Physiology: Thyroid Gland

Question 1

Thyroid Anatomy

Answer 1

• develops from endoderm of pharyngeal gut
• epithelial cells join to form thyroid follicles
• cells are polarize - apical side faces lumen, basal side faces blood vessels

Question 2

Thyroid Hormones

Answer 2

Thyroxine (T4) & Tri-iodothyronine (T3)

Question 3

Synthesis and secretion of thyroid hormones steps:

Answer 3

1. Iodine Trapping
2. Iodination of tyrosine on thyroglobulin forming mono or diiodotyrosine (MIT or DIT)
3. Coupling of MIT and DIT to form T3 and T4

Question 4

Iodine Trapping

Answer 4

• active transport against the gradient
• stimulated by TSH and cyclic AMP
• inhibited by thiocyanate and perchlorate
• iodine content in thyroid is 100x more than daily amount needed

Question 5

Iodination

Answer 5

• TG is synthesized in epithelium, exocytosed to lumen

- Apical peroxidase oxides iodide to iodine, iodinates tyrosine to form MIT or DIT

Question 6

Coupling

Answer 6

• MIT and DIT combine to form T3 or T4

- Iodinated TG is stored in the follicles as colloid

Question 7

Secretion of Thyroid Hormone

Answer 7

TSH stimulation -> pseudopodia extend into lumen -> TG is pinocytosed -> proteolytic cleavage -> secretion of T4 and T3 in the blood

liberation and recycling of free iodine and tyrosine for new hormone synthesis

Question 8

Feedback on Thyroid Hormone

Answer 8

Increased thyroid hormone levels inhibit TSH secretion, T3 is more potent than T4

Question 9

Autogregulation (Wolff-Chaikoff Effect)

Answer 9

Thyroid synthesis is directly proportional to iodide availability
When iodide intak exceeds 2 mg/day, free iodide inhibits iodide trapping, preventing synths and release

Question 10

Thyroid Hormone Transport

Answer 10

Enters target cells by carrier mediated active transport process
Target organ converts T4 to T3
T3 binds to nuclear receptors

Question 11

Functions: Growth

Answer 11

1. Endochondrial Ossification - maturation of epiphyses - linear growth
2. Induction of tooth development
3. Enhancement of normal growth cycle of epidermis and hair follicles
4. Inhibition of glycosaminoglycans
5. Maintains gonadal functions

Question 12

Functions: CNS

Answer 12

1. Proliferation of axons
2. Branching of dendrites
3. Myelination
4. Enhancement of cognitive behavior

Question 13

Functions: Cardiovascular

Answer 13

1. Increase cardiac output
2. Increase heart rate and contractility
3. Induce the synthesis of adrenergic receptors

Question 14

Functions: Metabolism

Answer 14

1. Increase GI glucose absorption
2. Potentiate effect of GH and catecholamines on gluconeogenesis, lipolysis, and proteolysis
3. Catabolic Effects

Question 15

Functions: BMR

Answer 15

1. Increase oxygen consumption (increases body temp, and synthesis of NA/K ATPase
2. Overall increase in basal metabolic rate

Question 16

Disposal of Thyroid Hormone

Answer 16

T4 and T3 circulate on thyroxine-binding globulin (TBG) - TBG has a large reservoir
T4 is converted to inactive reverse T3
T3 is inactivated to DIT and MIT

Question 17

Hyperthyroidism (Goiter)

Answer 17

Graves' Disease & Benign Neoplasms
Increased weight loss
Increase food intake
Increased heart rate
Exopthalmos

Question 18

Graves’ Disease

Answer 18

autoimmune in nature, antibodies bind to TSH receptor, constantly stimulate thyroid hormone synthesis and release

Question 19

Hypothyroidism Causes:

Answer 19

Lymphocyctic Thyroidities (Hashimoto’s Disease), Radiation Damage, Surgical Ablation, Hypothalamic or Pituitary Disorders, Iodine Deficiency, Excess iodine uptake

Question 20

Hashimoto’s Disease

Answer 20

autoimmune in nature - loose thyroid hormone synthesis

Question 21

Hypothyroidism Symptoms:

Answer 21

Low BMR, weakness, bradycardia, myxedema, damage to nerves, loss of hair, anemia, menstrual dysfunction, creatinism in infants