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Your Thyroid — The New Normal


a blog by Serena H. Chen, M.D., IRMS Reproductive Medicine at Saint Barnabas, January 31, 2012

How do you know what is “normal”? In medicine, “normal” can be defined in many different ways. For many lab values, “normal” is defined by the general population. What values do most people have for this particular lab test? A common criteria is to see what value do 95 percent of people have for this particular lab value? If you are outside the 95th percentile, then your lab value is “abnormal.” But what population do you choose? What if a lot of people in your population are actually abnormal?

The definition of normal for thyroid function has been very controversial in recent years. The problem with thyroid function is that we are finding the definition of normal is not so simple. It turns out the definition of normal may depend upon the situation.

For example: pregnancy. If you are pregnant, thyroid hormone is critical to the development of the fetal neural system. If your thyroid function is borderline, it is better to have more thyroid hormone on board, rather than less. On the other hand, if you are an older lady with a heart condition, too much thyroid hormone could increase your heart rate and put too much stress on your heart. If your thyroid function is borderline, unlike the pregnant lady, it is better for you to have less thyroid hormone on board, rather than more. So “normal” for an older lady with a heart condition is very different than “normal” for a young, pregnant woman.

One of the best ways to assess thyroid function is to measure your TSH level. TSH stands for Thyroid Stimulating Hormone. This hormone is made by your brain and stimulates thyroid hormone production by the thyroid gland in your neck. If your thyroid is not producing enough hormone, or you are hypothyroid, then your TSH level will increase as your brain tries to get your thyroid to work harder.

A recent large study of pregnant women raised the question of what is normal. Negro et al, in the September 2010 issue of the prestigious journal Journal of Clinical Endocrinology and Metabolism (JCEM), found that although the normal range of TSH is defined as 0.5 to 5.0, that pregnant women who had TSH levels of 2.5 to 5.0 had almost double the miscarriage rate (6.1 percent vs 3.6 percent) of women with TSH levels less than 2.5. What is more remarkable is that none of these women had thyroid antibodies — a marker for possible future thyroid problems.

No one really agrees about what to do with this new information. Many of us who care for high risk patients with a history of recurrent miscarriage and infertility have begun to give our patients with borderline TSH levels small amounts of thyroid hormone to nudge them into the “supernormal” zone, rather than the regular “normal” zone. The people in charge — like the Endocrine Society — who make the rules about “normal” still do not agree on what is “normal." For now, this has to be handled on an individual basis — the benefits of treatment should outweigh the risks and only your doctor can help you to decide that. If you are struggling to conceive, or have had two or more miscarriages, talk with your doctor about whether or not to consider thyroid supplementation.

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