Module 14: The Endocrine System: Thyroid Gland Hormones and Function

free T3

The amount of T3 that is floating freely in the bloodstream.

iodination of tyrosine

The side chain of tyrosine may pick up one or two iodine molecules; forms T1 or T2.

hypothyroidism

Decreased levels of thyroid function. Causes low basal metabolic rate, cold intolerant, constipation, decreased respiratory rate, low heart rate (bradycardia), weight gain and lethargy.

goiter

Enlargement of the thyroid; can be found in people with hypothyroidism, hyperthyroidism and euthyroidism. Usually due to iodine deficienty.

hyperthyroidism

Excessive levels of thyroid function. Causes high basal metabolic rate; heat intolerant, diarrhea, increased respiratory rate, high heart rate (tachycardia), weight loss and anxiety.

thyroid hormone secretion

Hypothalamus secretes TRH. The anterior pituitary responds by secreting TSH which stimulates the thyroid to secrete T3 and T4.

iodide trapping

Iodine circulates in the blood as iodide (I). Iodide is actively transported into the follicular cells.

thyroid follicles

Main functional unit of the thyroid gland. Consist of a central open space (lumen) surrounded by wall of follicular cells. Secretes hormones, T3 and T4.

graves disease

Most common form of hyperthyroidism. Autoimmune disease where antibodies target TSH receptors; antibodies act like TSH and stimulate thyroid hormone production. Symptoms: nervousness, weight loss, diarrhea, exophthalmamos due to periobrital edema.

pinocytosis and digestion of colloid

Once synthesized, TGB reenters the follicular cells where digestive enzymes break down the molecule and release T3 and T4.

thyroid sequence

Regulates oxygen use; increases basal metabolism rate (BMR); increases protein synthesis; increases carbohydrate and fatty acid catabolism; increases reactivity of the nervous system by increasing sensitivity to epinepherine and norepinepherine; works with hGH to control tissue growth and development.

parathyroid hormone (PTH)

Secreted by the parathyroid glands. Increases blood calcium levels by stimulating osteoclasts; increases bone resorption and releases more calcium ions into the blood. Also increases levels of calcitriol (vitamin D) released from the kidneys to release phosphate in the urine. Is antagonistic with calcitonin.

parathyroid glands

Small round masses of tissue attached to the posterior surface of the lateral lobes of the thyroid. One superior and one inferior attached to each lobe. Contain chief (principal) cells; secrete parathyroid hormone (PTH).

secretion of thyroid hormones

T3 and T4 are lipid soluble and can freely pass through the cell membrane into interstitial fluid and then the blood.

isthmus

Bridge like connection between the two lobes of the thyroid. Located anterior to the trachea.

thyroid gland

Consists of two lobes connected by an isthmus; is located inferior to the larynx. Stores about a 100 day supply of secretory products. Contains most of the iodide found in the body.

triiodothyronine (T3)

Contains 3 iodine molecules. More potent than T4.

congenital hypothyroidism

Thyroid dysfunction occurring during childhood. Caused by insufficient thyroid hormone during growth. Causes symptoms of hypothyroidism, mental retardation and dwarfism. Formerly called cretinism. Patient demonstrates a low growth rate, mental retardation and decreased development of the nervous and skeletal systems. Due to synergistic relationship between thyroid hormones and hGH.

myxedema

Thyroid dysfunction that occurs in adulthood. Caused by insufficient thyroxin during adulthood. Symptoms: brittle hair, dry skin due to decreased perspiration, edema, lethargy, low BMR, temperature and heart rates; weight gain, not mentally retarded but have diminished intelligence. Edema caused by deposition of excess connective tissue, causing water accumulation in the tissues.

coupling of T1 and T2 to make T3 and T4.

Two tyrosine molecules are joined to form either T3 or T4.

oxidation of iodide

Before iodide can bind to tyrosine, iodide must oxidize and combine with another iodide to form an iodine molecule (I2).

Transport of T3 and T4 in the blood

99% of T3 and T4 bind to transport proteins to travel through the bloodstream.

synthesis of thyroglobulin (TGB)

A glycoprotein produced by follicular cells. Contains large amounts of the amino acid tyrosine which binds with iodine to produce T3 and T4.

thyroglobulin (TGB)

A protein synthesized inside the thyroid follicular cells; consists of more than 5000 amino acids; 100 are tyrosine. TGB is released into the lumen where iodine atoms attach to the tyrosine. Forms T3 or T4 depending on the number of iodine atoms that attach.

tetraiodothyronine (T4)

Also called thyroxin. Contains 4 iodine molecules. Usually converted to the more powerful T3 molecule.

calcitonin

Also known as thyrocalcitonin; made by the parafollicular cells (C cells) of the thyroid). Action is to regulate calcium levels by inhibiting osteoclast action; promotes storage of calcium and phosphate in the bone matrix. "Tonin up the bones".