My Clinical Notes

• Composed of 2 distinct components;
  • Anterior lobe – adenohypophysis
  • Posterior lobe – neurohypophysis

 

Anterior pituitary

• Constitutes 80% of the gland
• Has a portal vascular system which transports hormone releasing factors from the hypothalamus
• The production of most pituitary tumours is controlled predominantly by positive acting releasing factors from the hypothalamus. The exception is prolactin which is inhibited by the action of dopamine from the hypothalamus
• There are five cell types in the anterior pituitary;
  • Somatotrophs – producing GH. Acidophilic cells
  • Lactotrophs – produce prolactin. Also acidophilic
  • Corticotrophs – basophilic cells which produce ACTH, pro-opiomelanocortin (POMC), melanocyte-stimulating hormone (MSH), endophins and lipotrophins
  • Thyrotrophs – basophilic cells which produce TSH
  • Gonadotrophs – basophilic cells which produce LH and FSH

 

Posterior pituitary

• Consists of modified glial cells and axonal processes extending from nerve cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus
• These neurones produce ADH and oxytocin
• Hormones are stored in axon terminals in the posterior pituitary and are released into the circulation in response to appropriate stimuli

 

Causes of hyperpituitarism;

• Pituitary adenoma
• Hyperplasia and carcinomas of the anterior pituitary
• Secretion of hormones due to nonpituitary tumours

 

Adenomas of the anterior pituitary

• May be functional (associated with clinical manifestions of hormone production) or silent (hormone production at the tissue level only without clinical symptoms)
• Usually result from a single cell type and produce a single type of hormone
• Classified based on the hormone(s) produced by the neoplastic cells
• Some may produce 2 hormones – the most common being GH and prolactin
• Rarely they are plurihormonal
• Pituitary hormones may be hormone negative producing nothing at all
• Both silent and hormone negative adenomas can cause hypopituitarism by encroaching and destroying the pituitary parenchyma

 

• Classification of pituitary adenomas;
  • Prolactin (lactotroph) cell adenoma
  • Growth hormone cell (somatotroph) adenoma
  • TSH cell (thyrothroph) adenoma
  • ACTH cell (corticotroph) adenoma
  • Gonadotroph cell adenoma
  • Mixed growth hormone-prolactin cell (mammosomatotroph) adenoma
  • Other plurihormonal adenomas
  • Hormone-negative adenomas

 

Prolacinomas

• Most frequent type of hyperfunctioning pituitary adenoma
• Increased serum levels of prolactin cause;
  • Amenorrhoea
  • Galactorrhoea
  • Loss of libido
  • Infertility
• May be treated by surgery or bromocriptine a dopamine receptor agonist

 

Growth hormone (somatotroph) adenomas

• Second most common type of functioning adenomas
• May be large before they come to clinical attention as the effects of excessive GH may be quite subtle
• The persistent secretion of GH stimulates the hepatic secretion of insulin-like growth factor 1 which causes many of the clinical manifestations;
  • Increase in body size with disproportionate arms and legs. If the increased levels of GH are around after the closure of epiphysis then patients develop acromegaly
  • Gonadal dysfunction
  • Diabetes mellitus
  • Generalised muscle weakness
  • Hypertension
  • Arthritis
  • Congestive heart failure and an increased risk of gastrointestinal tumours
• Diagnosis is based on elevated GH and IGF-1
• Failure to suppress GH levels in response to an oral dose of glucose is a sensitive test
• Tumour can be removed surgically or destroyed by radiation therapy, or GH secretion can be reduced by drug therapy

 

Corticotroph cell adenomas

• Excess ACTH by the corticotroph adenoma leads to adrenal hypersecretion of cortisol and the development of Cushing syndrome and therefore Cushing disease
• Large destructive adenomas can develop in patients who have had their adrenal glands removed due to the loss of the inhibitory effect if the adrenal corticotrophs on a pre-existing microadenoma. This is called Nelson syndrome. These patients present with a mass effect from their tumour
• There can also be hyperpigmentation because of the effect of other products of the ACTH precursor molecule on melanocytes

 

• Adenomas that secrete more than one hormone are usually aggressive

 

 

 

Gonadotroph adenomas

• Difficult to recognise because the secrete hormones inefficiently and variably and the secretory products don’t produce a recognisable clinical syndrome
• Paradoxically associated with secondary gonadal hypofunction. There can be reduced secretion of LH resulting in decreased libido and amenorrhoea
• FSH is usually the predominantly secreted hormone

 

Thyrotroph adenomas

• Rare, 1% of pituitary adenomas
• Rare cause of hyperthyroidism

 

Non-functioning pituitary adenomas

• Comprise both silent adenomas and hormone negative adenomas
• Comprise 25% of all pituitary tumours
• Typically present with mass effect and may compromise the residual pituitary function to result in hypopituitarism
• This may occur due to gradual enlargement of the adenoma or after abrupt enlargement of the tumour followed by acute haemorrhage – pituitary apoplexy
• Symptoms of mass effect include;
• Visual field defects including bitemporal hemianopia
• Headache, vomiting and nausea due to increased ICP

This entry was posted on Saturday, January 12th, 2008 at 3:00 pm and is filed under Endocrine. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.