What’s the Buzz

The Difference Between an Underactive vs. Overactive Thyroid

Thyroid research cartoon

Approximately 20 million Americans are living with thyroid disease. Many more are unaware they have a thyroid problem--up to 60% of people who have a thyroid disorder don’t know it. Women are 5-8 times more likely to have thyroid problems than men. Diagnosing problems with thyroid function is important because the thyroid gland affects every organ in the body. The two common types of thyroid disorders are hypothyroidism (underactive thyroid gland) and hyperthyroidism (overactive thyroid gland). Continue reading to learn more about hypothyroidism and hyperthyroidism and how their symptoms differ.

What is the thyroid gland? Why is it important?

The thyroid gland is a small gland located in the front of the neck. It is a part of the body’s endocrine system which secretes hormones. The pituitary gland in the brain controls the thyroid gland by secreting a hormone called the thyroid-stimulating hormone (TSH). TSH causes thyroid cells to make thyroid hormones. 

The thyroid gland produces two hormones, T4 and T3. These two thyroid hormones control many important processes, including the body’s metabolism, blood pressure, heart rate, and body temperature. 

When something goes wrong, the thyroid may not produce enough hormones, or produce too many hormones. This is called an underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism). The fluctuation in thyroid hormone levels can affect many organs and cause various symptoms.

Is underactive thyroid the same as hyperthyroidism?

No. An underactive thyroid is called hypothyroidism, or too few hormones. Hyperthyroidism refers to an overactive thyroid gland or too many hormones. The major difference between hypothyroidism and hyperthyroidism is the quantity of thyroid hormones being produced. 

In hypothyroidism or underactive thyroid, the thyroid gland does not produce enough thyroid hormone. In hyperthyroidism or overactive thyroid, the thyroid gland makes too much thyroid hormone. 

The two conditions have different signs and symptoms. However, sometimes they overlap. For instance, an enlarged thyroid gland (called goiter) can occur in both hypothyroidism and hyperthyroidism.

What causes thyroid disorders?

Causes of hypothyroidism thyroid disease

  • The most common cause is thyroiditis or inflammation of the thyroid gland, which can cause a decrease in thyroid hormone production.
  • Hashimoto’s thyroiditis is an inherited autoimmune disease. In this autoimmune disorder, the immune system mistakenly attacks and damages the thyroid gland, leading to hypothyroidism.
  • Some women develop hypothyroidism due to postpartum thyroiditis after childbirth, which is usually temporary.
  • Iodine is a mineral that is required to make thyroid hormones. An iodine deficiency can trigger hypothyroidism. 
  • In a small number of newborns, there is a missing thyroid gland, or the thyroid doesn’t work properly from birth. This is called congenital hypothyroidism. If left untreated, it can lead to serious physical and mental problems. All babies are screened for thyroid function in the hospital after birth.

Causes of hyperthyroidism thyroid disease

  • Graves’ disease, or diffuse toxic goiter (enlarged thyroid gland), is an autoimmune condition that can lead to an overproduction of thyroid hormone. 
  • Nodules in the thyroid gland can lead to increased thyroid hormone levels. Thyroid tissue with several thyroid nodules is called a toxic multinodular goiter. Most nodules are benign, but some can be thyroid cancer.
  • Thyroiditis can cause the thyroid gland to release stored hormones and cause hyperthyroidism. 
  • Too much iodine can cause the thyroid gland to make too many thyroid hormones.

How do I know if my thyroid is underactive or overactive?

A simple blood test for thyroid-stimulating hormone (TSH) can help to identify any problems with your thyroid levels. Doctors can also measure T3 and T4 thyroid hormone levels. Together, these tests are called thyroid function tests. Other tests like radioactive iodine can also be done if there are symptoms of hyperthyroidism.

TSH stimulates the production of thyroid hormones. Therefore, TSH is high when the body is not making enough thyroid hormones (in people with hypothyroidism or underactive thyroid). TSH is low when the body is making too much thyroid hormone (in people with hyperthyroidism or overactive thyroid).

How does an overactive thyroid (too much thyroid hormone) make you feel?

Some of the common hyperthyroidism symptoms (overactive thyroid symptoms) include:

  • Anxiety, nervousness, irritability
  • Hand tremors
  • Rapid heartbeat or irregular heartbeat
  • Weight loss
  • Frequent bowel movements
  • Trouble sleeping
  • Muscle weakness
  • Brittle skin
  • Thinning hair
  • Light or infrequent menstrual periods
  • Heat sensitivity

What are the symptoms of hypothyroidism (too little thyroid hormones)?

Common hypothyroidism symptoms (underactive thyroid gland symptoms) include:

  • Constipation
  • Cold sensitivity
  • Fatigue
  • Dry skin
  • Dry, coarse hair
  • Hoarse voice
  • Weight gain
  • Forgetfulness
  • Frequent or heavy menstrual periods

Which is worse, hypothyroidism or hyperthyroidism?

One condition is not worse or more dangerous than the other. There are treatment options for both thyroid conditions. Treating hyperthyroidism often involves treating the underlying cause using antithyroid medications, radioiodine treatment, or surgical removal with partial thyroidectomy. Hypothyroidism can be treated by replacing hormones with synthetic thyroid hormones to bring the levels into the normal range. 

Hypothyroidism is more common than hyperthyroidism. It also causes relatively fewer complications than hyperthyroidism. However, hypothyroidism can be more difficult to identify than hyperthyroidism, because hypothyroidism symptoms like weight gain and fatigue are more common and non-specific. 

Hyperthyroidism puts you at an increased risk for heart rhythm problems like atrial fibrillation. If left untreated, hyperthyroidism can lead to thyroid eye disease, osteoporosis, and bone fractures.

Can you have both hyperthyroidism and hypothyroidism? 

No, hypothyroidism and hyperthyroidism are two opposite things. You cannot have them at the same time because it is impossible to have too much hormone and too little hormone simultaneously. 

Can hyperthyroidism become hypothyroidism or vice versa? 

Yes, hyperthyroidism can become hypothyroidism or the other way around. This can occur if the medications used to treat an overactive thyroid are in excess, causing a significant drop in thyroid hormone levels. Similarly, if you take too much thyroid replacement medication for an underactive thyroid, it can result in hyperthyroidism. That’s why it is important to always get professional medical advice before adjusting your thyroid medications. 


  1. https://hartfordhealthcare.org/about-us/news-press/news-detail?articleid=32994&publicId=395
  2. https://www.thyroid.org/media-main/press-room/#
  3. https://my.clevelandclinic.org/health/diseases/8541-thyroid-disease