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
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)
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.
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.
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,
Is Secondary Hypothyroidism Treatable?
Fortunately, hypothyroidism is treatable with medication.
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.
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.
