Synthroid is contraindicated in patients with 

• Uncorrected adrenal insufficiency
• Acute myocardial infarction
• Acute myocarditis
• Pancarditis
• Active heart arrhythmias
• Thyrotoxicosis or hyperthyroidism
• Myxedema Coma

Thyroid hormone levels affect the action of many medications. Many medications are seen to increase or decrease the effects of Synthroid.

These medications may include:

• Calcium carbonate
• Stomach acid reducers such as lansoprazole, omeprazole, and esomeprazole
• Ferrous sulphate iron supplement
• Cholestyramine
• Proton pump inhibitors (Pantoprazole, Omeprazole)
• Antacids containing aluminum or magnesium such as Milk of Magnesia 3
• Seizure medications such as carbamazepine and phenytoin
• Tamoxifen
• Methadone
• Glucocorticoids
• Carbamazepine
• Rifampin
• Furosemide 
• Amiodarone
• Estrogens like in hormone replacement therapy (HRT)
• Antidiabetics
• Blood thinners such as warfarin

Many medications are found to interact with Levothyroxine. Not all possible interactions are mentioned here; other medicines may also interact. Inform your doctor regarding all the current medicines.

Synthroid, containing Levothyroxine, is indicated:

• As a hormone replacement therapy for the management of hypothyroidism
• As add-on therapy to surgery and radiotherapy in patients with thyroid cancer

Synthroid can also be prescribed for other indications. Patients must ask their healthcare providers for detailed information about the medicine.

Synthroid is available in many forms -- tablets, capsules, solution, and intravenous Levothyroxine. Read the patient information leaflet if available from the pharmacist before you start consuming the medication. If you have any questions, ask your physician or pharmacist. Here are the directions for its usage:

• Administer Synthroid on an empty stomach as a single dose, preferably one hour before breakfast.
• Avoid taking Synthroid along with other medicines, especially those containing iron and calcium. Take it four hours before or after taking the other medicines as they may affect the absorption of Synthroid.
• Take the doses recommended by your doctor, as the quantities are based on your age, body weight, and condition.
• Crush the tablet of Synthroid when it needs to be given to infants and children incapable of swallowing an intact tablet. Mix it in a small quantity (one to two teaspoonfuls) of water and give it using a spoon to the child. Never store the crushed tablet or water mixture, and do not give this medication with any soybean-based infant formula as it may lead to a severe interaction.3
• Avoid taking Synthroid along with antacids.
• You should use this medication daily or as prescribed by your physician to obtain the maximum benefit of the treatment.  Try to take the medication at the same time daily. 
• Do not stop taking this medication without consulting your doctor. Thyroid replacement treatment is often prescribed for a very long period.
• Do not switch to other brands without consulting your physician. 
• In case of missed dose: Take Synthroid the moment it is possible. However, leave out the missed dose if it is nearly the time for the next dose. Avoid taking two doses at one time.

Common

• Headache
• Hot flashes
• Temporary loss of hair
• Nausea
• Sleep problems
• Mood changes like irritability
• Changes in appetite or weight
• Fever

Rare

• Allergic reactions
• Trouble breathing
• Cardiac failure
• Abnormal heart rhythms
• Blurred or double vision
• Dizziness
• Eye pain
• Seizures
• Knee or hip pain

This is not a complete list of side effects, and others may take place. One must always check with their physician about other possible side effects.

• You should inform your pharmacist or physician if you are allergic to it or if you have any other allergies. 
• You should be careful about your dosage of Synthroid, especially if you are 50 years of age or more or have an underlying cardiac disease, as it is associated with an increased risk of cardiac adverse reactions.
• You should avoid using Synthroid if you suffer from uncorrected adrenal problems.
• You should not use Synthroid alone or with other drugs to treat obesity or weight loss.
• You should get your lab tests done regularly as too much Synthroid is associated with loss of bone mineral density, mainly in women undergoing menopause.
• If you have diabetes, you should get your blood sugar checked regularly as this medication may affect your blood sugar levels.
• You should avoid certain foods like soybean flour, cottonseed meal, walnuts, and products containing iron or calcium, as they are known to slow down the absorption of Synthroid. 
• You must inform your doctor if you are pregnant or planning to become pregnant as the dose of Synthroid needs to be increased during pregnancy.
• Levothyroxine passes into breast milk, it is unlikely to harm a nursing infant. It is best to discuss with the doctor before breast feeding.
• You must inform your doctor about all the other medications you are taking.
• You should inform your doctor or dentist about Synthroid before any surgery.
• Contact your doctor immediately if you experience any of the following: 

1. Rapid or abnormal heartbeat 
2. Chest pain 
3. Breathing difficulty 
4. Leg cramps 
5. Nervousness 
6. Headache 
7. Irritability 
8. Sleeplessness 
9. Shaking
10. Change in appetite 
11. Weight gain or loss 
12. Vomiting 
13. Diarrhea 
14. Increased sweating
15. Difficulty tolerating heat 
16. Fever 
17. Changes in the menstrual cycle 
18. Swollen red bumps on the skin (hives) or skin rash, or any other unusual medical event.

• Store Synthroid at room temperature 25°C (77°F).
• Keep Synthroid out of the reach of children.
• Avoid direct exposure to light and moisture.
• Discard the medication properly if it has expired or is no longer required.