In this article, I will discuss an important disorder of the antidiuretic hormone which is an important hormone for the regulation of water balance in the body as will be discussed later. This hormone has two clinical syndromes that are both involved in the control of the amount of water in the body.
The first disorder is called diabetes insipidus and has to do with deficiency in the amount of this hormone that is secreted by the pituitary gland into the circulation. This disorder has distinct clinical symptoms which will not be discussed here. The second no less in significance of this hormone disorder is the presence of excessive amount of this hormone in the blood. The causes and symptomes will be discussed only for this disorder.
The antidiuretic hormone is a nine amino acids polypeptide that is secreted by the supraoptic nuclei of the hypothalamus and is stored in the posterior part of the pituitary gland. It is secreted into the blood inresponse to hypovolemia or decreased blood volume. In addition it is also secreted in response to high blood osmolality.
The pituitary gland is divided into an anterior part and a posterior part. Both of these parts have important functional roles. Here, I am mainly concerned with the posterior part and in particular with the antidiuretic hormone. The posterior pituitary gland secretes in addition to this hormone another hormone and which is called oxytocin which is very similar to antidiuretic hormone in structure with different sequence of amino acids. This hormone has entirely different function in the body thatn the antidiuretic hormone and which has to do with muscles contraction.
Antidiuretic hormone is extremely important for the homeostasis of water in the body. Its concentration is tightly regulated by osmolality of the blood and by the level of blood volume in the blood vessels. Its deficiency causes polyurea with depletion of water from the body while its excess in the body causes retention of water with subsequent clinically significant symptoms.
In the case of excessive amount in the blood of the antidiuretic hormone there is excessive stimulation of the kidney tubules to reabsorb more and more of the filtered water in the glomeruli of the kidney. The excess of this hormone can be due to an inherent defect in the hypothalamic-pituitary axis that can cause the secretion of excessive amount of this hormone.
More often the disorder can be caused due to a tumour outside the pituitary gland and which is called ectopic secretion of antidiuretic hormone. Many malignant and nonmalignant neoplasms can cause ectopic secretion of this hormone. As an example lung cancer in additon to nonmalignant lung disorders such as tuberculosis can cause this syndrome of excessive secretion of this hormone.
Lymphomas and sarcomas at various sites in the body such as the pancreas and the duodenum can also trigger the ectopic secretion of this hormone. Trauma to the brain especially those involving the hypothalamus and the pituitary gland can also trigger this syndrome.
Many drugs that function by stimulating the release of this hormone can also trigger this condition. Examples of such drugs include clofibrate and chlorproamide. In addition to all the afore mentioned causes of this syndrome other endocrine disorders can also trigger this condition.
Examples of such disorders include liver failure in which case the excess amount of antidiuretic hormone does not stem from excessive secretion but due to the dysfunction of the liver. The liver cannot metabolize this hormone properly due to its failure. Thus causing its accumulation in the blood with subsequent development of the symptoms that are typical for excessive secretion of this hormone.
Clinical symptoms that can accompany this syndrome include hyponatremia due to the dilutional effect of water on sodium ions. In addition there is hypertension and edema that can also develop due to the conservation of water in the body.