at the same time.

Do not use Synthroid if:

• You have had an allergic reaction to levothyroxine, glycerol, or any type of thyroid hormone

• You have an uncorrected adrenal gland problem

• You have a cardiac condition

• You have an overactive thyroid gland

Tell your doctor if you are pregnant or breastfeeding.

Tell your doctor if you have recently received radiation treatment with iodine.

Check with your doctor before taking this medicine if you have:

• Heart or blood vessel disease

• High blood pressure

• Diabetes

• Kidney disease

• Liver disease

• Anemia

• Blood clotting problems

• Osteoporosis

• Pituitary gland problems

• Adrenal gland problems

This medicine may cause the following problems:

• Heart problems, including angina (chest pain) and heart arrhythmias

• Low bone density

This medicine should not be used to treat obesity or as part of a treatment plan for a weight control program.

Large doses of thyroid hormones, especially when taken with diet pills, can result in dangerous side effects, such as:

• Inability to tolerate heat

• Extreme nervousness, irritability, or tremors

• Severe diarrhea

• Intense sweating

Tell any doctor or dentist who treats you that you are taking this medicine.

Make sure you see your doctor for regular check-ups so that your condition can be followed closely.

Your doctor will do lab tests at regular visits to check on the effects of this medicine, usually by checking thyroid hormone levels, such as thyroid-stimulating hormone (TSH). TSH levels help determine someone’s thyroid function. Keep all appointments.

Do not stop using this medicine suddenly. Your doctor will need to slowly decrease your dose before you stop it completely.

Some foods and medicines can affect how levothyroxine works. Tell your doctor if you are taking any of the following:

• Amiodarone

• Asparaginase

• Carbamazepine

• Clofibrate

• Dexamethasone

• Digoxin

• 5-fluorouracil

• Furosemide

• Heroin

• Imatinib

• Ketamine

• Methadone

• Mitotane

• Nicotinic acid

• Phenobarbital

• Phenytoin

• Rifampin

• Tamoxifen

• Beta blockers

• Birth control pills (including estrogen)

• Blood thinners (including heparin and warfarin)

• Insulin or diabetes medicine

• Antidepressants (including amitriptyline and maprotiline)

• NSAIDs

• Salicylates

• Steroids (including dexamethasone, androgens, and anabolic steroids)

• Products used for colds, asthma, appetite control, hay fever, or sinus problems

In order to lessen the impact of drug interactions with Synthroid, take the following medications at least four hours before or four hours after you take Synthroid:

• Kayexalate

• Lanthanum

• Orlistat

• Sevelamer

• Sucralfate

• Antacids (including aluminum or magnesium hydroxide and simethicone) 

• Medicine to lower cholesterol (including cholestyramine, colesevelam and colestipol)

• Stomach medicine (including lansoprazole, omeprazole, and pantoprazole)

• Any medicine that contains calcium or iron

Cottonseed meal, dietary fiber, soybean flour (infant formula), or walnuts may decrease the absorption of this medicine. Talk with your doctor if you have questions.

Do not eat grapefruit or drink grapefruit juice while you are using this medicine.

Large doses of thyroid hormones, especially when taken with diet pills, can result in dangerous side effects.