It occurs in less than 5-10% of hypothyroidism cases.
Central hypothyroidism is defined as a reduction in circulating thyroid hormone as a result of inadequate stimulation of a normal thyroid gland by TSH. This condition means the pituitary gland fails to send a message via the TSH to the thyroid to produce. Central hypothyroidism may be secondary, due to pituitary disease, or tertiary, due to hypothalamic dysfunction. Causes include all pathologic processes that affect the hypothalamus or pituitary, including tumors, Sheehan's syndrome, idiopathic hypopituitarism and infiltrative diseases, such as sarcoidosis, histiocytosis and lymphocytic pituitary gland. Central hypothyroidism is common in patients irradiated for pituitary tumors, nasopharyngeal tumors, sphenoid and ethmoid sinuses tumors, and brain tumors. (Click Here)
The signs and symptoms of central hypothyroidism mimic those of the common hypothyroid conditions, and this disorder is therefore difficult to diagnose. A low free T4 and a low TSH in the setting of pituitary disease and signs and symptoms of hypothyroidism point a straightforward manner to the diagnosis of central hypothyroidism. Unfortunately, however, the TSH is most commonly in the normal range in cases of central hypothyroidism, creating a confusing picture. If a pituitary or hypothalamic cause is suspected, an MRI of the brain may be requested. As TSH is not a useful screen for the diagnosis of this disorder, a special test, known as the TRH test (Thyropropin Releasing Hormone), can help distinguish if the hypothyroidism is caused by a defect in the pituitary/hypothalamus. This test requires an injection of the TRH hormone. These investigations should be made on a case by case basis.