Calcitonin ,
In addition to secreting thyroid hormones, the thyroid gland SCCICICS
a hormone called calcitonin (kal-si-to'nin) produced by the parafol—
licular cells (see figure 15.176). Calcitonin secretion is directly regu-
lated by blood Ca2+ levels (figure 15.20). As blood Ca2+ concentration
increases, calcitonin secretion increases, and, as blood Ca2+ Concen»
tration decreases, calcitonin secretion decreases. Calcitonin binds to
membrane—bound receptors of osteoclasts and inhibits them, which
reduces the rate of bone matrix breakdown and the release of Ca2+
from bone into the blood. Calcitonin may prevent blood Ca2+ levels
From becoming overly elevated following a meal that contains a high
concentration of Ca2+.
- The role ofcalcitonin in humans is unclear. It may be important
in slowing bone turnover during periods of rapid growth. Calcironin
helps prevent elevated blood Ca2+ levels, but a lack of calcitonin
ssclrlenon does not result in a prolonged increase in blood Ca2+ levels_
r er mechanisms controlling blood Ca2+ levels, such as parathy-