Download Chap 76 - Thyroid Metabolic Hormones

THYROID METABOLIC HORMONES
Physiology III, Tri-4
Guyton & Hall, Chapter 76
Dr. Robyn: Strader
I. Thyroid Gland
A. located below larynx
B. anterior and lateral to trachea
C. secretes:
1. thyroxin (T4)
2. triiodothyronine (T3)
3. calcitonin
II. Formation and Secretion of Thyroid Hormones
A. secretion:
1. 90% thyroxine
TBG
2. 10% triiodothyronine = 4X more potent as thyroxine
B. anatomy:
1. closed follicles
I I
II-
2. secretory substance called colloid
3. lined with cuboidal epithelioid cells that secrete into follicle
4. thyroglobulin
a. major constituent of colloid
b. contains thyroid hormones
C. blood flow = 5X weight of gland
D. iodine
1. 50 mg / year
2. 1 mg / week
3. iodine trapping:
a. basal membrane pumps iodide actively to interior of cell
b. can concentrate up to 250X blood level
E. thyroid cells are protein-secreting glandular cells
F. thyroglobulin:
1. glycoprotein
2. secreted by endoplasmic reticulum and Golgi apparatus
3. secreted into follicles
I- I- T3 TBG
IT4
4. combines with iodine to form thyroid hormones
G. iodide ions:
1. oxidized via peroxidase
2. peroxidase is accompanied by hydrogen peroxide (H2O2)
H. organification = iodine binding with thyroglobulin
I. release of thyroxine and triiodothyronine
1. thyroglobulin not released into circulation
2. T3 & T4 cleaved form the thyroglobulin molecule
a. pinocytic vesicles ingested
b. lysosomes fuse with these vesicles
c. proteinases digest the thyroglobulin and release the thyroxine and
triiodothyronine
d. thyroid hormones are released into the blood
1. 90% is thyroxine
2. 10% is triiodothyronine
III. Transport
A. most all T3 & T4 bind to plasma proteins
1. 80% with thyroxine-binding globulin
2. 10 - 15% with thyroxine - binding prealbumin
3. remainder with albumin
B. half-life of thyroxine is 15 days, may have effects up to 2 months
C. half-life of triiodothyronine is 6 to 12 hours with activity up to 2 to 3 days
IV. Functions of the Thyroid Hormones
A. wholesale nuclear transcription of large numbers of genes
B. almost all cells increase:
1. protein enzymes
2. structural proteins
3. transport proteins
4. other substances
C. Thyroid hormone receptors are near DNA genetic strands
D. increased metabolic activities in almost all tissues
E. increased BMR
F. accelerated utilization of foodstuffs
G. increased protein synthesis and catabolism
H. accelerated growth rate
I. excited mental processes
J. increased endocrine activity
K. increases size, surface area, and number of mitochondria
L. increases transport of Na and K through cell membranes
M. increases heat production
N. mental and physical development
O. effects on carbohydrate metabolism
1. increased CHO uptake
2. enhanced glycolysis
3. enhanced gluconeogenesis
4. increased GI absorption
5. increased insulin secretion
P. fat metabolism increases:
1. depletion of fat stores
2. lipid mobilization
3. free fatty acid concentration in plasma
4. oxidation of free fatty acids by the cells
Q. increased thyroid hormone decreases the quantity of cholesterol,
phospholipids, and triglycerides in the plasma
R. increased need for vitamins
S. decreases body weight
T. blood flow, heart rate, contractility, BV and CO increase
U. increased respiration
V. increased GI and neural activity
W. muscle tremor
X. difficulty with sleep
Y. loss of libido
V. Regulation of Thyroid Hormone Secretion
A. TSH = thyrotropin
1. increased proteolysis of thyroglobulin
2. increased activity of iodide pump
3. increased iodination of tyrosine and increased coupling to form the thyroid
hormones
4. increased size and secretory activity of thyroid cells
5. increased number of thyroid cells, change from cuboidal to columnar cells
6. summary - TSH increases all the known activities of the thyroid glandular cells
B. thyrotropin-releasing hormone = TRH
1. stimulates anterior pituitary
2. release of TSH
3. exposure to cold
4. excitement and anxiety - decrease TRH
C. feedback of thyroid hormone on TSH
VI.
A.
B.
C.
Diseases of the Thyroid
hyperthyroidism
hypothyroidism
cretinism
Cold
TRH
TSH
T3 & T4
increased metabolism