MD, FACE
Category:
Hypothyroidism
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General info
Hyperthyroidism
Hypothyroidism
Nodular thyroid disease
Thyroid cancer
Thyroid hormone production is closely regulated by the pituitary and hypothalamus in the brain. These regions of the brain are able to detect when thyroid hormone levels drop below optimal levels. In response the hypothalamus releases thyrotropin releasing hormone (TRH) which activates the pituitary to release thyrotropin (TSH). TSH in turn activates the thyroid gland to increase production and release of thyroid hormone into the circulation.
Failure of the hypothalamus or pituitary to release TRH and TSH results in hypothyroidism.
| Pituitary Disorder | Hypothalamic Disorders |
| Mass lesions—adenomas, cysts | Benign Tumors (craniopharyngiomas) |
| Pituitary radiation | Metastatic Tumors (breast cancer, lung cancer, etc) |
| Pituitary surgery | Radiation |
| Sheehan syndrome | Infection (tuberculous meningitis) |
| Pituitary apoplexy | Ischemic encephalopathy |
| Prior head injury | Prior head injury |
| Infiltrative disease (lymphocytic hypophysitis, hemochromocytosis) | Infiltrative disease (sarcoidosis, histiocytosis) |
| Empty sella | Transient central hypothyroidism (post-treatment of hyperthyroidism: RAI, thionamides, or surgery; post T4 suppression in a nodular goiter) |
The symptoms of central hypothyroidism are the same as those of primary hypothyroidism. Additionally, patients may also manifest symptoms of central nervous system disease such as headache, visual disturbances, numbness or weakness. Most patients will also have clinical evidence of other pituitary hormone deficiency such as low testosterone in men or lack of menstrual cycles in women (low gonadotropins), adrenal insufficiency (ACTH deficiency), or frequent urination (diabetes insipidus from AVP deficiency). Some will manifest evidence of pituitary hormone excess such as acromegaly or elevated prolactin.
In primary hypothyroidism, TSH is elevated in virtually 100% of patients. In contrast, in central hypothyroidism levels of TSH are usually low or normal, with only 25% of levels being mildly elevated. Central hypothyroidism is associated with low levels of Free T4 AND a clinical history suggestive of central hypothyroidism. Concomitant abnormalities of the pituitary hormones are usually present. Often abnormalities will be evident on MRI of the brain/pituitary.
Patients are generally managed the same way patients with primary hypothyroidism are managed: levothyroxine. However, TSH is not a good gage of the adequacy of thyroid hormone replacement in these patients. Patients with central hypothyroidism may need slightly higher levels of free T4 in the circulation than normal subjects. We follow clinical symptoms of patients closely and usually aim for free T4 levels in the upper third of the normal range.
