is, a high level of TSH causes an increase in the amount of thyroid hormones produced.
Once the levels of the thyroid hormones reach a certain level in the bloodstream, the
amount of TSH secreted is reduced and the secretion of the thyroid hormones is
c. Pituitary Gonadotropic Hormones. The pituitary gonadotropic hormones
are three in number. These hormones control the development and function of the sex
glands (gonads). However, these hormones have differing effects in the different sexes.
Each of these hormones will be discussed below.
(1) Follicle-stimulating hormone. In the female, the follicle-stimulating
hormone (FSH)acts in the ovary to stimulate the growth and maturation of the ovarian
follicles that contain the ovum (egg). The FSH also stimulates the secretion of
estrogen, a female hormone, by the ovaries. In the male, the FSH acts on structures
called the seminiferous tubules in the testes to cause spermatogenesis (the production
(2) Luteinizing hormone. In the female, luteinizing hormone (LH) acts to
cause ovulation, the release of a mature egg from the ovary. In the male, LH is known
as the interstitial cell-stimulating hormone (ICSH). The ICSH controls the production of
testosterone, a male hormone, in the testes.
(3) Prolactin (luteotropic hormone). In the female, prolactin causes the
secretion of milk from the fully developed mammary gland (breast) after the breast has
been stimulated by progesterone and estrogen.
THE THYROID GLAND
The thyroid gland is located in the neck just below the larynx (voice box). The
thyroid gland secretes the hormone thyroxin.
a. Background. Thyroxin is synthesized within the thyroid gland by the
combination of several amino acids with four atoms of iodine. Once made, the hormone
is stored in the thyroid gland in combination with a protein. This protein-hormone
complex is called thyroglobulin. The hormone is released into the blood by breaking the
bonds between thyroxin and the protein. The thyroxin is then released into the
bloodstream. The release of the hormone is stimulated by the thyroid-stimulating
hormone from the anterior pituitary gland.
b. Effects of Thyroxin. When thyroxin reaches the cells of the body, it
stimulates them to use more oxygen. This increases the metabolic rate, or basal
metabolism, of the body. Basal metabolism is defined as the amount of oxygen the
body uses per unit of weight when the body is at rest. Thyroxin also functions to
regulate the growth of organs; aid in mental development; aid in sexual development;
and aid in the metabolism of water, electrolytes, proteins, glucose, and lipids. The
Basal Metabolic Rate test may be used to measure the effect of thyroxin on the body.