Endocrine System—Thyroid and Parathyroid Glands
Lecture 22
You may have noticed a peculiarity with the endocrine organs: We tend
to have a lot of roommates that have entirely different jobs. ... Now
we’re going to look at the thyroid and the parathyroid glands, which
also have completely separate functioning but are intimately related
in geography.
W
e nish our survey of the endocrine system with the thyroid and
parathyroid glands. The thyroid gland regulates the rate and
intensity of the body’s chemical reactions, and the parathyroid
glands regulate the amount of calcium and phosphorous in the blood.
Lecture 22: Endocrine System—Thyroid and Parathyroid Glands
For each of these glands, we brie y review the gross and microscopic
anatomy, the physiology of the gland, and the consequences of dysfunction.
Thyroid dysfunction (either hypo- or hyperthyroidism) can lead to cretinism,
myxedema, Graves’ disease, and other pathologies. Parathyroid dysfunction
(either hypo- or hyperparathyroidism) can lead to various disorders,
including overexcitement of the muscle and nervous systems, bony
demineralization, high calcium levels, duodenal ulcers, kidney stones, and
behavioral disorders.
Thyroid gland
The thyroid gland is the thermostat of the body, regulating the rate and
intensity of chemical (metabolic) reactions. It is an H- or butter y-shaped
gland located just below the larynx (voice box); the right and left lobes
are connected by an isthmus. It envelops the trachea anteriorly (clinically
signi cant in cases of thyroid cancer).
The gland is very richly vascularized via branches of the carotids and
the thyrocervical trunk, including the superior thyroid artery, the inferior
132
, thyroid artery, and multiple thyroid veins. The thyroid has clinically
important relationships with the recurrent and superior laryngeal nerves.
The thyroid gland is made primarily of spheres called follicles. Two types
of cells compose the follicles: The follicular cells produce thyroxine
(tetraiodothyronine or T4) and triodothyronine (T3). The parafollicular cells
(C cells) produce calcitonin.
Thyroid chemistry is iodine-based; iodine must be ingested because it is
an element. Benign goiter (enlargement of the thyroid) is common in areas
where iodine does not naturally occur in food. Western countries typically
include iodine in normal table salt. Cancerous thyroid tumors (nodules)
normally occur in patients whose faces have been irradiated, but these tumors
are easily curable.
Thyroid hormone synthesis
Hormones made in the cells are stored in the follicles in large quantities as
colloid for later use; about a three-month supply is stored. At all levels of
function, thyroid-stimulating hormone (TSH) from the anterior pituitary
regulates the processes via a negative feedback loop. Follicular cells actively
trap virtually all iodide molecules in the body.
Thyroglobulin is synthesized from amino acids and iodide and stored in the
follicles as colloid. When needed, T3 and T4 are released into the circulation
from the follicular cells. T4 is present in much greater quantity than T3, but
T3 is more potent. T4 is converted to T3 in the body cells.
The thyroid’s ability to trap iodine can be used clinically. Low levels of
radioactive iodine (I-131), combined with X-ray exposure, can map thyroid
function. Higher levels of I-131 will irradiate and destroy thyroid tissue,
when needed, without damaging surrounding cells.
Cancerous cells will not trap iodine when normal thyroid tissue is present.
Tumors must be surgically removed or thyroid tissue must be destroyed
before I-131 can destroy cancerous thyroid cells.
133
Lecture 22
You may have noticed a peculiarity with the endocrine organs: We tend
to have a lot of roommates that have entirely different jobs. ... Now
we’re going to look at the thyroid and the parathyroid glands, which
also have completely separate functioning but are intimately related
in geography.
W
e nish our survey of the endocrine system with the thyroid and
parathyroid glands. The thyroid gland regulates the rate and
intensity of the body’s chemical reactions, and the parathyroid
glands regulate the amount of calcium and phosphorous in the blood.
Lecture 22: Endocrine System—Thyroid and Parathyroid Glands
For each of these glands, we brie y review the gross and microscopic
anatomy, the physiology of the gland, and the consequences of dysfunction.
Thyroid dysfunction (either hypo- or hyperthyroidism) can lead to cretinism,
myxedema, Graves’ disease, and other pathologies. Parathyroid dysfunction
(either hypo- or hyperparathyroidism) can lead to various disorders,
including overexcitement of the muscle and nervous systems, bony
demineralization, high calcium levels, duodenal ulcers, kidney stones, and
behavioral disorders.
Thyroid gland
The thyroid gland is the thermostat of the body, regulating the rate and
intensity of chemical (metabolic) reactions. It is an H- or butter y-shaped
gland located just below the larynx (voice box); the right and left lobes
are connected by an isthmus. It envelops the trachea anteriorly (clinically
signi cant in cases of thyroid cancer).
The gland is very richly vascularized via branches of the carotids and
the thyrocervical trunk, including the superior thyroid artery, the inferior
132
, thyroid artery, and multiple thyroid veins. The thyroid has clinically
important relationships with the recurrent and superior laryngeal nerves.
The thyroid gland is made primarily of spheres called follicles. Two types
of cells compose the follicles: The follicular cells produce thyroxine
(tetraiodothyronine or T4) and triodothyronine (T3). The parafollicular cells
(C cells) produce calcitonin.
Thyroid chemistry is iodine-based; iodine must be ingested because it is
an element. Benign goiter (enlargement of the thyroid) is common in areas
where iodine does not naturally occur in food. Western countries typically
include iodine in normal table salt. Cancerous thyroid tumors (nodules)
normally occur in patients whose faces have been irradiated, but these tumors
are easily curable.
Thyroid hormone synthesis
Hormones made in the cells are stored in the follicles in large quantities as
colloid for later use; about a three-month supply is stored. At all levels of
function, thyroid-stimulating hormone (TSH) from the anterior pituitary
regulates the processes via a negative feedback loop. Follicular cells actively
trap virtually all iodide molecules in the body.
Thyroglobulin is synthesized from amino acids and iodide and stored in the
follicles as colloid. When needed, T3 and T4 are released into the circulation
from the follicular cells. T4 is present in much greater quantity than T3, but
T3 is more potent. T4 is converted to T3 in the body cells.
The thyroid’s ability to trap iodine can be used clinically. Low levels of
radioactive iodine (I-131), combined with X-ray exposure, can map thyroid
function. Higher levels of I-131 will irradiate and destroy thyroid tissue,
when needed, without damaging surrounding cells.
Cancerous cells will not trap iodine when normal thyroid tissue is present.
Tumors must be surgically removed or thyroid tissue must be destroyed
before I-131 can destroy cancerous thyroid cells.
133
