Human Physiology, 14th edition (2016)

342 Chapter 11

earlier in this chapter. Through the activation of genes, thyroid

hormones stimulate protein synthesis, promote maturation of

the nervous system, and increase the rate of cell respiration in

most tissues of the body. Through this action, thyroxine (after

it is converted into T 3 ) elevates the basal metabolic rate ( BMR;

chapter 19, section 19.1), which is the resting rate of calorie

expenditure by the body.

Calcitonin, secreted by the parafollicular cells, or C cells,

of the thyroid, inhibits the activity of osteoclasts, the bone cells

that promote dissolution of the calcium phosphate crystals of

bone. It also stimulates the urinary excretion of Ca^2 1 by the kid-

neys. These actions would lower the blood Ca^2 1 concentrations

and thus antagonize the effects of parathyroid hormone. How-

ever, whereas parathyroid hormone and vitamin D (discussed in

chapter 19, section 19.6) are necessary for Ca^2 1 homeostasis,

calcitonin is believed to have negligible importance in normal

human physiology.

Diseases of the Thyroid

Thyroid-stimulating hormone (TSH) from the anterior pitu-

itary stimulates the thyroid to secrete thyroxine; however, it

Production and Action of Thyroid Hormones

The thyroid follicles actively accumulate iodide (I^2 ) from
the blood and secrete it into the colloid. Once the iodide has
entered the colloid, it is oxidized and attached to a specific
amino acid (tyrosine) within the polypeptide chain of a protein
called thyroglobulin. The attachment of one iodine to tyro-
sine produces monoiodotyrosine (MIT); the attachment of two
iodines produces diiodotyrosine (DIT).
Within the colloid, enzymes modify the structure of MIT
and DIT and couple them together. When two DIT molecules
that are appropriately modified are coupled together, a mol-
ecule of tetraiodothyronine ( T 4 ), or thyroxine, is produced
( fig. 11.23 ). The combination of one MIT with one DIT forms
triiodothyronine (T 3 ). Note that at this point T 4 and T 3 are still
attached to thyroglobulin. Upon stimulation by TSH, the cells
of the follicle take up a small volume of colloid by pinocytosis,
hydrolyze the T 3 and T 4 from the thyroglobulin, and secrete the
free hormones into the blood.
The transport of thyroid hormones through the blood and
their mechanism of action at the cellular level was described

Figure 11.23 The production and storage of thyroid hormones. Iodide is actively transported into the follicular cells. In
the colloid, it is converted into iodine and attached to tyrosine amino acids within the thyroglobulin protein. MIT (monoiodotyrosine)
and DIT (diiodotyrosine) are used to produce T 3 and T 4 within the colloid. Upon stimulation by TSH, the thyroid hormones, bound to
thyroglobulin, are taken into the follicular cells by pinocytosis. Hydrolysis reactions within the follicular cells release the free T 4 and T 3 ,
which are secreted.

Blood plasma Thyroid follicle

Thyroid

uptake of

iodide

Thyroid

hormone

secretion

T 3

T 4

Plasma

carrier

protein

Colloid

I–

(Iodide in

plasma)

Peroxidase

H 2 O 2

Oxidized

iodide

+

Thyroglobulin

Bound to

thyroglobulin

Triiodothyronine

(T 3 )

Tetraiodothyronine

(T 4 )

Monoiodotyrosine

(MIT)

Diiodotyrosine

(DIT)

MIT + DIT DIT + DIT

I–

Endocytosis stim

ulated

by TSH