The thyroid is a gland of internal secretion whose chief function, so far as is known, is the elaboration and storage of its own peculiar hormone, namely, thyroglobulin, or the amino acid thyroxin contained therein. Originally it was a gland taking part in digestion. In the course of evolutionary changes, it has lost its connection with the alimentary tract. As Means1 aptly said, "For a rôle in digestion it has substituted a rôle in growth and metabolism." To fulfil these functions, it is endowed with tremendous capacities for increasing or decreasing its activities, manifested by changes in size, blood supply, microscopic appearance and hormone content. In man the thyroid attains its relative maximum size just prior to puberty, corresponding to the time of maximum load on the organ from factors of growth and development. Simple colloid goiter, so common at this age, is the clinical manifestation of this physiologic
LERMAN J. THE PHYSIOLOGY OF THE THYROID GLAND. JAMA. 1941;117(5):349–359. doi:10.1001/jama.1941.72820310005008
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