It's a small but mighty gland. Your thyroid — a butterfly-shaped gland that sits in the front of your neck just below your voice box (larynx) — is considered the main metabolism regulator of the body. 

Its primary job is to control your metabolism, the rate at which your body burns calories and uses energy. When your thyroid gland doesn't work properly, it can be due to a thyroid disorder, such as an underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism).

These thyroid disorders are common and can be treated, but they can impact your ability to gain or lose weight. An underactive thyroid can be linked to gaining weight.

"Screening for thyroid disorders should be part of the routine initial evaluation of unexplained weight gain," says Northwestern Medicine Endocrinologist Jared Friedman, MD.

What Is Hypothyroidism?

The thyroid gland is responsible for producing, storing and secreting thyroid hormone — an essential hormone for the function of virtually all parts of your body. Reduced production of thyroid hormone can be temporary, but when it lasts longer, the result is called hypothyroidism.

Low thyroid hormone levels slow metabolism, which can lead to weight gain.

"The thyroid gland is a huge determinant of the body's basal metabolic rate, so even subtle disruptions in thyroid hormone can slow metabolism and ultimately lead to weight gain," says Dr. Friedman.

About 5% of people in the United States, ages 12 and older, have some degree of hypothyroidism — the vast majority falling into what is known as "subclinical" or mild hypothyroidism.

When hypothyroidism is diagnosed early, it's called subclinical hypothyroidism. This happens when the thyroid-stimulating hormone (TSH) is slightly elevated, but the T3 (triiodothyronine) and thyroxine (T4) thyroid hormone levels are normal.

Risk Factors for Hypothyroidism

Anyone can develop an underactive thyroid, but hypothyroidism is most common in:

• Women
• People older than 60
• Those who have a history of thyroid problems
• People with certain medical conditions, such as celiac disease, diabetes, lupus or rheumatoid arthritis

Having hypothyroidism does not increase your risk of thyroid cancer. However, untreated hypothyroidism can affect blood sugar levels by increasing insulin resistance. This can also affect cholesterol levels, which can increase the risk of developing cardiovascular disease.

Causes of Hypothyroidism

You can have congenital hypothyroidism (present at birth) or acquired hypothyroidism (developed later in life).

Hashimoto's thyroiditis, also known as autoimmune thyroiditis, is the most common cause of hypothyroidism. It's an autoimmune disorder that impacts your thyroid function. Your immune system attacks your thyroid — specifically, your body's white blood cells and antibodies mistakenly attack the gland.

Other causes of hypothyroidism include:

• Thyroid surgery to remove your thyroid. Certain conditions may require surgery to remove all or part of the gland.
• Radiation therapy. During radiation therapy for head and neck cancers, the beams can damage the thyroid gland and surrounding tissues.
• Thyroiditis (inflammation of the thyroid, which can be caused by pregnancy or a viral infection)
• Conditions present at birth, like being born without a thyroid
• Certain medications like amiodarone (a medication that helps the heart beat more regularly)
• Iodine deficiency (This is rare in the U.S., but it's the most common cause of hypothyroidism worldwide, especially in areas where iodized salt is not available.)
• Pituitary gland disorders

Symptoms of Hypothyroidism

Symptoms of an underactive thyroid can be nonspecific or reflect other issues, including:

• Fatigue
• Brain fog (difficulty concentrating)
• Constipation
• Depression
• Anxiety
• Dry skin
• Feeling cold
• Increased cholesterol levels
• Weight gain
• More frequent or heavy menstrual periods
• Muscle aches or weakness
• Thinning hair
• Puffiness in face

Symptoms can depend on the severity of hypothyroidism, and an underactive thyroid can be mistaken for other illnesses because symptoms can be vague and overlap with other chronic conditions.

Getting the Right Diagnosis for Hypothyroidism

The only way to know for sure if weight gain or other symptoms are from hypothyroidism is through a simple blood test to check your TSH levels. After a physical exam, your physician can order labs to check your TSH.

Even small changes in your T4 and T3 can cause big changes in TSH levels. The normal range for TSH is between 0.4 and 4.0 milliunits per liter (mU/L). With hypothyroidism, TSH is higher than 4.0 mU/L, and with hyperthyroidism, TSH is lower than 0.4 mU/L.

TSH levels may vary depending on your life circumstances. For example, people older than 65 may have a slightly higher TSH since baseline TSH can increase with age. During pregnancy, the normal TSH range differs by trimester. If you are pregnant and have pre-existing thyroid disease, you will likely have to get labs once a month to monitor your TSH levels.

Treatment Options for Hypothyroidism

Once a hypothyroidism diagnosis is confirmed, your care team can work with you to create a treatment plan. There's no cure for hypothyroidism, but people with an underactive thyroid can manage the condition by taking the synthetic hormone levothyroxine (LT4). This replaces the T4 hormone your body is not producing.

Here's how T4 is taken:

• Available as a pill or in liquid form
• Typically taken daily first thing in the morning about an hour before breakfast (your physician will help you determine the dose, timing and frequency)
• Safe to take while pregnant and breastfeeding

Since hypothyroidism is considered a lifelong condition, most people who take levothyroxine take it for the rest of their lives.

As with any medication, there are side effects to levothyroxine, which are typically related to the dose being too high. You can develop symptoms of hyperthyroidism, such as an irregular heart rate, increased appetite and weight loss. These side effects should subside once your TSH is at an optimal level for you. It might take a few months and lab tests to get to the correct dose. Discuss any concerns with your care team.

 Untreated hypothyroidism can lead to:

• Cognitive changes
• Heart issues
• Infertility
• Goiter
• Myxedema coma, which is a life-threatening condition

The Link Between Thyroid and Your Weight

For some people, about 5 to 10 pounds of weight gain may be related to hypothyroidism. However, the thyroid is only one contributor to the regulation of body weight. It's possible for weight gain to not be thyroid-related.

Once hypothyroidism is treated with thyroid hormone, the basal metabolic rate and weight return to normal. If they do not, your care team can help address other causes of weight gain and help you with:

• Getting adequate sleep
• Reducing stress
• Making healthier food choices
• Incorporating more exercise in your routine

"It is important that thyroid health is optimized as part of a comprehensive approach to metabolic health," says Dr. Friedman.

When to See a Physician

If you are gaining weight, experiencing other symptoms of hypothyroidism or have concerns about your thyroid, speak with your primary care physician or endocrinologist (a physician who specializes in hormone issues) to see if a TSH blood test is right for you. Although weight gain may not always be explained by your thyroid, this test will help you eliminate it as the cause or start appropriate treatment to manage hypothyroidism.