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Thyroid Diseases

Thyroid Medication Dosage and Birth Control

12/05/2007

Question:

I have been hypothyroid since having mediastinal radiation as treatment of lymphoma 5 years ago. We were on top of checking my TSH and noticed when it started raising past "normal". I was initially started on .25 mcg and raised over the past 4 years to 137 mcg Synthroid. This entire time, I had been taking Yasmin birth control pill at the same time as my Synthroid - no one had ever told me not to. I have done some research recently that indicated that this may have been inhibiting my Synthroid from working correctly. My last TSH (2 weeks ago) was 2.45 and we had been aiming for .8-1.2. My doctor upped my dosage to 150 mcg Levoxyl. I have not picked up the Rx yet - this past week I`ve started taking my Yasmin at night and my Synthroid in the morning. I`ve already dropped 4 pounds. I`m worried that a) switching to a higher dose may over medicate me if the birth control has been the problem all along, and b) switching to a different brand will make a big difference at this point. What do you suggest I do?

Answer:

Assuming that you are not taking any other medications, estrogen (one component of yasmin) could raise the circulatory thyroid-binding globulin (TBG), which is the protein carrying the thyroid hormone in circulation. However, thyroid status is tightly regulated in order to keep the active form of thyroid hormone (unbound to TBG) within normal range. Therefore, estrogen could only increase the total thyroid hormone levels while the active form as well as TSH level remain unchanged. The optimal dose of thyroid hormone should be adjusted based on your TSH level, aiming for values from 1 to 3.

In order to keep your thyroid hormone absorption steady over time, you should take synthroid in the morning with empty stomach 30-60 minutes before your breakfast. Several drugs can interfere with gastrointestinal absorption of thyroid hormone and cause fluctuation in your thyroid status. Most importantly is calcium and iron, which should be taken at least 2-3 hours apart from synthroid.

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Response by:

Marzieh  Salehi, MD Marzieh Salehi, MD
Assistant Professor of Endocrinology
College of Medicine
University of Cincinnati