HYPOPITUITARISM; HYPOFUNCTION OF PITUITARY
In hypopituitarism, there is usually a deficiency of one or more of the pituitary hormones affecting either anterior pituitary or posterior pituitary and hypothalamus.
A. Hypofunction of Anterior Pituitary causing Hypopituitarism:
Adenohypophyseal hypofunction is indifferent due to the destruction of the anterior lobe of > 75%. Because the anterior pituitary own a large functional reserve.
This may result from anterior pituitary lesions or pressure and destruction from adjacent lesions. Lesions of the anterior pituitary include nonsecretory (chromophobe) adenoma, metastatic carcinoma, craniopharyngioma, trauma, postpartum ischaemic necrosis (Sheehan’s syndrome), empty-sella syndrome, and rarely, tuberculosis. Though a number of syndromes associates with deficiency of anterior pituitary hormones have been described, two important syndromes are panhypopituitarism and dwarfism.
The classical clinical condition of major anterior pituitary insufficiency known as panhypopituitarism. Three most common causes of panhypopituitarism are: non-secretory (chromophobe) adenoma (discussed later), Sheehan’s syndrome, and Simmond’s disease, and empty-sella syndrome.
SHEEHAN’S SYNDROME AND SIMMOND’S DISEASE.
Pituitary insufficiency occurring due to postpartum pituitary (Sheehan’s) necrosis known as Sheehan’s syndrome, whereas the occurrence of similar processes without preceding pregnancy as well as its occurrence in males is termed Simmond’s disease. The main pathogenetic mechanism underlying Sheehan’s necrosis is the enlargement of the pituitary occurring during pregnancy which may be followed by hypotensive shock precipitating ischaemic necrosis of the pituitary. Other mechanisms hypothesized are: DIC following delivery, traumatic injury to vessels, and excessive hemorrhage. Patients with long-standing diabetes mellitus appear to be at greater risk of developing this complication.
The first clinical manifestation of Sheehan’s syndrome is failure of lactation following delivery which is due to deficiency of prolactin. Subsequently, other symptoms develop which include loss of axillary and pubic hair, amenorrhoea, sterility and loss of libido. Concomitant deficiency of TSH and ACTH may result in hypothyroidism and adrenocortical insufficiency.
The morphologic features in the anterior pituitary in Sheehan’s syndrome during early stage are ischaemic necrosis and haemorrhage, while later necrotic tissue is replaced by fibrous tissue.
Empty-Sella syndrome is characterized by the appearance of an empty sella and features of panhypopituitarism. Most commonly, it results from herniation of subarachnoid space into the sella turcica due to an incomplete diaphragma sella creating an empty sella.
Other less common causes are Sheehan’s syndrome, infarction and scarring in an adenoma, irradiation damage, or surgical removal of the gland.
Severe deficiency of GH in children before growth complete results in retarded growth and pituitary dwarfism. Most commonly, isolated GH deficiency is the result of an inherited autosomal recessive disorder. Less often it may be due to a pituitary adenoma or craniopharyngioma, infarction, and trauma to the pituitary. The clinical features of inherited cases of pituitary dwarfism appear after one year of age. These include proportionate retardation in growth of bones, normal mental state for age, poorly-developed genitalia, delayed puberty, and episodes of hypoglycemia. The pituitary dwarf must be distinguished from hypothyroid dwarf (cretinism) in which there is achondroplasia and mental retardation.
B. Hypofunction of Posterior Pituitary and Hypothalamus causing Hypopituitarism:
Insufficiency of the posterior pituitary and hypothalamus is uncommon. The only significant clinical syndrome due to the hypofunction of the neurohypophysis and hypothalamus is diabetes insipidus.
Deficient secretion of ADH causes diabetes insipidus. The causes of ADH deficiency are:
- inflammatory and neoplastic lesions of the hypothalamohypophyseal axis
- destruction of neurohypophysis due to:
- head injury, and
- lastly, cases where no definite cause is known and labeled as idiopathic.
- The main features of diabetes insipidus are excretion of a very large volume of dilute urine of low specific gravity (below 1.010), polyuria, and polydipsia.