The thyroid gland is

in origin. It is comprised of large

that are made up from surrounding

cells.

Thyrocytes secreate

which is a hormone precursor and is secreated into the follicle lumen.

The activity of the thyroid gland is modulated by

released from the

The parathyroid glands are found to the posterior of the

They produce

which is critical in the maintenence of

levels in the blood.

Parathyroid glands also contain

cells, which are found in higher concentrations in older people an are thought to be transitional derviatives.

When Ca2+ levels in the blood fall,

cells of the

produce

which acts on the bones and kidney, causing an

plasma Ca2+ levels.

This hormone also causes an increase in plasma levels of

which increases the absobtion of Ca2+ and Mg2+ from the gut.

An increase in plasma Ca2+ stimulates the

to release

from parafollicular cells, which inhibits

and brings about a fall in plasma Ca2+ levels.