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Chapter 15
Section 15.3
Hormones that Affect Metabolism
Endocrine Glands

1.
Three glands of the endocrine system are responsible
for maintaining metabolism:
Thyroid Gland
- located at the base of the neck, immediately in front of trachea
(windpipe)
2.
Parathyroid Glands
- 4 parathyroid glands
- located inside the thyroid gland
3.
Anterior Pituitary
- front of pituitary in the brain
- REMEMBER = makes it’s own hormones
Endocrine Glands
Thyroid Gland
Thyroid Gland

Produces 3 hormones:
1. Thyroxine
(T4) – increases metabolism
and regulates growth.
2. Triiodothyronine
(T3) – increases
metabolisms and regulates growth; contains
3 iodine atoms.
3. Calcitonin
blood.
– lowers calcium levels in the
Feedback System for the Thyroid

Metabolic rate decreases.

Receptors in the hypothalamus are
activated.

Nerve cells in the hypothalamus secrete
thyroid releasing hormone (TRH).

TRH stimulates the pituitary to release
thyroid stimulating hormone (TSH).

TSH is carried by the blood to the
thyroid gland.

Thyroid gland releases T3 and T4.

T3 and T4 raise metabolism by stimulating
increased use of sugar by cells.

High levels of T3 and T4 cause the
pathway to be turned off by inhibiting the
release of TRH from the hypothalamus.
Diagram of Thyroid Feedback System
Anterior Pituitary
Feedback
Loop #5
TRH
T3
T4
Hypothalamus
Thyroid
TSH
= turns off
Goiters and the Importance of
Iodine

Iodine is normally actively transported
out of the blood into the follicle cells of
the thyroid.

This iodine enables the thyroid to
produce it’s hormones (3 iodine atoms
are needed to make T3).

Problems arise when iodine levels begin
to fall.

When inadequate amounts of iodine are
obtained from the diet, the thyroid
enlarges, producing a goiter.
Parathyroid Glands

Parathyroid does not respond to nerve or
hormone regulation.

It responds directly to chemical changes in
their immediate surroundings.

Produces 1 hormone:
Parathyroid Hormone (PTH) – increases
calcium levels in the blood and lowers the
phosphate levels
Feedback System for the
Parathyroid

Blood calcium levels fall.

Parathyroid glands release PTH.

Calcitonin is inhibited.

PTH acts on 3 organs:
Kidneys – reabsorb more Ca2+ before excreting
waste.
2. Intestines – absorb more Ca2+ from food.
3. Bones – release Ca2+ into blood.
1.

Blood calcium levels rise.

Thyroid gland releases calcitonin.

Calcitonin acts on the same 3 organs:
1.
Kidneys – reabsorb less Ca2+ before
excreting waste.
Intestines – absorb less Ca2+ from food.
Bones – deposit Ca2+.
2.
3.

Blood calcium levels fall and the cycle
begins again.
Diagram of Thyroid Feedback System
Feedback
Loop #6
Anterior Pituitary Gland
Produces :
1. Human Growth Hormone (hGH) –
stimulates elongation of skeleton.


Mainly affects the cartilage and bone cells.

Problems arise when the body produces too
much (hypersecretion) or too little
(hyposecretion).
Childhood

Hyposecretion of
hGH during childhood
can result in dwarfism.

Hypersectration of hGH
during childhood
can result in giantism.
Growth Hormone (GH, hGH, or somatotropin)
GH promotes protein synthesis and fat
metabolism in a wide variety of target cells.
 Excess GH in a young person can result in
gigantism (acromegaly) whereas a lack of GH
can cause dwarfism.

Adulthood

Hypersecretion in adulthood can cause
acromegaly, an abnormal bone growth in
the hands, feet and head.
What does hGH do?

Under the influence of hGH, cells of soft
tissues and bone begin to grow by
increasing the number of cells
(hyperplasia) and increasing the size of
cells (hypertrophy).

Increases cell size in muscle cells and
connective tissue by promoting protein
synthesis and inhibiting protein
breakdown.

hGH also plays a role in homeostasis.

It increases fatty acid levels in the blood
by promoting the breakdown of fats.

Muscles use these fatty acids for fuel
instead of glucose.

This helps to maintain blood glucose
levels.
Feedback
Loop #7
Summary
Gland
Hormone
Effect on Metabolism
Thyroid
Thyroxine (T4)and
Triiodothyronine (T3)
- Regulates the rate at which
glucose is oxidized within the
body cells
Thyroid
Calcitonin
- Lowers calcium levels in the
blood
Parathyroid
Glands
Parathyroid Hormone
(PTH)
- Raises calcium levels in the
blood
Anterior
Pituitary
Growth Hormone
(hGH)
- Promotes protein synthesis by
increasing uptake of amino acids
by cells
- Causes a switch in fuels 
glucose to fatty acids