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Endocrine system - Posterior Pituitary


Posterior Pituitary gland
  • Posterior pituitary gland (neurohypophysis) is arises from a ventral outgrowth of the primitive hypothalamus. The posterior pituitary gland secretes two key hormones: Antidiuretic hormone (ADH) and Oxytocin. 

Antidiuretic hormone (ADH)
  • Antidiuretic hormone (ADH), also known as vasopressin, is made by supraoptic vasopressinergic neurons. It acts at the kidney to increase the permeability of the collecting duct allowing it to reabsorb more water back into the serum. This prevents "diuresis" or dilute urine to be produced, instead the urine made is concentrated with higher osmolarity. It is produced in response to increased serum osmolarity and hypovolemia, and its production is inhibited by low serum osmolarity. ADH is very important in maintaining proper serum osmolarity. 
    • Syndrome of inappropriate antidiuretic hormone (SIADH): too much ADH is produced. Patients have urine with high osmolarity and serum with low osmolarity (low serum electrolytes including uric acid). There is dilute serum (with low Na+ concentration) and concentrated urine. The hyponatremia can cause swelling of neurons and cerebral edema which manifests as altered mental status and possibly seizures. This condition is seen in patients with CNS dysfunction, lung problems (pneumonia, small cell lung cancer), pain, or who are on certain medications. Since patients have dilute serum (excess free water), the treatment is water restriction. Other options include giving hypertonic saline (NaCl) to increase serum osmolarity but if done too quickly, it can result in central pontine myelinolysis. Another option is to induce nephrogenic diabetes insipidus by giving demeclocycline, an old tetracycline antibiotic that is also an ADH antagonist. Conivapta and tolvaptan are also ADH antagonists, and they work by blocking ADH at V2 receptor.
    • Central diabetes insipidus (CDI): lack of ADH production, resulting in large volumes of dilute urine. Can be idiopathic, genetic, or secondary to tumors, sarcoidosis, granulomatous diseases, or trauma. Patients are given ADH (desmopressin) either orally or using a nasal spray. 
    • Nephrogenic diabetes insipidus (NDI): kidneys do not respond to ADH, resulting in large volumes of dilute urine. This is often due to medications like lithium and demeclocycline. Patients are given a diuretic (e.g. hydrochlorothiazide). 
    • Psychogenic polydipsia: large volumes of dilute urine produced but not due to a disorder of ADH production. This is a psychiatric condition that results in excesive free water ingestion. It can be confused with diabetes inspidus but when restricted from drinking fluids these patients return to normal urine production.  
    • Differential diagnosis of polyuria with dilute urine: 
      • Water restriction - rules out psychogenic polydipsia
      • ADH (desmopressin, DDAVP) stimulation test -- patients who lack ADH production (CDI) will respond and produced more concentrated urine, whereas patients who are ADH resistant (NDI) will not. 
Oxytocin
  • Oxytocin is made by the paraventricular oxytocinergic neurons. It acts on the uterus and mammary glands to cause smooth muscle contraction leading to uterine contractions during delivery and milk ejection during breastfeeding. 

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