The two words are often used as if they mean the same thing. They do not, and the difference changes what a result means and what, if anything, needs to be done about it.
The short version: hypothyroidism describes what the thyroid is doing. Hashimoto's describes why.
Hypothyroidism is a state
Hypothyroidism, also called an underactive thyroid, means the thyroid gland is not making enough thyroid hormone. Nearly 5 in 100 Americans aged 12 and older have it, though most cases are mild or cause few obvious symptoms. Women are much more likely than men to develop it, and it is more common after age 60.
When symptoms do appear, they tend to develop slowly. They can include fatigue, weight gain, trouble tolerating cold, joint and muscle pain, dry skin or thinning hair, heavy or irregular periods, a slowed heart rate, and depression.
Hypothyroidism has several possible causes. Hashimoto's is the most common one in the United States, but others include inflammation of the thyroid, surgery or radiation treatment involving the thyroid, and some medicines.
Hashimoto's is a cause
Hashimoto's disease is an autoimmune condition. The immune system produces antibodies that target the thyroid, and over time that can reduce the gland's ability to make hormone. It affects women about 4 to 10 times more often than men, and it most often develops between the ages of 30 and 50. A family history of thyroid disease, or another autoimmune condition such as celiac disease, type 1 diabetes, or rheumatoid arthritis, raises the likelihood.
Here is the part that surprises many people. Having Hashimoto's does not always mean having hypothyroidism. Many people have no symptoms at first, and thyroid hormone levels can stay in range for years. Some will develop hypothyroidism over time, and some will not.
Why the difference matters
The two can appear in any combination:
Hashimoto’s antibodies present, thyroid hormone levels in range: The autoimmune process is present, but the thyroid is still keeping up. A clinician may simply recheck symptoms and hormone levels at regular intervals.
Hashimoto’s antibodies present, hypothyroidism present: The most common cause of an underactive thyroid in the United States. Treatment addresses the low hormone.
Hypothyroidism present, no antibodies: The underactive thyroid may have another cause, which is worth identifying.
How each is identified
Both are identified mainly through blood work, alongside a history and examination. Thyroid-stimulating hormone, or TSH, and free T4 show how the thyroid is functioning. Thyroid antibodies, most often thyroid peroxidase antibodies, help identify Hashimoto's as the cause. If Hashimoto's is suspected but antibodies are absent, an ultrasound of the thyroid may be used.
A diagnosis of hypothyroidism cannot be based on symptoms alone, because so many of its symptoms overlap with other conditions, including the menopause transition.
How treatment works
Treatment is aimed at the hormone level, not the antibodies. When hypothyroidism is present, the standard treatment is levothyroxine, a medicine identical to a hormone a healthy thyroid makes. It is usually taken on an empty stomach in the morning, 30 to 60 minutes before eating, and a blood test is repeated about 6 to 8 weeks after starting or after any dose change.
When Hashimoto's is present without hypothyroidism, treatment may not be needed. Monitoring is a reasonable plan, not a missed opportunity.
One practical caution
People with Hashimoto's may be sensitive to iodine. Federal guidance advises talking with your clinician before taking iodine supplements, kelp or seaweed supplements, or iodine-containing medicines. Some supplements contain iodine, so read the label before you start one.
What to ask
If you have been told you have one of these, three questions clarify where you stand. Was the diagnosis based on hormone levels, antibodies, or both? Is my thyroid currently making enough hormone? And if no treatment is needed now, when should it be rechecked?
The Energē Editorial answers one question like this each month, with sources printed in full. Join the Editorial.
If you are located in Virginia and would like your own picture evaluated, request an Energē Introduction.
This article is general education. It is not medical advice and does not replace care from your own clinician.
Sources
National Institute of Diabetes and Digestive and Kidney Diseases. Hypothyroidism (Underactive Thyroid). Last reviewed March 2021.
National Institute of Diabetes and Digestive and Kidney Diseases. Hashimoto's Disease. Last reviewed June 2021.
American Thyroid Association. Thyroid Function Tests. 2019.
