8. Hormonal Fluctuations
Taking estrogen in any form, whether as hormone replacement therapy or in birth control pills, can affect thyroid test results. For example, some women taking supplemental estrogen may need to take more thyroid replacement hormone. Estrogen increases a particular protein that binds thyroid hormone to it, making the thyroid hormone partially inactive. Thyroid tests can end up showing falsely increased total T-4 levels. For a woman without a thyroid gland in particular, this can increase the dosage requirement slightly, as there is no thyroid to compensate.
Menopause, with its fluctuating hormone levels, can also impact your TSH levels. (Read The Menopause Thyroid Connection.)
The intense surge in estrogen during early pregnancy can increase your TSH and your body's need for thyroid hormone. It's particularly important for a woman to have her TSH tested periodically in early pregnancy, to ensure that dosages can be modified accordingly. TSH will frequently drop after delivery as well, in response to these shifts. See Pregnancy & Thyroid Disease Information Center.)
10. Herbs/Supplements/Drugs You Are Taking
Some herbal supplements can have an impact on thyroid function. Herbs such as the ayurvedic herb "guggul," supplements such as tyrosine, products containing iodine such as vitamins, or kelp and bladderwrack supplements have the potential to increase or decrease thyroid function. Starting or stopping one of a number of prescription drugs can also affect thyroid levels. A partial list of medications that can affect thyroid levels include: antidepressants, cholesterol-lowering drugs, corticosteroids, lithium and amiodarone.
11. Changing Course of Your Thyroid Disease
You may have been diagnosed with autoimmune Hashimoto's disease a year ago, prescribed thyroid hormone, gone back six weeks later, got rechecked and your TSH was 2. The doctor decided that your levels were fine and told you to come back in a year to be retested. This year's test shows your TSH at 5.7. This sort of increase may reflect the progression of the autoimmune process. As antibodies further attack the thyroid, it can become less and less able to produce thyroid hormone on its own; therefore, T4 and T3 levels drop and TSH rises.
This same process works in the reverse with Graves' disease, where the same dose of antithyroid drugs that kept you in the normal range six months ago is now leaving you still hyperthyroid, as the thyroid becomes even more overactive. In some cases, after months or more on antithyroid drugs, some Graves' disease patients also go into remission, so you find that your antithyroid drug dose can decrease or even be eliminated at times.
In addition, some women develop thyroiditis after pregnancy. For the majority of these women, the condition will resolve itself, meaning that over time, the thyroid will attempt to return to normal and blood test levels will reflect these changes. Drug dosages need to be changed accordingly.Sources:
Levothyroxine Sodium Tablets: USP Monograph Online: http://www.usp.org/USPNF/notices/levothyroxineSodiumTablets.html
Braverman, MD, Lewis E., and Robert D. Utiger, MD. Werner and Ingbar's The Thyroid: A Fundamental and Clinical Text. 9th ed. Philadelphia: Lippincott Williams & Wilkins (LWW), 2005.