You probably know that feeling tired all the time and not being able to lose unwanted weight are classic red flags that you could have an underactive thyroid gland (hypothyroidism), the most common thyroid condition. What you might not realize is that this disease also can cause plenty of other symptoms—some of which seem, at first glance, to be unrelated to the thyroid.

Why is that? The thyroid, a two-inch, butterfly-shaped gland tucked below the voice box, makes hormones that help regulate metabolism—how your body uses energy from the foods you eat—and crucial processes throughout the body such as body temperature, muscle strength and the functioning of such vital organs as the heart and brain.

To ensure that a sluggish thyroid isn’t secretly harming your health, here are the facts you need…

    • Symptoms of hypothyroidism can be subtle and often develop gradually. While you’re likely to notice if your bathroom scale shows that you’re putting on some unexplained weight, other symptoms of hypothyroidism often are overlooked as normal signs of aging. Symptoms, which are sometimes mild, may include depression, forgetfulness, a puffy face, joint and muscle pain, constipation and dry skin. You might also begin to feel a bit colder than you used to, and your hair may start to thin—typically on the eyebrows, but it may also occur on the scalp. To complicate matters, not everyone with hypothyroidism gains weight.
    • Men can be affected by hypothyroidism, too. While it’s true that 80% to 90% of hypothyroidism cases occur in women, an estimated 1.5 to 3 million men in the US also have the condition. Women are more likely than men to be affected by hypothyroidism because they are more susceptible to autoimmune disease. An autoimmune disorder known as Hashimoto’s disease is the main cause of hypothyroidism. With Hashimoto’s, the immune system produces antibodies that attack the thyroid gland, interfering with its ability to produce thyroid hormone.

When men develop symptoms of hypothyroidism—especially weight gain, depression and muscle pain—they often are shrugged off as signs of aging. Over time, low levels of thyroid hormone also can interfere with a man’s libido and/or lead to premature ejaculation.

    • Medications can cause hypothyroidism. The use of certain drugs can trigger the disease by interfering with the production of thyroid hormone. These medications include amiodarone (Cordarone), a heart drug…interferon alpha, a cancer drug…lithium (Eskalith), a drug for treating bipolar disorder…and interleukin-2, a drug taken for kidney cancer.

Paradoxically, some conditions that cause the thyroid to produce too much hormone also can lead to underproduction. In addition, treatment for overactive thyroid could later make you hypothyroid.

    • Doctors don’t routinely test for hypothyroidism. You may assume that the blood tests you receive at your routine doctor visits always check how well your thyroid is functioning. But that’s not true.

Be sure to ask your doctor for a blood test that measures levels of thyroid-stimulating hormone (TSH)—the most accurate indicator of thyroid activity. Higher-than-normal TSH readings indicate hypothyroidism.

Important: Not all TSH testing kits use the same reference ranges—the measurements that are considered normal. For TSH, the most commonly used reference range is 0.5 mIU/L (milli-international units per liter) to 5.0 mIU/L.

However, there is no clear consensus in the medical community on when treatment for hypothyroidism should begin. Some doctors will advise treatment for a reading of 2.5 or above, while others might not unless the reading exceeds 10—the definition of primary hypothyroidism, according to guidelines from the American Association of Clinical Endocrinologists. I typically begin treatment if the TSH level exceeds 4.0 and there is a positive test result for antibodies indicating Hashimoto’s. If the test for Hashimoto’s is negative, I retest the person’s TSH to see if the level normalizes within two to four months before beginning treatment.

Even though there are no definitive answers on when to start treatment, we know that people whose TSH is higher than 2.0 are more likely to eventually develop clear-cut hypothyroidism than those with lower test scores. On the other hand, research has shown that older adults with slightly elevated TSH tend to live longer than those with lower levels.

Tests that may be used in addition to TSH to help diagnose hypothyroidism include those that measure levels of thyroxine (T4) and/or triiodothyronine (T3), two hormones produced by the thyroid…and a thyroid auto-antibody test, which looks for antibodies that are markers for Hashimoto’s disease.

    • Hypothyroidism can elevate cholesterol levels . Even in the early stages, low thyroid can cause an increase in LDL “bad” cholesterol. I’ve seen patients with untreated hypothyroidism whose cholesterol levels were 20 to 50 points higher than they would be otherwise.

Important : If your thyroid levels are low and you also have high cholesterol, be sure to get your thyroid disease treated. It’s possible that your cholesterol will fall (sometimes within a matter of weeks) once the thyroid disease is treated.

    • Hundreds of medications interact with the medication used to treat hypothyroidism. Levothyroxine (Synthroid) is a synthetic version of thyroid hormone. Hypothyroidism is typically well-controlled with this medication—if it’s taken daily as prescribed.

While levothyroxine is unlikely to cause side effects at the proper dose, you can expect to have some side effects until your doctor determines the best dose for you. Too much of the drug can cause increased appetite, shakiness or insomnia. Too little means that your symptoms won’t get better. Follow-up blood tests will be needed until the proper dose is determined and periodically repeated thereafter.

Important: More than 500 medications, from antacids to diabetes drugs, can interfere with levothyroxine—and affect the dose you’ll need. For a list of these medications, go to (click on “Interactions Checker” and enter “levothyroxine). Levothyroxine should be taken first thing in the morning, 30 to 60 minutes before having food, supplements, vitamins or minerals—all of which interfere with the absorption of thyroid hormone.