Biology 12

Chapter 6 The Endocrine System • MHR 177

Figure 6.12The release of the hormone oxytocin during
infant feeding strengthens maternal bonding with a newborn.

You will now examine the other major endocrine
structures in the body. These include the thyroid,
parathyroid, pancreas, pineal, thymus, and
adrenal glands.

The Thyroid and Parathyroid

The activity of the thyroid glandillustrates the
role of negative feedback loops in controlling
metabolism and maintaining homeostasis in the
body. This gland, with its distinctive “butterfly”
shape, is located above the trachea in the neck.
Embedded in the thyroid glands are four small
parathyroid glands. They produce a hormone
called parathyroid hormone (PTH). The function of
these small glands will be discussed in more detail
later in this section.
The primary function of the thyroid gland is the
production of the hormone thyroxine, a non-steroid
peptide molecule made from the amino acid
tyrosine. Each thyroxine molecule contains four
atoms of iodine. Receptors for thyroxine are found
on most cells in the body.
Thyroxine increases basal metabolic rate and
oxygen consumption, especially in the heart, skeletal
muscle, liver, and kidney. In comparison, metabolic
activity of the brain, spleen, and reproductive
organs is less affected by thyroxine. The increased

oxygen demand is due to the stimulation of sodium

potassium pump activity in the cell membranes of

target cells. The additional oxygen is consumed in

the production of ATP that is required to drive the

sodium potassium pump. Heat is given off as a

byproduct of this process.

Thyroxine secretion is governed by the anterior

lobe of the pituitary gland, which produces

thyroid-stimulating hormone (TSH). As you can

see in Figure 6.13, the interaction between thyroxin

and TSH takes the form of a negative feedback

loop. The two hormones, thyroxine and TSH,

interact to adjust the levels of thyroxine in

response to the body’s constantly changing needs.

Caffeine in some beverages we consume reduces

glucose metabolism in the cells of the body by

inhibiting TSH production, which in turn

suppresses thyroxine secretion.

Thyroid hormones are carried through the blood

by carrier proteins, as are steroid hormones.

Thyroid hormones (circulating in the blood) then

feed back to the pituitary gland, where they

suppress the secretion of TSH. In the thyroid gland,

TSH stimulates an increase in iodine uptake from

the blood, and the synthesis and secretion of

thyroxine hormone. About 30 percent of the iodine

in blood is consumed by the thyroid gland to be

used in the synthesis of thyroxine. The rest is

eventually excreted from the body.

Figure 6.13This diagram shows how negative feedback by

hormones keeps the amount of thyroxine at a level suitable

to the body’s needs.

less

thyroxine

secreted

hypothalamus

anterior pituitary

larynx

thyroid

trachea

more

releasing

hormone

less

releasing

hormone

more TSH

secreted

less TSH

secreted

more

thyroxine

secreted