What is the most commonly prescribed brand name medication in America?

No, it's not a blood thinner, an antidepressant or a cholesterol-lowering drug—it's Synthroid, a medication used to treat underactive thyroid (hypothyroidism). Nearly 30 million Americans—most of them women—have been diagnosed with either hypothyroidism or hyperthyroidism (overactive thyroid).

"Women who come to clinical attention are just the tip of the iceberg," says R. Mack Harrell MD, president of the American Association of Clinical Endocrinologists. Most women with either an over- or underactive thyroid have no definitive symptoms, according to Harrell. But a lack of outward indications may mask the more serious underlying health consequences of a thyroid that's on the fritz.

What is the thyroid and what does it do?

The thyroid, a small endocrine gland located just below the Adam's apple, is responsible for making and releasing the hormones that manage the rate of energy dispersal in the body.

The most crucial of these hormones is called thyroxine (T4). Thyroxine influences a person's metabolism, body temperature, muscle control, cardiovascular functioning, and bone and brain health. Too much T4 can cause heart palpitations, weight loss and extreme anxiety, while too little can cause an abnormally slow heart rate, weight gain and depression.

Over time, unbalanced thyroid hormone levels can lead to an increased risk of thyroid cancer and atrial fibrillation—a dangerous type of heart arrhythmia.

How do I know if I have a thyroid condition?

The two main thyroid conditions—hypo- and hyperthyroidism—have opposite sets of symptoms.

People with hypothyroidism tend to experience weight gain, irregular menstrual cycles, slow heart rate, muscle aches, constipation, voice changes and the loss of body hair.

Conversely, people with hyperthyroidism tend to experience weight loss, irregular menstrual cycles, rapid heart rate, emotional instability, and increased nervousness and anxiety.

The nonspecific nature of these symptoms can make thyroid conditions difficult to diagnose, according to Harrell. "Endocrinologists are constantly getting calls from people who 'don't feel right,' " he says. "The unfortunate thing is that most of these people don't have low thyroid levels. Lots of other things can cause these symptoms."

If you feel you may be having problems with your thyroid, Harrell suggests bringing them to the attention of your primary care doctor. He or she will perform a physical exam, taking your vitals and feeling your throat and neck for suspicious nodules.

You may also be asked to undergo bloodwork to check the levels of T3 (another thyroid hormone) and T4 in your blood, as well as the amount of TSH (thyroid-stimulating hormone) you're secreting. TSH is an important measure because it's essentially your body's impression of whether you have enough thyroid hormone to fuel your organs and cells.

What causes the thyroid to malfunction?

Most thyroid malfunctioning is caused by an autoimmune response. The immune system of someone with Hashimoto's Thyroiditis (hypothyroidism) sees the thyroid as a threat and begins to attack it, causing inflammation and damage that leads to a decrease in hormone production. The immune system of someone with Grave's disease (hyperthyroidism) compels the thyroid, unnecessarily, to over-produce hormones.

A prevailing theory behind why women are more susceptible to thyroid disorders is because the female immune system is more pliable than the male immune system, capable of revving up or winding down in response to certain biological events, specifically pregnancy. When a woman becomes pregnant, the baby is seen as a foreign object in the body. In order to prevent the growing fetus from being damaged, the mother's immune system must be suppressed.

According to Harrell, women frequently experience some form of transient hypothyroidism after birth, which can sometimes be confused with postpartum depression, since the two conditions share similar symptoms.

Does stress affect the thyroid?

A question that's sure to be at the top of many a caregiver's list is whether long-term stress can cause a person's thyroid to behave erratically. Many endocrinologists say their female patients report an increased amount of psychosocial stress in the months leading up to their diagnosis, according to Harrell, though experts have yet to determine the cause of this trend.

How are thyroid disorders treated?

The majority of hypothyroid conditions are treated with a prescription medication called levothyroxine. This drug helps bridge the gap between the amount of T4 being released by the thyroid and what a person's body needs to function optimally. For patients on levothyroxine medication, their primary care physician or endocrinologist will conduct periodic blood tests to monitor and tweak the dosage over time.

Synthroid is the brand name version of levothyroxine. However, several generic varieties are also available on the market.

It's vital for anyone who is taking these medications to remember three simple words: same, same, same. "Take the same preparation at the same time and in the same fashion, every day," Harrell says. "Thyroid hormone is a bit persnickety. There are a lot of compounds that can interfere with its absorption."

Generally, thyroid medications should be taken at least 30 minutes prior to eating breakfast or before bedtime, and hours away from ingesting foods or vitamins that contain soy, calcium or iron.

People who use health insurance to help cover the cost of generic levothyroxine should also be aware of any substitutions that are being made with their medications. In an effort to curb costs, insurance providers may switch between different generics, sometimes as frequently as month-to-month. While all generic forms of levothyroxine must be internally consistent, Harrell cautions that the seemingly minor variations in the dosage amount of these medications that are allowed by the Food and Drug Administration (FDA) can cause trouble for some people.

For example, if company A makes a 100 microgram (mcg) levothyroxine tablet that's actually 104 mcg (within the FDA-allowed parameters of five percent fluctuation) and company B makes a 100 mcg tablet that's actually 96 mcg, it could be enough to mess up a patient's TSH levels.

Each month, people taking generic levothyroxine should look to see if the number stamped on their pills have changed. "This is the only way to tell whether a generic has changed," Harrell says. "This process is confusing for patients—most never even know they've been switched."

Arm yourself with knowledge

If you or a loved one has thyroid concerns, it's important to know how to talk to your doctor and pharmacist about these issues. Harrell and his colleagues at the AACE offer this list of 10 Questions to Ask about Your Thyroid Health:

  1. Where is the thyroid located, and what does it do?
  2. What are the differences between hypothyroid and hyperthyroid patients, and what are the symptoms of each?
  3. What is Thyroid Stimulating Hormone (TSH), how is it measured, and what should my target number be?
  4. What else besides TSH levels is important for making sure my thyroid condition is under control?
  5. Why are more people than ever being diagnosed with thyroid cancer, and should I be checked for it?
  6. What is the difference between a generic thyroid hormone pill and a brand name thyroid hormone pill?
  7. Will you notify me in advance if you switch my thyroid medicine from the brand name I normally use to a generic?
  8. What time of day is best to take my thyroid hormone pill?
  9. May I take my thyroid medication with food, other medications, vitamins or supplements?
  10. Can any of my other medications affect my thyroid?