B

secretion of thyroid hormones. As a result, the serum TSH level increases, stimulating thyroid growth.17 The increase must be relatively short lived and intermittent because most patients have normal serum TSH levels. Other growth factors are obviously involved since the sizes of various nodules vary considerably in the same patient. Furthermore, goiters may grow despite administration of T4 in doses that reduce the serum TSH level to a subnormal level or in patients with toxic nodular goiter. Thus, thyroid growth-modulating factors in addition to TSH are involved in thyroid growth. Some growth factors (e.g., insulin-like growth factor 1, epidermal growth factor, and fibroblast growth factor) have a growth-promoting effect, whereas others (e.g., transforming growth factor [TGF]-(3 and activin A) inhibit growth.18 Increased expression of ras and other protooncogenes may also contribute to goiter growth.19

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