Fatigue, weight gain and feeling cold all the time are some of the most common reasons people ask for a thyroid test. Sometimes the thyroid is the cause. Often it is not. The good news is that one inexpensive blood test, the TSH, answers the first question for most people, and the condition it finds is very treatable.

What the thyroid does, and what happens when it slows down

The thyroid is a small, butterfly-shaped gland in the front of the neck. Its hormones set the pace at which the body uses energy, so they touch nearly every organ, including the heart. When the gland does not make enough, the condition is called hypothyroidism, or underactive thyroid, and many body functions slow down. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) estimates that nearly 5 out of 100 Americans age 12 and older have hypothyroidism, though most cases are mild.

It is much more common in women and in people over 60. The NIDDK also lists a higher risk if you have had a thyroid problem or thyroid surgery before, radiation to the neck or chest, a family history of thyroid disease, or conditions such as type 1 or type 2 diabetes, celiac disease, rheumatoid arthritis or lupus. The most common cause in the United States is Hashimoto’s disease, in which the immune system attacks the thyroid. Some heart, cancer and bipolar disorder medicines can also slow the gland.

The symptoms the NIDDK lists are easy to mistake for ordinary life: fatigue, weight gain, trouble tolerating cold, joint and muscle aches, dry skin or thinning hair, a slow heart rate and low mood. Because the condition usually develops slowly, people may not notice it for months or years. An underactive thyroid can also raise cholesterol, which is why a high cholesterol result is one reason to check thyroid function.

How the TSH test works

TSH stands for thyroid stimulating hormone. It is made by the pituitary gland in the brain and works like a thermostat signal, telling the thyroid how much hormone to make. According to the NIDDK’s guide to thyroid tests, doctors usually check TSH first. When the thyroid is underactive, the pituitary keeps pushing, so a high TSH most often means hypothyroidism. A low TSH usually points the other way, toward an overactive thyroid.

If the TSH is abnormal, the next step is usually a free T4 level, which measures the hormone the thyroid actually releases. Thyroid antibody tests can show whether an autoimmune condition such as Hashimoto’s is behind it. A diagnosis is never made on symptoms alone, since so many of them overlap with other conditions, and a borderline result is often repeated before anyone starts medicine.

Should everyone be screened?

Not necessarily. The U.S. Preventive Services Task Force concluded in 2015 that the evidence is insufficient to recommend for or against screening adults who have no symptoms, and that statement is still current. In practice, doctors test when there are symptoms, risk factors, abnormal cholesterol, or other reasons to look.

A common gray zone is subclinical hypothyroidism: a TSH that is somewhat high while free T4 is still normal. This is especially common with age. A large trial of adults 65 and older with this pattern, published in the New England Journal of Medicine in 2017, found that thyroid hormone pills did not improve symptoms or tiredness after a year compared with placebo. That is why a mildly high TSH in an older adult is often rechecked and watched rather than treated right away.

Treatment and follow-up

When treatment is needed, it replaces the hormone the thyroid can no longer make. The standard medicine is levothyroxine, identical to the hormone a healthy thyroid produces. The NIDDK describes the usual routine on its hypothyroidism page:

  • Take it as directed, often in the morning before eating.
  • Expect a repeat blood test about 6 to 8 weeks after starting or changing the dose.
  • Once the dose is right, the test is usually repeated at 6 months and then once a year.
  • Do not stop the medicine without talking to your doctor.

More is not better. Too much thyroid hormone can cause serious problems, including atrial fibrillation and bone loss, which is why the dose is guided by blood tests. Some supplements get in the way: the NIH Office of Dietary Supplements notes that taking iron with levothyroxine can make the medicine less effective. Bring every supplement you take to your visits, and ask how far apart to space them. The NIDDK also cautions that large amounts of iodine, from kelp or iodine supplements, can worsen hypothyroidism in people with Hashimoto’s.

When to call

Call your doctor if you have new fatigue, weight change, feeling cold, or a slow pulse that does not go away, or if you find out you are pregnant while taking thyroid medicine, since the dose often needs to rise. Severe, untreated hypothyroidism can rarely become a medical emergency called myxedema coma, in which the body’s functions slow to a life-threatening point. That needs 911.

A TSH is part of the blood work Dr. Shtern can order and review with you, with results explained in plain language. Our labs and tests page covers how testing works at the practice, and our article on medication reviews explains why supplements belong on the list. If you would like to meet Dr. Shtern first, the first visit is a complimentary meet and greet.

This article is general information, not advice for your situation. Members can call the office with questions about their own care.