DEAR DOCTOR K:
I have hypothyroidism and take synthetic thyroid hormone. Is it OK to switch brands or switch to a generic?
It’s OK to switch, so long as your doctor monitors your thyroid blood tests. It is very important to get your dose right: Your thyroid gland influences your metabolism. It affects everything from body temperature to body weight, energy level, even fertility.
Thyroid hormone comes in two forms, called T4 and T3. Although there is some controversy on this point, most thyroid experts here at Harvard Medical School think that T4 and T3 pills are equally effective. Most often, doctors treat with a synthetic form of T4 called levothyroxine sodium.
Hypothyroidism occurs when the thyroid gland is not making enough thyroid hormone. Two blood tests are used most often to diagnose hypothyroidism; they measure the levels of the T4 and thyroid-stimulating (TSH) hormone. TSH is a brain hormone. When the brain senses your T4 levels are low, it makes more TSH. The TSH then signals the thyroid to make more T4.
So, paradoxically, when your thyroid gland starts to not make enough T4, your T4 level is low but your TSH level is high. These tests help diagnose hypothyroidism, and enable your doctor to determine if your treatment dose is correct. (See the illustration below, which explains how the thyroid works.)
Normally, the hypothalamus sends a signal in the form of thyrotropin-releasing hormone (TRH) that enables the pituitary gland to secrete thyroid-stimulating hormone (TSH). In response, the thyroid gland releases T4 and a small amount of T3. These travel to the liver and other organs, where T4 is converted to T3 and enters the bloodstream. Like a heating system with a thermostat set at a constant temperature, a normally functioning thyroid operates at a steady pace without much variation.
If your thyroid gland can’t make enough T4, you need to take some thyroid hormone in pill form. Finding the right dose of treatment involves some trial and error. Your initial dose is an educated estimate. To check if it’s correct, your doctor will measure your blood levels of TSH (and, sometimes, also T4). If your TSH level still is high, you need a higher dose of T4. If the TSH level is normal, you’re on the right dose. If the TSH level is too low, your dose is too high.
There are many brands of synthetic T4, and each is as good as the next. However, problems can occur when you switch brands. Small differences between brands can significantly impact how much T4 your body absorbs. Even slight changes in amounts of T4 absorbed may affect your overall health.
If you must switch — if your health plan changes its list of approved drugs, or if another brand is cheaper — tell your doctor. He or she should measure your TSH level after you start the new brand and make necessary adjustments to your dosage.
I also wouldn’t recommend taking a generic version of synthetic T4. You’re not guaranteed to receive the same generic version every time you refill your prescription. This inconsistency may throw off your thyroid hormone levels. What’s more, the FDA allows a certain degree of difference between a generic and its brand-name equivalent. This slight difference can have a significant impact on thyroid hormone levels.
If you must switch to a generic, your doctor should regularly monitor your TSH levels.