As discussed in Chapter 1, it's not uncommon for the thyroid to enlarge and/or develop lumps or nodules that make their own supply of thyroid hormone—in addition to the usual supply made by the rest of the thyroid gland. As soon as the nodule(s) starts to make more than normal amounts of thyroid hormone, it suppresses the pituitary's production of TSH. This causes the normal (non-nodular) portions of the thyroid gland to "go to sleep" since there is no TSH to wake them up. The degree of thyrotoxicosis depends upon the ability of this autonomous thyroid nodule to make thyroid hormone.
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