What Is Secondary Hypothyroidism?

Secondary (central) hypothyroidism occurs when the pituitary gland does not produce or release enough thyroid-stimulating hormone (TSH). TSH is required for normal stimulation and functioning of the thyroid.

The thyroid—a small, butterfly-shaped gland at the front of your neck—produces the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are released into the bloodstream and carried to the body’s organs and tissues. Thyroid hormones regulate your metabolism and affect nearly every process in the body, including body temperature, heart rate, appetite, blood sugar, cholesterol levels, and the central nervous system.

This article explores secondary hypothyroidism symptoms, causes, treatments, and more.

Secondary Hypothyroidism Symptoms

Secondary hypothyroidism can affect virtually all body functions and cause various symptoms ranging from mild to severe. 

Symptoms of secondary hypothyroidism include:

• Brittle nails 
• Cold intolerance
• Constipation 
• Depression 
• Fatigue
• Irregular menstrual cycles 
• Joint pain 
• Muscle cramps 
• Pale, dry skin 
• Weight gain 

Causes

Secondary hypothyroidism occurs when an otherwise normal, healthy thyroid gland does not get enough thyroid-stimulating hormone (TSH) from the pituitary gland to stimulate hormone production. Anatomical differences, or disorders of the pituitary gland and/or hypothalamus (a part of the brain that regulators numerous body functions), are associated with secondary hypothyroidism. 

Causes of secondary hypothyroidism include:

• Pituitary tumor 
• Hypothalamic lesions 
• Genetics/gene mutations
• Breech delivery (a baby being born feet or buttocks first), particularly if an infant is born with the condition
• External beam radiation therapy to the brain
• Sheehan’s syndrome (a type of hypopituitarism, sometimes caused by excessive bleeding during childbirth)

Diagnosis

If you have symptoms of hypothyroidism, talk to your healthcare provider. They will perform a physical examination, take your vital signs, and manually check the size of your thyroid gland.

If your thyroid is small or you have a slow heart rate, low temperature, or low blood pressure, your healthcare provider may order blood tests to determine whether you have primary or secondary hypothyroidism. 

Thyroid blood tests include:

• Free T4 index: Measures how much thyroxine (T4) you have in your blood
• Total T3 levels: Measures how much triiodothyronine (T3) you have in your blood 
• Serum TSH: Low TSH levels indicate pituitary gland dysfunction and are a key indicator of secondary hypothyroidism

If you are diagnosed with secondary hypothyroidism, your healthcare provider may order imaging tests, such as magnetic resonance imaging (MRI), to check for tumors on the pituitary gland. Genetic testing may be ordered to determine if gene mutations are the cause of secondary hypothyroidism. 

Treatment

Secondary hypothyroidism is treated by thyroid hormone replacement therapy. Synthroid, Levoxyl, Unithroid, and Levothroid (levothyroxine), synthetic versions of thyroxine (T4), are the most common hypothyroidism treatment. This oral medication is taken daily to restore and stabilize TSH and T4 levels to normal levels. If levothyroxine alone does not eliminate hypothyroidism symptoms, the medication Cytomel (liothyronine sodium), synthetic T3, may also be prescribed.

Once treatment begins, symptoms of secondary hypothyroidism may resolve within several weeks. Your healthcare provider will check your TSH levels six to 10 weeks after starting treatment to see how well the medication works and make dosage adjustments as needed. Once thyroid hormone levels stabilize, your healthcare provider will most likely check your levels once a year. 

There is no cure for hypothyroidism, so lifelong treatment may be necessary. It’s important to continue taking your prescribed medication, even when your symptoms subside. 

If a pituitary or hypothalamic disorder causes secondary hypothyroidism, treatments for the underlying cause may be needed before hormone replacement therapy begins. A pituitary tumor, for example, should be removed during a surgical procedure before hormone replacement therapy begins. 

Other Types of Hypothyroidism

There are three other types of hypothyroidism:

Primary hypothyroidism: Occurs when the thyroid gland fails to produce sufficient thyroid hormones. Approximately 99% of all people with hypothyroidism have this type.

Tertiary hypothyroidism: Occurs when the pituitary gland secretes deficient levels of TSH as a result of the hypothalamus producing insufficient levels of thyrotropin-releasing hormone (TRH). 

Subclinical hypothyroidism: Also known as mild thyroid failure, this type is associated with slightly elevated TSH levels and normal levels of other thyroid hormones (e.g., T3, T4). 

Summary 

Secondary hypothyroidism is a rare form of hypothyroidism caused by an underactive pituitary gland and/or hypothalamus. Symptoms include fatigue, dry and brittle hair, skin, and nails, cold intolerance, joint and muscle pain, and weight gain.

A pituitary tumor is the most common cause of secondary hypothyroidism. Other causes include a personal or family history of thyroid disease, previous radiation therapy to the head or neck, and hypothalamus dysfunction.

Thyroid hormone replacement therapy is the standard line of treatment for secondary hypothyroidism. Levothyroxine is an oral medication commonly prescribed to help restore thyroid hormone levels to the normal range and reduce or eliminate symptoms.

A Word From Verywell

Secondary hypothyroidism can make you feel fatigued and generally unwell. The good news is that the condition is highly treatable. Thyroid hormone replacement therapy can help restore and stabilize thyroid hormone levels to help you feel your best.