Hashimoto thyroiditis is an autoimmune disorder that is the most common cause of hypothyroidism in the United States.

Hypothyroidism causes low levels of thyroid hormones, and several underlying issues can cause it.

The thyroid gland is a butterfly-shaped organ in the neck that produces important hormones. These support functions across the body, including how the heart works. Any condition or treatment that damages tissue in the thyroid gland can affect its hormone output, leading to hypothyroidism.

One such disorder is Hashimoto thyroiditis, a condition in which the immune system attacks healthy tissue in the thyroid and causes inflammation.

This article explains the connections and differences between Hashimoto thyroiditis and hypothyroidism.

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Hypothyroidism refers to low blood levels of thyroid hormones, particularly thyroxine (T4). These hormones are vital for controlling heart rate and other cardiovascular functions, the movement of food through the gut, and the reproductive system.

Other glands, such as the pituitary and hypothalamus, release hormones that trigger the production of T4. Damage to the thyroid, pituitary, or hypothalamus can lead to hypothyroidism.

Hypothyroidism due to thyroid damage — primary hypothyroidism — accounts for over 99% of all hypothyroidism diagnoses.

Around 5% of people in the United States have hypothyroidism. Several health problems can lead to low thyroid levels, including:

  • Hashimoto thyroiditis
  • congenital hypothyroidism, in which an infant has a partially formed or ineffective thyroid
  • thyroiditis, or thyroid inflammation leading to thyroid hormone leakage
  • removal of thyroid tissue during surgery
  • the side effects of radiation therapy
  • the adverse effects of medications, including some heart medications, cancer treatments, and medications for bipolar disorder

Symptoms

The symptoms of hypothyroidism generally develop slowly. They may also vary among individuals, be hard to detect, or be easy to confuse with other health conditions.

Common hypothyroidism symptoms include:

Hashimoto thyroiditis, also known as Hashimoto disease, develops when the immune system mistakenly produces excess white blood cells and antibodies to attack an otherwise healthy thyroid gland. It is one of the most common autoimmune diseases.

White blood cells build up in the thyroid, damaging the hormone-producing tissue. The damage means that the thyroid becomes less able to produce thyroid hormones. Researchers are still investigating the exact cause of Hashimoto thyroiditis. However, genetics and viruses, including hepatitis C, may factor into the development.

Hashimoto thyroiditis is the most common cause of hypothyroidism in the United States.

Symptoms

Hashimoto thyroiditis may cause any hypothyroidism symptoms.

However, it can also cause hyperthyroidism for some people, in which the thyroid produces too much thyroxine.

The thyroid may swell, causing a noticeable bulge in the front of the neck, known as a goiter.

The main difference is that Hashimoto thyroiditis is an autoimmune condition. Hypothyroidism is a hormonal condition that may occur for several reasons, including physical trauma to the thyroid, surgery, or treatment side effects.

Hashimoto thyroiditis also leads to the creation of specific antibodies, such as antithyroid peroxidase (anti-TPO), in around 90% of people with the condition. These are not present in the blood of people with a cause that does not relate to Hashimoto thyroiditis.

How are they connected?

Hashimoto thyroiditis is a common cause of hypothyroidism linked to the immune system.

However, hypothyroidism can occur due to other reasons, and Hashimoto thyroiditis may also cause hyperthyroidism (overactive thyroid).

Hashimoto thyroiditis and hypothyroidism have a close link, but they are not the same condition.

Conditions with symptoms that resemble those of Hashimoto thyroiditis include:

However, the common symptoms of hypothyroidism as the result of Hashimoto thyroiditis, such as fatigue, weight gain, and joint pain, can also resemble the effects of other conditions. They are often subtle and slow to develop. As a result, a healthcare professional may misdiagnose them, especially early on.

Learn more about diagnosing hypothyroidism.

The course of treatment for Hashimoto thyroiditis depends on whether the disorder causes hypothyroidism. However, the treatment for hypothyroidism is generally standard.

Hypothyroidism

A healthcare professional will typically prescribe levothyroxine for people with low T4. This artificial but identical version of T4 is available as a pill, liquid, or gel capsule.

Those with hypothyroidism will need to take levothyroxine for the rest of their lives, as it supplements the lost T4 in the body.

Hashimoto thyroiditis

Hashimoto thyroiditis may produce the related antibodies without damaging the thyroid enough to bring down T4 levels.

People with Hashimoto thyroiditis that does not lead to hypothyroidism may only need to undergo regular testing to check thyroid levels and monitor any ongoing symptoms without active treatment.

The following are answers to some questions people frequently ask about Hashimoto thyroiditis and hypothyroidism.

Does Hashimoto thyroiditis turn into hypothyroidism?

Hashimoto thyroiditis is the most common cause of hypothyroidism. However, hypothyroidism does not always occur due to Hashimoto thyroiditis. According to a 2023 review, people with Hashimoto thyroiditis who test positive for anti-TPO antibodies have a higher risk of developing hypothyroidism.

Can you have Hashimoto thyroiditis without hypothyroidism?

Healthcare professionals can detect Hashimoto thyroiditis through anti-TPO antibodies in the blood, even though it may not have damaged the thyroid glands. In these circumstances, healthcare professionals will continue to monitor the disorder through blood testing and consultations without actively providing treatment.

Hashimoto thyroiditis and hypothyroidism have a close connection, but they are different health problems. Hashimoto thyroiditis is the most common cause of hypothyroidism and produces specific antibodies in the blood, but other conditions and physical injuries can also reduce thyroid hormone levels.

Hashimoto thyroiditis also rarely causes hyperthyroidism, and it may not cause hypothyroidism if thyroid damage does not occur. If hypothyroidism does develop, an individual will need to take levothyroxine for the rest of their lives to restore thyroid hormone levels.