What Happens In Secondary Hypothyroidism

[Kody Chavva]

Thethyroid gland is an important organ of the endocrine system, located in the front of the neck just below the voicebox. The thyroid secretes the hormones thyroxine (T4), triiodothyronine (T3), and calcitonin, which control body metabolism and regulate calcium balance. The secretion of T3 and T4 by the thyroid is controlled by an endocrine feedback system involving the pituitary gland and the hypothalamus (structures in the brain). Lowered levels of these thyroid hormones result in increased levels of pituitary and hypothalamic hormones. The reverse is also true -- when levels of the thyroid hormones rise, pituitary and hypothalamic hormones fall back. This helps keep levels appropriately balanced.

Since the thyroid gland is regulated by the pituitary gland and the hypothalamus, thyroid disorders may result not only from defects in the thyroid itself but also from the disruption of the control system in these other organs. Thyroid disorders caused by overproduction of thyroid hormones are called hyperthyroidism, and underproduction of these hormones is known as hypothyroidism.

The cause of secondary hypothyroidism is failure of the pituitary gland to secrete thyroid stimulating hormone (TSH). This is usually caused by a tumor in the region of the pituitary. Rarely the cause is an infiltration of the pituitary by inflammatory cells from the immune system or foreign substances (such as iron in hemochromotosis). Hypothyroidism may cause a variety of symptoms and can affect all body functions. The body's normal rate of functioning slows, causing mental and physical sluggishness. Symptoms vary from mild to severe. The most severe form is called myxedema, which is a medical emergency and can lead to coma and death.

A physical exam usually reveals a small thyroid gland. Vital signs (temperature, pulse, rate of breathing, blood pressure) reveal a slow heart rate, low blood pressure, and low temperature. A chest x-ray may reveal an enlarged heart. Laboratory tests to determine thyroid function include:

The purpose of treatment is to replace the deficient thyroid hormone. Levothyroxine is the most commonly used medication. The lowest dose effective in normalizing thyroid function is used. Life-long therapy may be necessary. Medication must be continued even when symptoms subside. After replacement therapy has begun, report any symptoms of increased thyroid activity (hyperthyroidism), such as restlessness, rapid weight loss, and sweating.

A high-fiber, low-calorie diet and moderate activity will help relieve constipation and promote weight loss if weight was gained during the time when thyroid activity was low. In individuals with accompanying hypoadrenalism, steroid replacement must be instituted before thyroid replacement is begun. In patients who have hypothyroidism caused by a pituitary tumor, surgery may be required. However, surgery may not cure the hypothyroidism, and thyroid replacement will still be needed.

Myxedema coma is treated by intravenous (IV) thyroid replacement and steroid therapy. Supportive therapy of oxygen, assisted ventilation, fluid replacement, and intensive care nursing may be indicated.

Calling your health care provider:

Call your health care provider if signs of hypothyroidism are present, or if chest pain or rapid heartbeat occur. Call your provider if restlessness, rapid weight loss, sweating, or other symptoms occur after beginning treatment for this disorder. Call your provider if headache, visual loss, or breast discharge occur.

If left untreated, hypothyroidism may lead to myxedema coma, a rare complication associated with progressive cognitive deterioration, skin swelling, hypothermia, an enlarged tongue, coarse hair, and organ abnormalities. Myxedema can be life-threatening and requires immediate medical attention and treatment.

Primary hypothyroidism is caused by a dysfunctional thyroid and is associated with high levels of thyroid-stimulating hormone (TSH). Secondary hypothyroidism is caused by a diseased or dysfunctional pituitary gland or hypothalamus and is associated with low TSH levels.

There is no cure for secondary hypothyroidism, but it is a highly treatable condition. Levothyroxine is the most commonly prescribed medication for hypothyroidism. Taken orally once a day, levothyroxine helps restore and stabilize thyroid hormone levels to reduce or eliminate symptoms of hypothyroidism. Lifelong treatment may be required for secondary hypothyroidism.

Dr. Kelly Wood is an ABMS board certified endocrinologist and internal medicine physician who treats adults with diabetes, thyroid disease, osteoporosis, and other hormonal conditions. She achieved her fellowship in endocrinology from University of Wisconsin Hospital and Clinics.

Heather Hobbs (she/her) is an editor at Healthline Media and RVO Health. She has also worked in content creation, marketing, copy editing, and search engine optimization (SEO) for brands, agencies, and nonprofit organizations. A former educator, she earned her MA in teaching as well as a BA with a double major in English writing and Spanish, along with a minor in French. You can find her on LinkedIn.

A problem with any of these three glands can lead to hypothyroidism. The vast majority of hypothyroidism diagnoses are caused by problems with the thyroid gland and are categorized as primary hypothyroidism.

To diagnose pituitary hypothyroidism, a doctor will likely start by asking about your symptoms and reviewing your personal and family medical history. They may perform a physical exam where they feel your thyroid gland for abnormal changes.

For example, some people may also have secondary adrenal insufficiency that may require glucocorticoid treatment. Secondary adrenal insufficiency is when your pituitary gland does not produce enough adrenocorticotropic hormone (ACTH), which leads to low cortisol levels.

Secondary hypothyroidism is a rare form of hypothyroidism caused by your pituitary gland underproducing TSH. It makes up less than 1% of cases of hypothyroidism and can cause symptoms like fatigue, poor cold tolerance, and weight gain.

Children that show the symptoms described above should be examined by a special doctor called a pediatric endocrinologist to be tested for secondary hypothyroidism. Regular examinations, testing, and early treatment will prevent the damage caused by this condition.

Children who have hypothyroidism due to a pituitary or hypothalamus gland problem should take thyroid hormone replacement. Sometimes children with secondary hypothyroidism have problems with other endocrine glands, such as the adrenal gland. One of the effects of taking hormone replacement is an increased metabolic rate. This can trigger a severe or even life-threatening condition called addisonian (adrenal) crisis. Because of this possible condition, your child should be examined frequently by a special doctor called a pediatric endocrinologist. Before beginning hormone replacement therapy, the pediatric endocrinologist should perform a test that stimulates the release of adrenal hormones. If not enough adrenal hormone is present, in most cases, before hormone replacement therapy begins, children are treated with cortisone acetate, a stress hormone similar to the adrenal hormone.

Several causes of hypothyroidism are ranked based on where the problem originates in the body. Because thyroid hormone levels are controlled by endocrine glands in the brain, it may be compromised by other malfunctioning organs that tell it what to do.

The first and most common cause of hypothyroidism outside the U.S. is a lack of dietary iodine. The thyroid needs iodine to make thyroid hormones. When iodine is lacking and thyroid hormone cannot be produced, feedback systems from the brain tell the thyroid it needs to produce even more hormone. This can cause a proliferation of thyroid cells and the development of goiter, an enlarged thyroid. In the U.S., the advent of iodized salt in the 1920s significantly reduced levels of iodine deficiency. Still, globally, a lack of iodine currently remains a significant risk for underactive thyroid and goiter, as well as severe disabilities in babies born to women with severe hypothyroidism and iodine deficiency during pregnancy.

Sometimes, secondary hypothyroidism also encompasses problems with the hypothalamus, the endocrine gland that controls the pituitary. However, hypothalamic problems that lead to hypothyroidism are sometimes referred to as tertiary hypothyroidism.

After a diagnosis of hypothyroidism, the primary course of treatment is a medication known as thyroid hormone replacement therapy. Your doctor will put you on a dose that will resolve the hypothyroidism and keep your thyroid levels optimal. If other organs, such as your adrenals, are affected, you may need additional treatment to keep your endocrine glands functioning appropriately.

In addition to treatment for hypothyroidism, supportive lifestyle measures can make living with secondary hypothyroidism easier. For example, a high-fiber diet can help relieve constipation, while exercise can help keep your weight in check.

Hypothyroidism results from low levels of thyroid hormone with varied etiology and manifestations. Hypothyroidism is primarily categorized as primary and secondary (ie, central) hypothyroidism. In primary hypothyroidism, the thyroid gland cannot produce adequate amounts of thyroid hormone. The less commonly seen secondary or central hypothyroidism occurs when the thyroid gland functions normally; however, hypothyroidism results from the abnormal pituitary gland or hypothalamus function. Autoimmune thyroiditis and iodine deficiency are the most common causes of the disease. Central hypothyroidism is rare.

The drug of choice for the treatment of hypothyroidism of any etiology is thyroid hormone replacement. Untreated hypothyroidism increases morbidity and mortality. This activity for healthcare professionals aims to enhance learners' competence in selecting appropriate diagnostic tests, managing hypothyroidism, and fostering effective interprofessional teamwork to improve outcomes.

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