Chapter 18. The Pituitary Gland
An endocrine physiologist is studying communication between the hypothalamus and pituitary in a rat model. She interrupts blood flow emanating from the median eminence and then measures circulating levels of pituitary hormones following appropriate physiologic stimulation. Secretion of which of the following hormones will be unaffected by the experimental manipulation?
C. Thyroid-stimulating hormone (TSH)
D. Follicle-stimulating hormone (FSH)
The correct answer is E. Secretion of hormones from the anterior pituitary (including growth hormone, prolactin, TSH, and FSH) is regulated either positively or negatively by factors delivered from the hypothalamus in the portal hypophysial circulation. Thus, the secretion of all anterior pituitary hormones will be influenced by the manipulation described, although it is important to note that secretion of prolactin will be increased rather than decreased since the dominant influence of the hypothalamus on its secretion is inhibitory (mediated by dopamine) (rules out options A, B, C, and D). Conversely, vasopressin is released from the endings of hypothalamic nerves that make up the posterior pituitary and should be unaffected.
A 20-year-old African American woman is seen by her primary care physician for evaluation of patches of skin on her face and hands that have lost pigmentation. She denies any injuries to the affected areas and is otherwise healthy. The symptoms have developed over the last few weeks. Blood tests reveal the presence of autoantibodies to tyrosinase. The most likely diagnosis is
C. primary adrenal insufficiency.
The correct answer is D. Vitiligo is an autoimmune condition that develops at varying times after birth and results from patchy destruction of melanocytes by antibodies directed against their cellular components (such as tyrosinase, the enzyme responsible for the synthesis of melanin from tyrosine). It can be distinguished from piebaldism, which it otherwise may resemble, because piebaldism is present from birth and results from developmental defects in migration of neural crest cells that become melanocytes (rules out option B). Albinism results in a generalized, rather than patchy, loss of pigmentation from birth, ascribable to a genetic defect in tyrosinase activity (rules out option A). Pallor does occur in hypopituitarism due to a loss of circulating ACTH (the circulating trigger for melanin synthesis in adults), but again it is generalized (rules out option E). In primary adrenal disease, hyper- rather than hypopigmentation occurs because ACTH ...