668 DUMONT, LAMY, ROGER, AND MAENHAUT Volume 72 roid weighs-0.2 g at 20-25 wk in the fetus and-15 g in the adult and assuming a grossly similar tissue composition, this growth requires at least six to seven cell divisions. Human thyroid cells divide about five times in adulthood, which demonstrates that there is a slow constant turnover of these cells, with cell division and cell death compensating each other. When corrected for the life of the animal, the evaluated cell turnover is about five in adulthood for humans and for animals as different as dogs, rats, and mice (49). The calculation of the cell turnover in the whole gland does not depend on any assumption about the homogeneity of the cell population. However, if only a fraction of the population was involved in the turnover (eg, f= 0.4)(45,298), the number of possible divisions in these cells (life span) would be correspondingly higher (n= 5/f= 12.5). Although in adults, under constant conditions, the thyroid maintains its size with a slow cell turnover, it retains the capacity to grow by cell hypertrophy and proliferation in response to a stimulus. The size and function of the thyroid are controlled by a physiological negative feedback mechanism: the thyroid cell secretes thyroid hormones that inhibit the secretion by pituitary thyrotrophs of thyrotropin (TSH), the thyroid-stimulating hormone. Whenever thyroid hormone secretion decreases, as in iodine metabolism defects, iodine deficiency, or after goitrogen or antithyroid drug administration, TSH secretion increases, causing an activation of thyroid function and growth (77, 82, 88, 177).