What is hyperthyroidism?

The thyroid is a small gland in the front of the neck that produces two hormones: thyroxine (T4) and triiodothyronine (T3), referred to simply as “thyroid hormone.” These hormones influence the body’s metabolism in two ways: by spurring almost every tissue that the body needs to function and by increasing the amount of oxygen that cells use.ⁱ

The actions of the thyroid hormone on the body’s cells have many downstream effects, including regulating heart rate, changing how calories are burned and food digests, stimulating growth, maintaining healthy skin and hair, and protecting fertility.

Hyperthyroidism is a condition that occurs in the presence of an overactive thyroid gland, or one that has a higher-than-normal thyroid production.ⁱⁱ

How common is hyperthyroidism?

The number of people who experience symptoms from an overactive thyroid is low: only 0.2 to 1.3 percent of people in the world.ⁱⁱⁱ However, residents of certain countries are more likely than others to experience symptomatic hyperthyroidism. For example, the prevalence is around 3 percent in countries with iodine-deficient diets^iv because iodine is required to make thyroid hormone. A lack of iodine can lead to both hyper- and hypothyroidism.^v

An overactive thyroid is more common in females compared to males.^vi Patients with autoimmune diseases are also more likely to experience hyperthyroidism. Studies indicate that 10 percent of patients with Graves' disease, an autoimmune condition causing hyperthyroidism, also have another autoimmune disease.^vii

A person's chance of developing hyperthyroidism increases in some instances:^viii

• If they’re iodine deficient
• If they consume too much iodine
• If they have a family member with autoimmune thyroid disease
• If they smoke (it doubles the risk)
• If they take certain medications, such as amiodarone (heart medication) and lithium (psychiatric medication)

Hyperthyroidism vs. hypothyroidism

Hyperthyroidism and hypothyroidism both affect the thyroid gland and hormone production, but in different ways. With hyperthyroidism, the gland produces too much thyroid hormone; with hypothyroidism, the gland produces too little. If a person has signs of thyroid problems, a practitioner will check for an enlarged thyroid gland, and recommend they undergo testing to evaluate their hormone levels in addition to other assessments.

The first test used to measure thyroid hormone levels and function is called the thyroid-stimulating hormone (TSH) test, which measures the level of the hormone that stimulates the thyroid to produce thyroid hormone.^ix The normal value for TSH is between 0.4 and 4 mIU/L (though sometimes the upper limit of normal is 5 mIU/L in certain laboratories).^x

If the TSH is above the upper limit of normal, the patient may have an underactive thyroid or primary hypothyroidism. TSH that is below the lower limit of normal is a potential indicator of an overactive thyroid or hyperthyroidism.^xi

What are the symptoms of hyperthyroidism?

While testing is required for a definitive diagnosis of hyperthyroidism, people can also have symptoms that indicate a thyroid disorder. The symptoms of hyperthyroidism occur because organs of the body are exposed to too much thyroid hormone, or because TSH receptor antibodies cause inflammation. Here are some possible hyperthyroidism symptoms:^xii

• Psychiatric and nerve symptoms such as anxiety, lethargy, and a tremor in the hand or fingers
• Heart and blood vessels issues leading to heart palpitations, an abnormal heartbeat, and high blood
• Unintentional weight loss
• Sweating and a feeling of being overheated
• Pain or swelling in the thyroid gland (located in the neck)
• Diarrhea
• Thickened red skin (with Graves’ disease only)
• Bulging eyes (with Graves' disease only)
• Insomnia

Females may experience irregular menstrual cycles while males may suffer from erectile dysfunction, both of which could contribute to problems with conception.^xiii

Because many of the above symptoms can be caused by other conditions, it is not uncommon for patients with hyperthyroidism to have a delay in diagnosis.^xiv

What causes hyperthyroidism?

The most common cause of hyperthyroidism is Graves’ disease. Graves’ disease is an autoimmune disorder, or a condition where the patient’s immune system targets its own healthy cells or tissues.^xv In Graves’ disease, the immune system creates antibodies to the TSH receptors in the thyroid, which subsequently stimulate the thyroid gland to grow and produce an excess of thyroid hormone. These TSH receptor antibodies can also cause inflammation in the thyroid (known as thyroiditis) and in other parts of the body where TSH receptors are present, leading to bulging eyes (Graves’ orbitopathy) and raised darkened skin, most commonly over the shins (infiltrative dermopathy).

Medications such as amiodarone (a heart medication) can also lead to thyroiditis and cause thyroid gland pain and hyperthyroidism, as well as hypothyroidism.^xvi

Another common cause of hyperthyroidism is the growth of benign (non-cancerous) masses in the thyroid (called “nodules”). These thyroid nodules can produce excess thyroid hormone, even without normal stimulation by TSH.^xvii This uncontrolled production of thyroid hormone then leads to hyperthyroidism.

Masses outside the thyroid are another, much rarer, cause of hyperthyroidism. In central hyperthyroidism, a mass (typically benign) grows in the pituitary gland, which is the part of the brain that secretes TSH. This mass can secrete excess TSH, which then over-stimulates the thyroid gland to make thyroid hormone.^xviii It is also possible to have cancers of the ovary or other parts of the body that produce TSH. In these cases, the thyroid gland itself is normal, but there is an excess of the hormone that normally stimulates the thyroid gland, leading to hyperthyroidism.^xix

Finally, it is important to note that iodine is essential to the thyroid’s ability to create thyroid hormone, and a lack of sufficient iodine intake is associated with both hypothyroidism and hyperthyroidism. For this reason, in the U.S., Canada, and much of the rest of the world, salt is fortified with iodine to ensure there is enough iodine in people’s diets. In countries where iodine is deficient in the diet and is not supplemented, the rate of hyperthyroidism increases.^xx

How is hyperthyroidism diagnosed?

Diagnosing hyperthyroidism and determining its cause requires testing. A person with symptoms of hyperthyroidism can have their doctor order a simple blood test to measure the level of thyroid stimulating hormone (TSH) and determine if it is within the normal range.^xxi In patients with hyperthyroidism, the TSH value is typically low because the thyroid is producing extra thyroid hormone, which sends a signal to the brain to reduce the production of TSH. Normally, TSH would stimulate the thyroid to produce thyroid hormone, but when there is already too much thyroid hormone in the body, the brain makes less TSH in an attempt to decrease erroneous thyroid hormone production.^xxii

If the TSH value is below the normal level, the thyroid hormone (free T4 and T3) is then measured. If the thyroid hormone is elevated, a diagnosis of primary hyperthyroidism is confirmed.

After the initial diagnosis, a physician will conduct further testing to determine the cause of hyperthyroidism. First, they will measure specific antibodies that may be stimulating the thyroid gland to produce too much thyroid hormone (such as the antibodies produced in Graves’ disease).^xxiii

Sometimes, endocrinologists will order a special test called a thyroid scan or a radioactive iodine uptake test to check for nodules in the thyroid.^xxiv During the test, the patient is given a radioactive tracer that binds to iodine, which is then pulled into the thyroid gland to help produce thyroid hormone. If there are areas of the gland that are producing extra thyroid hormone, these parts will light up brighter as they take up more of the tracer, suggesting the presence of nodules. In some cases, such as if a patient is pregnant, a radioactive uptake test cannot be performed. An ultrasound can then be used instead to locate any thyroid masses.

Often, doctors will also order other lab work such as blood tests to measure white and red blood counts, or liver function. From this testing, a physician can identify abnormal lab values that are seen in patients with hyperthyroidism.^xxv When a physician finds nodules in the thyroid gland, a biopsy is sometimes performed to determine if the thyroid nodules are benign or cancerous.^xxvi

After determining the root cause of the condition, a physician can determine an appropriate course of action to control the overproduction of thyroid hormone.

How does hyperthyroidism impact fertility and pregnancy?

Although hypothyroidism is more common and thus more likely to cause infertility or miscarriages, overt hyperthyroidism may also impact fertility and pregnancy.^xxvii

Approximately 6 percent of females with hyperthyroidism experience fertility issues,^xxviii which is a similar percentage to patients without hyperthyroidism. This number is reflective of the widespread diagnosis and treatment of hyperthyroidism, as treatment can reverse female infertility.^xxix

Untreated hyperthyroidism can affect fertility because it can cause abnormal hormone levels, which interfere with ovulation and menstruation. Research shows that 60 percent of females with hyperthyroidism have infrequent or absent menstrual cycles and 5 percent have abnormally heavy menstrual bleeding.^xxx

The thyroid antibodies found in autoimmune hyperthyroidism (e.g., Graves’ disease) may also affect fertility directly, though it is unclear how this occurs.^xxxi

During pregnancy, uncontrolled hyperthyroidism can lead to problems for both the mother and fetus. In very severe cases, a pregnant woman can develop heart failure, or a condition called “thyroid storm,” where extremely high levels of thyroid hormone can lead to confusion, fast heart rate, and organ failure. Hyperthyroidism is also associated with preeclampsia, a condition characterized by high blood pressure in pregnancy.^xxxii

With regard to fetal outcomes, miscarriage, low birth weight, and preterm birth may be more common in patients with hyperthyroidism.^xxxiii Maternal thyroid hormone is essential to the normal development of the growing fetus, particularly for the growth and maturation of the fetal brain. If the mother’s thyroid gland produces too much — or too little — thyroid hormone, the fetus may fail to develop normally, leading to spontaneous miscarriages or birth defects.^xxxiv

Hyperthyroidism can also affect fertility in males, since an overactive thyroid can cause issues with both achieving and maintaining an erection. It can also cause premature ejaculation.^xxxv Fortunately, with proper treatment for high thyroid hormone levels, it is possible to successfully reverse these causes of infertility.^xxxvi

Can IVF drugs cause thyroid problems?

While there is some evidence that fertility treatments can lead to an increase in thyroid stimulating hormone (TSH) and asymptomatic (sub-clinical) hyperthyroidism, these effects are typically self-limiting, meaning they resolve after hormone treatment.^xxxvii Currently, it is unclear how treatment-stimulated hyperthyroidism affects the success of IVF, particularly when compared to people who do not experience the condition.^xxxviii

What hyperthyroidism treatments exist and how successful are they?

Appropriate hyperthyroidism treatment depends on the underlying cause of the condition and its severity, which are determined during a thorough medical history evaluation and extensive testing.

Sometimes, medications can be given to treat hyperthyroidism symptoms. For example, if the patient experiences an irregular heartbeat from the disease, a doctor may prescribe a beta-blocker, which is a medication that regulates heart rate.

Treatment may also address the underlying cause of hyperthyroidism. There are three main ways to treat hyperthyroidism:^xxxix

• Anti-thyroid medications
• Surgery, which involves removal of the whole thyroid gland or any overactive thyroid nodules
• Treatment with radioactive iodine

Each option is effective but has side effects that should be discussed with a physician to determine the treatment option most preferred by the patient.

Anti-thyroid medications

One of two medications can be used to suppress the creation of excess thyroid hormone: methimazole or propylthiouracil (PTU). Both of these medications interfere with the ability of the thyroid gland to release thyroid hormone and can be used indefinitely, or for a short period of time until radioactive iodine ablation or surgery to remove the thyroid can be performed. Approximately 50 percent of patients stop responding to long-term treatment with these medications and need alternate therapy in the form of surgery or radioactive ablation.^xl

Surgery

The surgical approach, which is a permanent cure, involves removing either the entire thyroid gland or the part of the gland that is producing excess hormone (in the case of an overactive nodule).^xli If the entire gland is removed, permanent hypothyroidism will result, which requires lifelong thyroid hormone supplementation.^xlii

Radioactive iodine

For Graves’ disease and toxic nodular goiter, radioactive iodine may be the preferred treatment.^xliii With this treatment, iodine is tagged with radiation and then injected into the bloodstream. The thyroid takes up the iodine, after which the radiation kills some of the thyroid cells thereby leading to the involution or “death” of much of the thyroid gland. Some patients do not require thyroid supplementation after treatment, though many patients do need to take lifelong thyroid supplements to replace the role of the thyroid gland that was rendered non-functional from treatment.^xliv

Treatment during pregnancy

Hyperthyroid treatment options are limited in pregnancy because radioactive iodine can lead to birth defects, while surgery comes with anesthesia risks (so is often deferred until after the pregnancy). That means oral medications are often the treatment of choice. The type of oral medication used depends on the trimester, due to the benefits and risks of different medications. In general, propylthiouracil (PTU) is used in the first trimester and either continued throughout the pregnancy or switched to methimazole in the second trimester.

Conclusion

Thyroid hormone at the correct levels is necessary to perform certain functions in the body. When levels are abnormal, a person may experience issues or complications that can lead to more serious consequences if left untreated. An overactive thyroid, for example, can cause infertility in both males and females. However, it can be treated with proper medical intervention.