The information provided here is meant to give you a general idea about each of the medications listed below. Only the most general side effects are included. Ask your doctor if you need to take any special safety measures. Use each of these medications only as recommended by your doctor. Use them according to the instructions provided. If you have any questions about usage or side effects, contact your doctor.
Levothyroxine is taken once a day. The dose of levothyroxine is based on your thyroid function tests, age, and weight. It is also based on whether you have any heart conditions or any remaining thyroid gland. Getting the right dose can take time. The dosage is slowly increased every 6-8 weeks until blood levels of TSH are in the normal range.
If you are elderly or have a heart condition, your doctor will start with a lower dose. This will give your body a chance to get used to the higher thyroid hormone level. After you have reached normal levels of TSH, your doctor may want to see you at least twice a year to make sure your levels continue to be normal. Thyroid blood levels can get higher or lower over time. It may be necessary to increase or decrease the dosage of medication.
You may begin to feel better within 1-2 weeks after you start taking medication. It may take some people 3-6 months after your blood levels are normal before you really feel well. You must keep taking levothyroxine. It is the only way to manage the condition. Remember to take your thyroid medication on an empty stomach with a full glass of water. Calcium, iron, soy, and multivitamins in the stomach will interfere with its absorption.
After you start on a certain brand of levothyroxine, you should stay on that brand. The same dose of another brand may be absorbed differently in your body. If you change your dose or brand, you will be asked to have your blood levels of TSH checked about 6 weeks later.
Liothyronine (T3) is a very short-acting thyroid hormone. It is not prescribed as often as levothyroxine (T4). This medication is often given to patients after their thyroid is removed surgically for thyroid cancer. It is sometimes prescribed along with levothyroxine in a few select cases.
There is little concern for side effects of either form of thyroid hormone replacement. The only concern is overdosing. Both of these synthetic hormones are forms of thyroid hormones that are natural and needed in your body.
If you take too much of either of these synthetic hormones, it may cause the following problems:
If you are taking medications, follow these general guidelines:
Alexander EK, Marqusee E, Lawrence J, et al. Timing and magnitude of increases in levothyroxine requirements during pregnancy in women with hypothyroidism. N Engl J Med. 2004;351(3):241-249.
Bell DS, Ovalle F. Use of soy protein supplement and resultant need for increased dose of levothyroxine. Endocr Pract. 2001;7(3):193-194.
Hypothyroidism (underactive thyroid). National Institute of Diabetes and Digestive and Kidney Diseases website. Available at: https://www.niddk.nih.gov/health-information/endocrine-diseases/hypothyroidism. Updated August 2016. Accessed February 20, 2017.
Hypothyroidism in adults. EBSCO DynaMed Plus website. Available at:http://www.dynamed.com/topics/dmp~AN~T115914/Hypothyroidism-in-adults. Updated July 12, 2016. Accessed February 20, 2017.
Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA. 2000;283(21):2822-2825.
Last reviewed February 2017 by James P. Cornell, MD Last Updated: 3/15/2015