Central hypothyroidism is a thyroid disorder caused by a dysfunction in the hypothalamus or pituitary gland. Symptoms include weight gain, cold intolerance, and fatigue. Diagnosis is done through blood tests and imaging. Treatment depends on the cause and may include surgery or medication.
Hypothyroidism is a thyroid disorder caused by low levels of circulating thyroid hormone. Central hypothyroidism is a subtype of this thyroid disorder, involving the hypothalamus or pituitary gland rather than the thyroid gland. Like other forms of hypothyroidism, central hypothyroidism is characterized by weight gain, cold intolerance, decreased sweating, rough skin, hair loss, tiredness, and weakness. Proper identification and diagnosis of central hypothyroidism from other forms is essential in therapy and is done through laboratory and imaging tests.
Regulation of thyroid hormone secretion occurs through endocrine signaling in the pathway called the hypothalamic-pituitary-thyroid axis. First, the hypothalamus releases a hormone called thyrotropin-releasing hormone (TRH). When this substance reaches the pituitary gland, cells called thyrotrophs secrete another hormone called thyroid stimulating hormone (TSH) into the bloodstream. The blood then carries the TSH to the thyroid gland, also called the target organ.
The amount of TSH determines how much the thyroid gland is stimulated. Adequate production levels of TSH will lead to sufficient secretion levels of T3 and T4. These hormones, especially T3, are important in maintaining the body’s metabolic processes. If these hormones are secreted at an inappropriate rate, metabolic processes slow down, leading to the symptoms of hypothyroidism.
The hypothalamus and pituitary are considered central for two reasons. First, they are parts of the brain. Second, they are at the top of the endocrine system hierarchy because they deliver stimulating signals to endocrine glands such as the thyroid, adrenal glands, ovaries, and testicles. Thus, when a defect in the signaling pathway occurs due to a disturbance in the functioning of the hypothalamus and pituitary gland, central hypothyroidism occurs.
Secondary and tertiary classifications of central hypothyroidism are also diagnosed. The secondary form occurs when only the pituitary gland is affected, while the tertiary form occurs when only the hypothalamus is affected. Conversely, hypothyroidism due to thyroid disease, such as Hashimoto’s thyroiditis, thyroid adenoma, or thyroid cancer, is called primary or peripheral hypothyroidism.
Causes of central hypothyroidism include pituitary adenoma and brain tumors compressing the hypothalamus. Drugs such as dopamine or lithium can also cause this disease by inhibiting the functions of the hypothalamus. Traumatic injuries or lack of blood supply to the brain can also lead to this condition.
Blood tests to determine TSH and thyroid hormone levels are the first step in diagnosing central hypothyroidism. If the doctor finds that the TSH is low or normal and the T3 and T4 levels are reduced, he would suspect central causes. An imaging test, such as computed tomography (CT) or magnetic resonance imaging (MRI), is then done.
Treatment of central hypothyroidism depends on its cause. If the cause is a tumor or adenoma, surgical removal is preferable. On the other hand, if a drug is causing the condition, it may be given at a lower dosage or it may be stopped. These measures, however, may not restore function to the hypothalamus or pituitary gland. In this case the synthetic thyroid hormone levothyroxine is prescribed.
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