A 34-year-old man comes to the clinic due to headaches and mild confusion over the past week. The patient has a seizure disorder and recently began taking carbamazepine. Vital signs are within normal limits. Physical examination shows no abnormalities. Laboratory studies reveal decreased serum sodium concentration. The patient is diagnosed with a condition caused by increased release of a hormone that is co-secreted with neurophysins. This hormone is most likely secreted into the systemic circulation at which of the following sites?
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This patient with new-onset hyponatremia after drug exposure most likely has the syndrome of inappropriate antidiuretic hormone secretion (SIADH). This condition can be caused by a multitude of drugs, including carbamazepine, cyclophosphamide, and selective serotonin reuptake inhibitors.
Posterior pituitary hormones (eg, ADH, oxytocin) are synthesized within magnocellular neurons found in the supraoptic and paraventricular nuclei of the hypothalamus, with the former primarily producing ADH and the latter oxytocin (Choices D and F). Once translated, these hormones are attached to carrier proteins known as neurophysins. The complexes are packaged into vesicles and travel down axon projections via anterograde transport into the posterior pituitary (neurohypophysis).
At their terminal endings, the axons become neurosecretory terminals known as Herring bodies, temporarily storing secretory vesicles. Pituicytes (glial cells) regulate the permeability of the hypophysial capillaries, facilitating secretion of ADH and oxytocin from the posterior pituitary into the hypophysial vein.
(Choice A) The anterior pituitary (adenohypophysis) is the glandular portion of the pituitary that lies closest to the optic chiasm. It is composed of various hormone-secreting cells, including lactotrophs (prolactin), corticotrophs (ACTH), somatotrophs (growth hormone), thyrotrophs (TSH), and gonadotrophs (FSH, LH).
(Choice B) The intermediate lobe of the pituitary gland is a vestigial region derived from the posterior wall of the Rathke pouch during embryonic development. Although predominantly nonfunctioning, the remainder produces melanocyte-stimulating hormone and endorphins.
(Choice C) The median eminence is a small ridge of hypothalamic tissue that secretes regulatory hormones (both releasing and inhibitory) that modulate the anterior pituitary. These include corticotropin-releasing hormone, gonadotropin releasing hormone, thyrotropin-releasing hormone, and growth hormone–releasing hormone.
Educational objective:
Vasopressin and oxytocin are synthesized and packaged with carrier proteins (neurophysins) within neurons found in the hypothalamus. The hormones are then transported down axonal projections to the posterior pituitary (neurohypophysis), where they are secreted into circulation.