Hypothyroidism Explained: Causes, Symptoms, And Treatments

If you're constantly exhausted, need a sweater even in the summer, and have gained a bit of weight that you can't seem to lose, you may have hypothyroidism (via the National Institute of Diabetes and Digestive and Kidney Diseases). Also known as an underactive thyroid, hypothyroidism is a condition in which your thyroid gland doesn't produce enough thyroid hormones. Since these hormones control a number of critical functions — including how every cell in your body uses energy — not having enough of them can have a significant effect on your health.

Hypothyroidism is quite common and affects about 5% of Americans over the age of 12. While the condition can happen to anyone, women are much more likely than men to have an underactive thyroid, and the risk of hypothyroidism increases with age. Those with past thyroid issues or a family history of thyroid disease are, not surprisingly, also at greater risk. Having certain conditions, including diabetes, celiac disease, rheumatoid arthritis, and lupus, can also up your chances for hypothyroidism. The good news is that most cases of hypothyroidism are mild. Even thyroids that are moderately or severely underactive can be treated with medication. Although hypothyroidism can't be cured, it can be effectively managed, keeping people symptom-free and able to live a normal life.

The thyroid gland and thyroid hormones

In order to understand hypothyroidism, you need to know a little bit about the thyroid gland and what it does. The thyroid is a butterfly-shaped gland located in front of the windpipe and below the larynx in the throat (via Endocrineweb). If your metabolism is a factory where workers (the body's cells) turn food into energy, then the thyroid gland is the foreman, supervising and regulating everything. It does this by producing two hormones: T3 (triiodothyronine) and T4 (thyroxine). These hormones affect every cell in the body and dictate how quickly or slowly that cell uses energy. Iodine is an essential component of these hormones, and the thyroid extracts and stores this mineral. In a healthy thyroid, about 80% of the hormones produced are T4 and 20% are T3. The thyroid also produces calcitonin, a hormone that helps regulate levels of calcium in the blood. When thyroid hormone levels drop too low, the hypothalamus (a small structure in the brain) produces tryrotropin-releasing hormone (TRH), which triggers the pituitary gland (also located in the brain) to secrete thyroid-stimulating hormone (TSH) (via You and Your Hormones). TSH is what tells the thyroid to make more T3 and T4. In hypothyroidism, either the thyroid can't respond properly to TSH, or there's a problem with the release of TRH or TSH.

In addition to controlling how quickly cells use energy, thyroid hormones influence body temperature, heart rate, the speed at which food moves through the digestive tract, how muscles contract, and the rate at which dying or damaged cells are replaced (via University of Michigan Health).

Hashimoto's thyroiditis

Hypothyroidism can have many underlying causes, but Hashimoto's thyroiditis (also known as Hashimoto's disease) is by far the most common (via Endocrineweb). Hashimoto's affects about 14 million Americans and is the most prevalent form of thyroid disease in the United States. It's an autoimmune condition in which the immune system becomes confused and begins attacking the thyroid, causing inflammation and interfering with the gland's ability to produce thyroid hormones. It's unclear what exactly triggers the autoimmune reaction, but there may be many contributing factors. Having a pre-existing condition like type 1 diabetes increases the risk of developing Hashimoto's. And because Hashimoto's often runs in families, genetics may play a role (via WebMD). Hormones may also be involved, since women are seven times more likely than men to have the condition. Excessive amounts of iodine and exposure to radiation may also cause the immune system to go rogue and begin attacking the thyroid.

Another potential trigger for the autoimmune reaction that causes Hashimoto's disease (and autoimmune conditions in general) is a viral or bacterial infection (via the National Center for Biotechnology Information). In the course of fighting the infection, the immune system may accidentally produce cells that attack healthy human tissue, and these cells continue to multiply long after the original infection has been cleared.

Treating hyperthyroidism can lead to hypothyroidism

On the other end of the spectrum, some people have hyperthyroidism, a condition in which the thyroid is overactive and produces too much T3 and T4 (via the Mayo Clinic). Hyperthyroidism can be treated, but sometimes those treatments are a little too effective and end up overcorrecting the imbalance, leading to an underactive thyroid. There are three options for treating hyperthyroidism. Radioactive iodine is taken orally and, once absorbed by the thyroid gland, causes it to shrink and become less active. Anti-thyroid medications such as methimazole and propylithiouracil can also block the thyroid from producing excess amounts of thyroid hormones. The last resort is surgery to remove the thyroid (thyroidectomy).

All three treatments for hyperthyroidism can lead to hypothyroidism. In fact, according to the American Thyroid Association, "the most common result of surgery and radioactive iodine therapy is hypothyroidism." The association also noted that "hypothyroidism can also develop if the dose of antithyroid drug is too high." The effects on the thyroid after surgery or radioactive iodine treatment are irreversible, but hypothyroidism caused by overuse of anti-thyroid medication is usually resolved once the dosage is adjusted. Thyroid removal may also be performed if someone has thyroid cancer, a goiter (noncancerous enlargement of the thyroid), or suspicious nodules that might be cancerous (via the Mayo Clinic). If only one lobe of the thyroid is removed during surgery (lobectomy), patients have a 30–50% chance of developing hypothyroidism (via Columbia Surgery).

Some medications can cause hypothyroidism

Certain drugs besides those used to treat hyperthyroidism may also cause hypothyroidism. A 2010 paper published in Best Practice & Research Clinical Endocrinology & Metabolism outlined a number of these. Some drugs can adversely affect how much T3 and T4 the thyroid is able to secrete. These include lithium (used to treat mood disorders such as bipolar disorder), amiodarone (used to treat irregular heartbeat), and aminoglutethimide (used to treat Cushing's disease, in which the body produces too much cortisol). Other medications may cause inflammation in the thyroid (thyroiditis), which impacts its ability to produce hormones. Interferons and sunitinib, both used to treat certain types of cancer, can cause thyroiditis and subsequent hypothyroidism. Medication-induced hypothyroidism can also occur if a drug interferes with the pituitary gland's ability to secrete TSH. Glucocorticoids (anti-inflammatory drugs), dopamine agonists (used to manage Parkinson's disease), metformin (a diabetes medication), and rexinoids (a new group of drugs with promising anti-cancer properties) can all suppress the pituitary gland's ability to produce the TSH that the thyroid gland uses as a signal to make more thyroid hormones.

For those with hypothyroidism, some supplements and drugs may interfere with levothyroxine, the medication used to treat underactive thyroid. Large amounts of iron and calcium can prevent absorption of levothyroxine, as can colestipol (used to treat high cholesterol) and raloxifene (used to treat and prevent osteoporosis). Others, like carbamazepine (used to treat epilepsy) and propranolol (prescribed for high blood pressure) can negatively impact how levothyroxine is metabolized by the liver.

Hypothyroidism can result from iodine deficiency

The thyroid needs iodine to create T3 and T4, so not getting enough of this important mineral can cause the thyroid to become underactive. As the National Institutes of Health explains, "In the absence of sufficient iodine, TSH levels remain elevated, leading to goiter, an enlargement of the thyroid gland that reflects the body's attempt to trap more iodine from the circulation and produce thyroid hormones." Adults need very little iodine to maintain healthy thyroid function. The recommended dietary allowance (RDA) for adult men and nonpregnant women is only 150 mcg, while pregnant women should get 220 mcg and breastfeeding mothers need 290 mcg. Iodine is present in varying amounts in soil around the world, and the iodine content of foods is reflective of where they came from. Because of how iodine bioaccumulates as it moves up the food chain, meat, dairy products, and seafood are generally higher in iodine than plant foods (with the exception of iodine-rich seaweed). Iodine is also commonly added to table salt. A quarter teaspoon of iodized salt contains 76 mcg of iodine.

Iodine deficiency used to be prevalent in North America. According to the American Thyroid Association, "before the 1920s, iodine deficiency was common in the Great Lakes, Appalachian, and Northwestern U.S. regions and in most of Canada." The area around the Great Lakes was even referred to as the "goiter belt." But the introduction of iodized salt has almost completely eliminated iodine deficiency in the United States. Globally, however, 30% of the world's population remain at risk for iodine deficiency.

Cancer treatment can lead to hypothyroidism

Cancer treatment is no walk in the park. Chemo and radiation can leave people weak, exhausted, nauseous, and losing their hair (per the Centers for Disease Control and Prevention). And, as a 2014 paper published in Oncologist explained, both treatment options can damage the thyroid, leading to hypothyroidism. In particular, the treatment of thyroid cancer and head and neck cancer is most likely to lead to an underactive thyroid, thanks to the proximity of the cancer to the thyroid gland. In the case of radiation therapy, the radiation damages the cells of the thyroid and causes acute and chronic inflammation. But it takes a while for hypothyroidism to appear — usually two to seven years after radiation therapy. Chemotherapy drugs for certain types of cancer can lead to hypothyroidism in as many as 70% of patients. In some cases the hypothyroidism resolves once the drugs are stopped, but in other cases the damage is permanent. And, in the case of thyroid cancer, surgical removal of the thyroid will, of course, cause irreversible hypothyroidism.

Each year, more than 44,250 people are diagnosed with thyroid cancer (via Cancer.Net). Women are three times more likely than men to get thyroid cancer, and it's the seventh most common cancer among women. Each year about 2,200 people die of thyroid cancer, but because the thyroid is an organ that can be removed relatively easily, the five-year survival rate for thyroid cancer is 98%. Changes in medical technology have also made it easier for doctors to diagnose thyroid cancer at a very early stage. Head and neck cancer accounts for about 4% of all cancers in the United States, with more than 66,000 new diagnoses each year. Men are significantly more likely than women to get head and neck cancer (via Cancer.Net).

Problems with the pituitary gland or hypothalamus can also cause hypothyroidism

Most cases of hypothyroidism happen at the level of the thyroid gland itself, but in some instances the problem lies farther upstream, with one of the two organs that control the thyroid gland. As a 2011 paper published in the Indian Journal of Endocrinology and Metabolism explained, the term "central hypothyroidism" refers to hypothyroidism caused by issues with either the pituitary gland (known as secondary hypothyroidism) or the hypothalamus (tertiary hypothyroidism). While primary (regular thyroid-based) hypothyroidism is relatively common, central hypothyroidism is quite rare, affecting approximately 1 in 80,000–120,000 individuals. Central hypothyroidism is usually accompanied by other hormone and neurological issues, since the pituitary and hypothalamus play a number of important roles in the body.

Central hypothyroidism can have many causes. Some are genetic, such as mutations in the genes that control the production of TSH and TRH or the receptors that they bind to. Brain tumors can also impair the normal functioning of the pituitary and hypothalamus. Brain hemorrhage and stroke can damage the pituitary or hypothalamus, as can viral and bacterial infections. Hemochromatosis, a condition in which iron builds up in the body to toxic levels (via Cleveland Clinic), can also lead to central hypothyroidism. Other causes include damage resulting from addiction to morphine or huffing glue, head trauma, and certain medications. In cases of central hypothyroidism, rather than treating patients with the TSH or TRH they aren't producing on their own, doctors simply cut out the middleman and use synthetic thyroid hormones (via Drugs.com).

Hypothyroidism may be present from birth

In rare cases, a person may be born with hypothyroidism. Known as congenital hypothyroidism, the condition affects 1 in every 2,000–4,000 infants (via MedlinePlus). In 80–85% of cases, the thyroid gland is either totally absent, extremely small, or not located in the correct place. In the other 15–20% of cases, the thyroid is normal or even enlarged, but thyroid hormones either aren't produced at all or are produced in too small a quantity. This occurs if there's a problem with any of the several steps required to create T3 and T4. Congenital hypothyroidism can happen on its own or as part of a larger syndrome that adversely affects other organs as well.

Initially, newborns with congenital hypothyroidism may not show any signs of their condition (via the Mayo Clinic). If symptoms are present, they usually include jaundice, a large tongue that protrudes from the mouth, difficulty breathing, and hoarse crying. These infants will likely have trouble feeding and may not grow and develop normally. Constipation, excessive sleepiness, and poor muscle tone can also occur as a result of the underactive thyroid. If not adequately treated, even mild cases of hypothyroidism in babies can lead to significant physical and mental impairments.

Hypothyroidism during pregnancy carries unique risks

As if moms-to-be didn't have enough to worry about, add hypothyroidism to the list of conditions that may arise during or complicate a pregnancy. During a normal pregnancy, thyroid hormones may fluctuate because of the influence of human chorionic gonadotropin (hCG) and increased estrogen levels (via the American Thyroid Association). The thyroid may also increase in size, although this usually only occurs among women who are iodine deficient. Hashimoto's thyroiditis is the most common cause of hypothyroidism in pregnancy, and some experts suspect that the hormonal changes that occur during gestation are the trigger of this autoimmune response. If hypothyroidism during pregnancy isn't adequately treated, it increases the mother's risk for miscarriage, anemia, muscle pain and weakness, congestive heart failure, pre-eclampsia, problems with the placenta, and postpartum hemorrhage. Severe untreated hypothyroidism during pregnancy can impair the baby's brain development. Fortunately, most pregnant women with an underactive thyroid can safely take thyroid medication to balance their hormone levels, although their dose may need to be increased by 25–50%.

According to a 2012 paper published in the Indian Journal of Endocrinology and Metabolism, about 0.3–0.5% of pregnant women will experience symptomatic hypothyroidism during their pregnancy, while 2–3% have subclinical (symptomless) hypothyroidism.

Symptoms of hypothyroidism

Everyone with an underactive thyroid is affected by the condition differently. Which symptoms someone has, and the degree to which they have symptoms at all, can vary significantly from person to person (via UpToDate). If hypothyroidism develops gradually, symptoms are usually more mild, whereas rapid-onset hypothyroidism is likely to produce more dramatic symptoms. Symptom severity usually directly corresponds to how underactive the thyroid is, so mild hypothyroidism often produces no or few symptoms. To make matters more complicated, the symptoms of hypothyroidism are pretty nonspecific, and many of them are also a common part of normal aging. This means people experiencing symptoms are more likely to write them off as no big deal and thus never get a formal diagnosis. Common symptoms include fatigue, sluggishness, cold intolerance, dry skin, brittle nails, reduced sweating, facial swelling, slowed heart rate, decreased lung function, constipation, and menstrual irregularities.

According to a 2017 study published in the Journal of Clinical and Diagnostic Research, the symptom people are most likely to associate with hypothyroidism is weight gain. In reality, however, the slowed metabolism an underactive thyroid causes usually leads to only modest weight gain of just 5–10 pounds (via the American Thyroid Association). What's more, this weight gain is mostly retained salt and water, not fat.

Complications of untreated hypothyroidism

Untreated hypothyroidism, especially if it's severe, can lead to a number of complications. Goiter (an enlarged thyroid) is one of the most common (via the Mayo Clinic). When an underactive thyroid is constantly being bombarded with TSH (the signal to create more thyroid hormones) but it can't produce what's needed, the gland may grow in an attempt to get the job done (via Everyday Health). In addition to negatively impacting a person's appearance, large goiters can make swallowing and even breathing difficult. Hypothyroidism can also elevate levels of LDL ("bad") cholesterol, which in turn raises an individual's risk for heart disease (via Mayo Clinic). Depression, impaired thinking, nerve damage, and infertility can also arise if hypothyroidism isn't addressed.

Myxedema coma is a rare but life-threatening complication of untreated severe hypothyroidism (via WebMD). If you have an extremely underactive thyroid, myxedema can be triggered by forgetting to take your thyroid medication or, if you aren't on medication, by an infection, heart failure, stroke, physical trauma, surgery, or certain drugs. Symptoms include extreme weakness and lethargy, low body temperature, confusion or non-responsiveness, and difficulty breathing. Individuals with myxedema are near or in a comatose state and, if not treated promptly in an emergency room, the condition can be fatal.

Diagnosing hypothyroidism

If your doctor suspects you may have hypothyroidism, they'll order blood tests to check your TSH and T4 levels (via the Cleveland Clinic). And, since thyroid issues are so common, some doctors routinely test the TSH levels of non-symptomatic patients just to make sure everything's normal. TSH is usually the initial screening test, and high values indicate an underactive thyroid. If TSH levels are elevated, T4 will then be examined (T3 levels are normally only tested when diagnosing and managing hyperthyroidism). Lower-than-average levels of T4 indicate hypothyroidism. 

If a person has high TSH levels but normal levels of free T4 and few if any symptoms, they have subclinical hypothyroidism (via the Merck Manual). Subclinical hypothyroidism is quite common, affecting about 15% of older women and 10% of older men. If these individuals have a TSH level greater than 10 mU/L, there's a good chance their subclinical hypothyroidism will progress to full-blown hypothyroidism (with low T4 and more symptoms) within the next 10 years. Individuals with subclinical hypothyroidism are usually treated with the same drug used to treat overt hypothyroidism.

Treating hypothyroidism

The good news for people with hypothyroidism is that the condition can be completely controlled with levothyroxine, a synthetic form of T4 (via the American Thyroid Association). There are many brand names for levothyroxine (including Unithroid, Levo-T, and Synthroid), but all contain the same active ingredient with a differing list of inactive ingredients. Thyroid medication only needs to be taken once a day, ideally first thing in the morning on an empty stomach. Synthetic T4 is extremely safe, and the only side effects arise if someone takes more or less than they need to stabilize their hormone levels. Even though T3 is the more powerful of the two thyroid hormones, this usually doesn't need to be supplemented, since the thyroid converts T4 into T3.

Figuring out the correct dose of levothyroxine to bring T4 (and thus also TSH) levels back into the normal range can be tricky and require some trial and error. An initial dosage will be calculated based on a person's hormone levels, age, sex, weight, and other medical conditions. TSH and T4 levels are checked six to eight weeks later and the dosage is adjusted as necessary (via the Mayo Clinic). Because the optimal dosage will likely change over time, hormone levels are usually rechecked every year to make sure they're still in the normal range.

Alternative treatments for hypothyroidism aren't a good idea

Many people assume that there are natural or alternative treatments for hypothyroidism. In fact, in a 2017 study published in the Journal of Clinical and Diagnostic Research, researchers found that 45% of hypothyroid patients surveyed believed alternative treatments could treat their condition. But, as GoodRX explains, "prescription medication is usually the key to managing hypothyroidism — everything else is either risky or complementary." Some people believe that taking supplemental iodine will jumpstart their underactive thyroid, but unless they have an iodine deficiency, this mineral is no miracle cure. Another commonly touted but potentially dangerous treatment is ingestion of dried pig or cow thyroid tissue. Herbs such as ashwagandha, gum guggal, and coleus forskohlii are purported to stimulate thyroid function, but more scientific research is needed to support these claims. Essential oils, coconut oil, yoga, hot and cold therapy, and acupuncture have also been labeled as "cures" for hypothyroidism, and while some may make you feel a little better, none of them actually stimulate the thyroid to produce more hormones.

Can hypothyroidism be managed by eating or avoiding certain foods? The answer is no. According to WebMD, "there's no special meal plan for managing thyroid disease. Still, a well-balanced diet can help you feel good and ward off disease." Soy products, however, may interfere with your body's ability to absorb levothyroxine, so it's best to avoid eating them at the same time you take your thyroid medication.