Cushing disease and hyperthyroidism are two different endocrine disorders that affect the production and regulation of hormones in the body. However, they can have some overlapping symptoms and complications, as well as some interactions between them. In this article, we will explore the causes, signs, diagnosis, and treatment of both conditions, and how they may be related.
What is Cushing Disease?
Cushing disease is a rare condition that occurs when the pituitary gland, a small gland at the base of the brain, produces too much adrenocorticotropic hormone (ACTH). ACTH stimulates the adrenal glands, which are located above the kidneys, to produce cortisol, a hormone that helps the body respond to stress and inflammation. When there is too much cortisol in the blood, it can cause various symptoms and health problems, such as:
- Weight gain, especially in the face, neck, and abdomen
- Thin and fragile skin that bruises easily
- Purple or pink stretch marks on the skin
- Muscle weakness and bone loss
- High blood pressure and high blood sugar
- Mood changes, such as depression, anxiety, and irritability
- Irregular or absent menstrual periods in women
- Reduced sex drive and fertility in men
Cushing disease is different from Cushing syndrome, which is a broader term that refers to any condition that causes excess cortisol in the body. Cushing syndrome can be caused by other factors besides pituitary tumors, such as adrenal tumors, ectopic ACTH production by other tumors, or long-term use of corticosteroid medications.
What is Hyperthyroidism?
Hyperthyroidism is a condition that occurs when the thyroid gland, a butterfly-shaped gland in the front of the neck, produces too much thyroid hormone. Thyroid hormone regulates the metabolism and energy of the body. When there is too much thyroid hormone in the blood, it can speed up the body’s processes and cause various symptoms and complications, such as:
- Weight loss without trying
- Fast or irregular heartbeat
- Tremors or shaking of the hands
- Heat intolerance and sweating
- Nervousness, anxiety, and irritability
- Difficulty sleeping and concentrating
- Enlarged thyroid gland or goiter
- Eye problems, such as bulging eyes or vision loss
Hyperthyroidism can be caused by several medical conditions that affect the thyroid gland. The most common cause is Graves’ disease, an autoimmune disorder that causes the immune system to attack the thyroid gland and stimulate it to produce more hormone. Other causes include thyroid nodules or tumors, thyroid inflammation or infection, excessive iodine intake, or too much synthetic thyroid hormone.
How are Cushing Disease and Hyperthyroidism Related?
Cushing disease and hyperthyroidism are not directly related, but they can have some indirect effects on each other. For example:
- Hypercortisolism can impair the function of the hypothalamus-pituitary-thyroid axis, which regulates the production and feedback of thyroid hormone. This can result in central hypothyroidism, a condition where there is low thyroid hormone due to insufficient stimulation from the pituitary gland. Central hypothyroidism can cause similar symptoms as primary hypothyroidism (low thyroid hormone due to thyroid gland dysfunction), such as fatigue, weight gain, cold intolerance, dry skin, hair loss, and depression. According to Healio, central hypothyroidism is prevalent in about 50% of adults with Cushing disease.
- Hypercortisolism can also suppress the immune system and mask the development of autoimmune thyroid diseases, such as Graves’ disease or Hashimoto’s thyroiditis. These diseases can cause fluctuations in thyroid hormone levels and lead to hyperthyroidism or hypothyroidism. According to MDPI, autoimmune thyroid diseases increase in prevalence after the resolution of hypercortisolism.
- Hyperthyroidism can worsen some of the symptoms and complications of hypercortisolism, such as high blood pressure, heart problems, osteoporosis, mood disorders, and infections. Hyperthyroidism can also interfere with some of the medications used to treat hypercortisolism, such as ketoconazole or metyrapone.
- Some of the treatments for hyperthyroidism can affect cortisol levels or adrenal function. For example, radioactive iodine therapy can destroy some of the adrenal tissue and reduce cortisol production. According to Oxford Academic, this can lead to adrenal insufficiency or crisis in patients with Cushing disease who undergo this treatment.
How are Cushing Disease and Hyperthyroidism Diagnosed?
The diagnosis of Cushing disease and hyperthyroidism requires a careful evaluation of the medical history, physical examination, blood tests, and imaging tests. Some of the tests that may be used include:
- For Cushing disease: ACTH test, cortisol test, dexamethasone suppression test, 24-hour urinary free cortisol test, late-night salivary cortisol test, CRH stimulation test, pituitary MRI, and inferior petrosal sinus sampling.
- For hyperthyroidism: TSH test, free T4 test, free T3 test, thyroid antibodies test, radioactive iodine uptake test, and thyroid scan.
How are Cushing Disease and Hyperthyroidism Treated?
The treatment of Cushing disease and hyperthyroidism depends on the cause, severity, and individual factors of each patient. Some of the treatment options include:
- For Cushing disease: Surgery to remove the pituitary tumor or the adrenal glands, radiation therapy to shrink the tumor, medication to block cortisol production or action, or bilateral adrenalectomy followed by hormone replacement therapy.
- For hyperthyroidism: Medication to reduce thyroid hormone production or action, radioactive iodine therapy to destroy thyroid cells, or surgery to remove part or all of the thyroid gland.
Cushing disease and hyperthyroidism are two distinct endocrine disorders that can have some common symptoms and complications. They can also affect each other in various ways, such as causing central hypothyroidism, masking autoimmune thyroid diseases, worsening cardiovascular and bone health, and interacting with medications and treatments. Therefore, it is important to check the thyroid function in patients with Cushing disease, both during the active disease and after its remission. Likewise, it is important to monitor the cortisol levels and adrenal function in patients with hyperthyroidism who undergo treatments that may affect them. By doing so, the diagnosis and management of both conditions can be improved and the quality of life of the patients can be enhanced.