Hormones play an important role in human development. The main hormones involved are growth hormone, thyroid hormone, sex hormones like testosterone and estrogen, and pituitary gonadotropic hormones. Growth hormone stimulates growth of cells and tissues. Thyroid hormone is necessary for normal growth and brain development. Testosterone promotes male sexual development and the growth spurt during puberty. Estrogens stimulate female sexual development and changes at puberty like breast growth. The pituitary also secretes follicle-stimulating hormone and luteinizing hormone which regulate the development and function of the ovaries and testes.

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Hormones as Mediators of Development
Hormones are chemical messengers secreted by endocrine cells, they are released
directly in the blood where they circulate the entire body but act only on their target cells
(which have compatible receptors).
Hormones Involved in Development:
The main hormones concerned with development and growth are pituitary growth
hormone, thyroid hormone, the sex hormones testosterone and estrogen, and the
pituitary gonadotropic (sex-gland-stimulating) hormones.
Numerous factors may retard maturation or prevent normal growth, including hormonal
disorders, metabolic defects, hereditary conditions, and inadequate nutrition.
(1) Growth Hormone / Somatotrophin Hormone:
Pituitary growth hormone is a protein that is secreted by the pituitary gland throughout
life. It is necessary for growth in children; without it dwarfism results. Growth hormone
increases the number and size of almost all cells of the body. During fetal life, it seems
not to be necessary, though normally present.
It is not secreted at a constant rate all day but in small bursts of activity. The normal
stimulus for secretion is not certain, but a sharp and “unnatural” lowering of blood sugar
will cause growth hormone to be secreted. The hormone decreases the amount of fat
and causes protein to be laid down in muscles and viscera.
If the concentration of growth hormone deviates from normal, the person will not be able
to develop normally, i.e. in case of under-secretion, the person will undergo a condition
called dwarfism. In case of over-secretion before adulthood, the person will undergo a
condition called gigantism, if the over-secretion takes place after adulthood, the person

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will undergo a condition called acromegaly (abnormally large hand, feet, lower jaw,
etc.).
(2) Thyroid Hormones:
Thyroid hormone from the thyroid gland in the neck is necessary for normal growth,
though it does not itself stimulate growth, for example, in the absence of pituitary growth
hormone, no growth happens. Without thyroid hormone, cells do not develop and
function properly, especially in the brain.
Babies who lack thyroid hormone at birth are small and have insufficiently developed
brains; they are known as cretins (a condition called cretinism). Thyroid lack may also
develop later in childhood, when it causes a slowing of growth rate; full catch-up follows
prompt treatment. Thyroid hormone is also very important for metamorphosis in
amphibians, without it, tadpole larva cannot transform into an adult frog but rather it just
grows into a bigger tadpole.
(3) Testosterone:
Testosterone, secreted by the interstitial cells of the testis, is important not only at
puberty but before. Its secretion by the fetal testis cells is responsible for the
development of certain parts of the male genital apparatus.
If testosterone is not secreted at a particular time, the genitalia develop into the female
form (hence it is important for the development of male embryos, otherwise they will be
female, i.e. it determines the developmental pathway and even the sex of the unborn
baby).
Only small amounts of testosterone circulate between birth and puberty, but at puberty,
the interstitial cells develop greatly in response to pituitary luteinizing hormone, and
testosterone is secreted in large amounts, bringing about most of the changes of male
puberty. It acts on a widespread series of receptors—for example, the cells of the penis,

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the muscles, the skin of the face, the cartilages of the shoulder, and certain parts of the
brain. Most of the adolescent growth spurt (sudden growth) is due to testosterone.
(4) Estrogens:
The female sex hormones, collectively called estrogens, are first secreted in quantity at
puberty by cells in the ovary. They cause growth of the uterus, vagina, and breast; they
act also on the bones of the hip, causing the specifically female widening.
(5) Follicle Stimulating Hormone & Luteinizing Hormone:
The pituitary secretes two other hormones concerned in development: one, follicle-
stimulating hormone (FSH), causes growth of the main portions of the ovary in the
female and the sperm-producing cells in the testis of the male; the other, luteinizing
hormone (LH), causes growth and secretion of the testosterone-secreting cells of the
male and has an action in controlling the menstrual cycle in the female. The pituitary is
caused to secrete gonadotropins by substances called releasing factors that come to it
from adjacent areas of the brain, where they are made.