Hashimoto’s disease: It’s not just “low thyroid”

Published April 7, 2026

Hashimoto’s disease is a chronic autoimmune disease affecting millions of Americans, yet many live with it for years without knowing.¹ Understanding what’s happening, how it develops, and what to look for may help bring clarity to symptoms that are often overlooked. 

What is Hashimoto's disease?

Hashimoto’s disease (also called Hashimoto’s thyroiditis) is an autoimmune disease where the immune system mistakenly targets and attacks the thyroid gland. Over time, this causes chronic inflammation and scarring, which can eventually lead to a loss of thyroid function and hypothyroidism.² 

Are Hashimoto’s disease and hypothyroidism the same thing?

They are linked, but they are not the same. Hypothyroidism, or underactive thyroid, is a condition where the thyroid doesn’t make or release enough thyroid hormone. Hashimoto’s disease is the most common cause, but not the only one. Hypothyroidism can also develop after thyroid surgery, radiation therapy, iodine deficiency, or from certain medications.

What makes Hashimoto’s disease different is that it’s autoimmune. The immune system makes specific antibodies that target the thyroid and cause damage over time, which can lead to hypothyroidism.³

Are Hashimoto’s disease and hypothyroidism the same thing?

They are linked, but they are not the same.

Hypothyroidism, or underactive thyroid, is a condition where the thyroid doesn’t make or release enough thyroid hormone. Hashimoto’s disease is the most common cause, but not the only one. Hypothyroidism can also develop after thyroid surgery, radiation therapy, iodine deficiency, or from certain medications.

What makes Hashimoto’s disease different is that it’s autoimmune. The immune system makes specific antibodies that target the thyroid and cause damage over time, which can lead to hypothyroidism.³

Having hypothyroidism doesn’t automatically mean you have Hashimoto’s disease, and not everyone with Hashimoto’s develops hypothyroidism. However, it remains the most common thyroid disorder overall and the leading cause of hypothyroidism in the US. It occurs 4 to 10 times more often in women and can develop at any age, but it’s most often diagnosed between ages 30 and 50.²

What does the thyroid do? How does Hashimoto’s disease affect thyroid function?

The thyroid gland makes and releases thyroid hormones, mostly T4 (thyroxine), along with a smaller amount of T3 (triiodothyronine). It does this in response to thyroid-stimulating hormone (TSH), a signal from the brain that tells the thyroid to get to work. Your energy, body temperature, mood, heart rate, digestion, and many other bodily functions depend on this process running smoothly. 

To make T3 and T4, the thyroid relies on 2 key components: thyroglobulin (Tg) and thyroid peroxidase (TPO). Tg acts as the physical foundation for building the hormones, while TPO helps attach iodine to Tg, allowing T3 and T4 to form. Without both working together, the thyroid cannot properly make thyroid hormones.

In Hashimoto’s disease, the immune system mistakenly identifies Tg and TPO as threats and creates antibodies against them, turning the very components the thyroid needs to function into targets for the immune system to eliminate.³

When Tg and TPO antibodies signal the immune system to attack, they trigger a large buildup of white blood cells (lymphocytes) in the thyroid, causing ongoing inflammation and tissue damage. Over time, this can take a toll on thyroid function and lead to hypothyroidism, although not everyone with Hashimoto’s disease will develop it.³,⁴

Learn more → The importance of thyroid testing

What causes Hashimoto’s disease? 

Researchers don’t fully understand what causes Hashimoto’s disease, but it’s thought to involve a combination of genetic and environmental factors.²,⁴

Who it affects more often

• Women 
• People with a family history of Hashimoto’s disease or other thyroid conditions
• People who have another autoimmune disease

Other potential factors

• Hormonal changes
• Certain medications
• Certain nutritional imbalances
• Exposure to toxins or radiation
• Viral or bacterial infections

Autoimmune diseases often overlap, and several are associated with Hashimoto’s disease, including celiac disease, lupus, rheumatoid arthritis, Sjögren syndrome, and type 1 diabetes.²

What are the symptoms of Hashimoto’s disease? 

Symptoms may vary, develop slowly over time, and resemble other health conditions, making them easy to overlook. Many people have no noticeable symptoms at first. As thyroid hormone levels begin to drop, symptoms of hypothyroidism may appear, including¹,⁵

• Fatigue that sleep doesn’t fix
• Brain fog or difficulty concentrating
• Depression or low mood
• Weight gain or trouble losing weight
• Feeling cold
• Dry skin or hair loss
• Puffiness in the face and around the eyes
• Muscle aches or joint pain
• Constipation
• Heavy or irregular periods
• Enlarged thyroid gland (goiter)
• Elevated cholesterol levels

If you are experiencing any of these symptoms, explore testing options at questhealth.com or speak to a healthcare provider to discuss appropriate next steps.