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Chapter 19. The Thyroid Gland

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A 40-year-old woman comes to her primary care clinician complaining of nervousness and an unexplained weight loss of 20 pounds over the past 3 months despite her impression that she is eating all the time. On physical examination, her eyes are found to be protruding, her skin is moist and warm, and her fingers have a slight tremor. Compared to a healthy individual, a biopsy of her thyroid gland would most likely reveal which of the following:

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A. Decreased numbers of reabsorption lacunae

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B. Decreased evidence of endocytosis

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C. A decrease in the cross-sectional area occupied by colloid

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D. Increased levels of NIS in the basolateral membrane of thyrocytes

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E. Decreased evidence of lysosomal activity

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The correct answer is C. The patient’s symptoms are consistent with a diagnosis of hyperthyroidism. Because thyroid hormones are secreted in excess in this patient, the thyroid will show evidence of activation of thyrocytes, including an increase in the numbers of resorption lacunae at their apical membranes as colloid is absorbed, as well as increased endocytosis, and lysosomal activity (rules out options A, B, and E). However, NIS expression and translocation to the basolateral membrane is regulated by TSH, and TSH levels will likely be decreased secondary to feedback inhibition (rules out option D). On the other hand, increased thyroid hormone secretion is accompanied by active resorption of colloid and a decrease in its proportion of the gland as a whole.

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A scientist studying the normal biosynthesis of thyroid hormones creates a series of mouse models that are deficient in various putative contributors to hormone production. Of the animals tested, those lacking which of the following factors should exhibit normal levels of thyroid hormone synthesis?

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A. Iodine

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B. Ferritin

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C. Thyroglobulin

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D. Protein synthesis

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E. TSH

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The correct answer is B. Thyroid hormones are synthesized when tyrosine residues in the thyroglobulin molecule are iodinated in the thyroid gland (rules out options A and C). Further, even though thyroid hormones are not themselves proteinaceous, production of thyroglobulin, as well as thyroid peroxidase that mediates the iodination reaction, requires protein synthesis (rules out option D). Thyroid hormone production is also under the control of TSH from the anterior pituitary, and reduced in hypopituitarism (rules out option E). On the other hand, ferritin is involved in cellular storage of iron, which is not directly involved in thyroid hormone biosynthesis.

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