Tsh levels in patients with secondary hypothyroidism are

Pituitary hypothyroidism is a rare condition characterized by low levels of thyroid hormone due to failure of the pituitary gland. The pituitary gland is a pea-sized structure in the brain that releases thyroid-stimulating hormone (TSH). This hormone circulates in the blood and tells the thyroid to release its own hormones.

Low thyroid levels can be due to failure of the thyroid gland itself (primary hypothyroidism) or failure of the pituitary gland (secondary hypothyroidism). Pituitary failure can manifest with several potential symptoms.

Read on to learn more about the symptoms, diagnosis, and treatment of secondary hypothyroidism.

Secondary Hypothyroidism Symptoms

Thyroid hormones are involved in many metabolic processes throughout the body. Symptoms of hypothyroidism generally include the following:

• Thin, brittle nails
• Hair thinning or hair loss
• Fatigue
• Weight gain
• Low energy levels
• Constipation
• Feeling cold all the time
  
• Dry skin
• Menstrual irregularities

The symptoms listed above are common to primary and secondary hypothyroidism. Some additional symptoms are due to specific causes of pituitary failure (such as brain tumor) or due to low levels of other hormones. These symptoms can include:

• Headache
• Vision changes
• Low blood pressure
• Lack of a menstrual period (amenorrhea)
• Increased thirst
• Frequent urination
• Muscle pain

Undiagnosed Hypothyroidism

At its most severe, undiagnosed hypothyroidism can manifest as myxedema. This life-threatening condition is characterized by swelling, slowed heart rate, low blood pressure, confusion, low body temperature, and fluid buildup around the heart. It requires hospitalization for urgent treatment.

What Causes Secondary Hypothyroidism?

The body functions best when hormone levels are maintained in the correct range. To accomplish this, the brain and endocrine organs work together through complex feedback loops.

The process starts in the part of the brain called the hypothalamus. The hypothalamus releases a hormone called thyrotropin-releasing hormone (TRH). This hormone signals the pituitary gland to release thyrotropin, also known as thyroid-stimulating hormone (TSH).

TSH then signals the thyroid gland to release its own hormones, triiodothyronine and thyroxine (called T3 and T4 for short). T3 and T4 circulate in the blood and affect many biological processes, resulting in symptoms when levels are either too high or too low. The T3 and T4 then interact with the hypothalamus, telling it to secrete more or less thyrotropin-releasing hormone.

When any parts of the feedback loop are not functioning appropriately, thyroid levels can become either too high or too low, resulting in various symptoms. For example, pituitary failure results in inadequate TSH release, and subsequently low thyroid hormone (T3 and T4) levels.

Pituitary failure may be caused by:

• Tumor of the pituitary gland
• Injury to the pituitary gland
• Bleeding in the pituitary gland
• Head trauma
• Injury due to excessive blood loss during childbirth (Sheehan's syndrome)
• Infiltration of the gland, such as in melanoma, sarcoidosis, lymphoma, or hemochromatosis
• Infection
• Certain medications
• Genetic conditions
• As a complication of brain surgery
• Radiation treatment
• Hypophysitis (a condition in which the pituitary gland becomes inflamed)

Hormones Involved in Thyroid Regulation

The hormones involved in thyroid regulation include:

• Thyrotropin-releasing hormone: TRH is released by the hypothalamus to stimulate the pituitary gland.
  
• Thyroid-stimulating hormone: TSH is released by the pituitary gland in response to thyrotropin-releasing hormone. TSH stimulates the thyroid gland itself to release its own hormones).
  
• T3 and T4: These hormones, which are released by the thyroid gland, are involved in many metabolic processes in the body.

How Is Secondary Hypothyroidism Diagnosed?

Hypothyroidism is generally diagnosed with blood tests. Measuring low thyroid levels and circulating hormones can help determine where the problem is.

A low T3 and T4 and inappropriately low or normal TSH localizes the problem to the brain, either the pituitary gland or hypothalamus. Brain imaging is performed using magnetic resonance imaging (MRI) to evaluate for underlying causes and to rule out other possibilities.

When secondary hypothyroidism is diagnosed, evaluating other hormone systems, like cortisol, growth hormone, and sex hormones, is necessary,

Classification of Hypothyroidism

Hypothyroidism can be classified according to which part of the feedback loop is affected, such as:

• Primary hypothyroidism occurs when the thyroid gland does not release enough thyroid hormones.
  
• Secondary hypothyroidism occurs when the pituitary gland does not release enough TSH.
  
• Tertiary hypothyroidism occurs when the hypothalamus does not release enough thyrotropin-releasing hormone.

Is Secondary Hypothyroidism Treatable?

Fortunately, hypothyroidism is treatable with medication.

Thyroid hormones can be replaced with a synthetic thyroid hormone called levothyroxine. Alternatively, desiccated thyroid hormone derived from thyroid glands of animals (known as Armour thyroid) can be administered. In secondary hypothyroidism, hormone levels are monitored to ensure appropriate dosing of thyroid hormone.

The underlying cause of secondary hypothyroidism also needs to be addressed. Pituitary tumors, one of the more common causes of secondary hypothyroidism, may require surgery.

Outlook for Secondary Hypothyroidism

While the management of hypothyroidism is straightforward and achievable with medication, the overall prognosis of secondary hypothyroidism is variable, depending on the underlying cause. Conditions like cancer, infection, head trauma, or bleeding need to be addressed to truly understand the overall prognosis.

Summary

Secondary hypothyroidism, also known as pituitary hypothyroidism, is a condition in which failure of the pituitary gland results in low thyroid stimulating hormone (TSH) and, subsequently, low thyroid hormone levels. Symptoms include fatigue, weight gain, constipation, menstrual irregularities, and skin, hair, and nail changes.

Diagnosing secondary hypothyroidism involves laboratory blood testing, and brain imaging is required to evaluate any underlying causes of pituitary failure. Hypothyroidism is treatable with medication, including synthetic thyroid hormones. The overall prognosis depends on the underlying cause.

A Word From Verywell

Because pituitary hypothyroidism is not common, achieving a diagnosis can be a stressful and complex experience. Know that low thyroid hormone disorders are treatable, though it can take time to start feeling better. Work with your healthcare provider to determine the appropriate dose of thyroid replacement medication so you can feel better sooner.

Frequently Asked Questions

• What are the symptoms of a malfunctioning pituitary gland?

  The pituitary gland is responsible for secreting hormones that interact with the kidneys, sex glands, adrenal glands, and thyroid gland. With pituitary gland failure, symptoms include fatigue, low blood pressure, irregular or absent menstrual cycles, infertility, decreased libido, mood changes, weight changes, muscle weakness, and skin, hair, and nail changes.

• Can hypothyroidism cause an enlarged pituitary gland?

  In primary hypothyroidism, a problem involving the thyroid gland itself, the pituitary gland can become enlarged due to enlargement (hyperplasia) of cells that secrete thyroid stimulating hormone (TSH). This is because there is excess thyrotropin stimulating hormone, which tells the pituitary gland to secrete TSH to normalize thyroid levels.

• How is pituitary hypothyroidism diagnosed?

  Pituitary hypothyroidism is diagnosed using laboratory tests for thyroid hormone levels (T3 and T4) as well as thyroid stimulating hormone (TSH) levels. Low T3 and T4 levels with normal or low TSH levels indicate a problem with the brain. A brain MRI is needed for further evaluation.

By Angela Ryan Lee, MD
Angela Ryan Lee, MD, is board-certified in cardiovascular diseases and internal medicine. She is a fellow of the American College of Cardiology and holds board certifications from the American Society of Nuclear Cardiology and the National Board of Echocardiography. She completed undergraduate studies at the University of Virginia with a B.S. in Biology, medical school at Jefferson Medical College, and internal medicine residency and cardiovascular diseases fellowship at the George Washington University Hospital. Her professional interests include preventive cardiology, medical journalism, and health policy.

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