Fact Sheet 2 – Medications for Hypothyroidism and Interaction of Thyroxine with Other Medications
Medications for Hypothyroidism
Thyroid hormones are essential for normal growth, development and metabolism. However, not everyone has the proper level of these hormones. Insufficient thyroid hormone is called hypothyroidism.
Hypothyroidism is treated by giving the patient sufficient thyroxine (thyroid hormone), the most commonly prescribed in Canada being Synthroid and Eltroxin. The average daily dose is 0.125 milligrams (125 micrograms); it is rare to require more than 0.2 milligrams (mg).
In the case of an overdose of thyroxine, symptoms of Hyperthyroidism will occur, i.e. nervousness, tachycardia, irregular heart rhythms, angina, weight loss, increased sweating, fever, diarrhoea and abdominal cramps. Immediately notify your physician if these symptoms occur.
It is very important to tell your physician if you are taking thyroid medication, so he can take this into consideration when ordering other medications.
The correct dosage of thyroxine to treat primary hypothyroidism is determined by the TSH (about 0.3-4.5milliU/L, but can vary slightly depending on the laboratory and testing method. For most patients the target TSH will be within the lower half of the reference range, but may differ for certain populations (e.g. pregnancy, elderly, thyroid cancer patients).
These tests should be done every two or three months until correct dosage has been determined. Once a correct dosage is determined, a yearly check-up is all that is necessary. Hypothyroid patients should not stop taking thyroid hormone or alter the dose without consulting their physicians.
- You may nurse your baby if you are taking thyroxine.
- You must take your thyroid pill during pregnancy.
- You may take your thyroid pill before you go for blood work.
- The Red Cross will accept you as a blood donor if you are taking thyroxine and are healthy. Check with your physician.
- If you have coronary heart disease, your doctor will start you on a low dose of thyroxine and increase it gradually so as not to strain your heart.
Interaction of Thyroxine with Other Medications
Thyroxine and Pregnancy, Estrogen or Birth Control Pills:
Estrogen increases proteins that bind thyroid hormones. In people taking thyroid hormone replacement increases in estrogen levels (pregnancy, starting birth control pills) may increase thyroid hormone dose requirements and decreases in estrogen levels (menopause, post pregnancy, stopping birth control pills) may decrease thyroid hormone dose requirements.
Thyroxine and Dilantin:
This combination lowers the blood level of thyroxine (T4), but the free thyroxine remains normal.
Thyroxine and Supplements (calcium, iron), Metamucil, Coffee, Alcohol:
Thyroxine and anything that affects the digestive system should be taken as many hours apart as possible to ensure better absorption of the thyroid medication.
Thyroxine and Cholestyramine (Questran):
This combination lowers the absorption of thyroxine, therefore the two should not be taken together. A space of three to four hours between each is recommended with thyroxine being taken first.
Lithium can cause hypothyroidism, people taking lithium should be monitored periodically for hypothyroidism.
Thyroxine Calcium and Iron:
Calcium, iron and other vitamin supplements can reduce the absorption of thyroid hormone and thyroid hormone should be taken up to four hours apart from these supplements and high calcium containing foods.
Thyroxine and Food:
Many foods can alter the absorption of thyroid hormones. It is best to take thyroid hormone replacement on an empty stomach and not eat for about an hour after taking thyroid hormone.
Thyroxine and Stomach Acid Medications:
Changes in the amount of stomach acid can affect the absorption of thyroid hormone. Thyroid hormone dose requirements may change after starting or stopping stomach acid suppressing medications (e.g. proton pump inhibitors).
Reviewed and updated in February 2016 by Dr. Deric Morrison, MD FRCPC, ECNU, Endocrinologist, Assistant Professor, Division of Endocrinology, Department of Medicine, University of Western Ontario, London ON