Thyroid Physiology

Question 1

T3

Answer 1

Triiodothyronine

Question 2

T4

Answer 2

Thyroxine

- more abundant

Question 3

Thyroid hormones are composed of ____ and _____

Answer 3

Tyrosine; iodine

Question 4

Functions of thyroid hormones

Answer 4

• increased metabolic rate

- protein, lipid, CHO

Question 5

Thyroid anatomy

Answer 5

Bi-lobed gland in cervical neck

• vascular!
• parathyroid glands (2 per lobe) = calcium metabolism

Question 6

What is the functional unit of the thyroid

Answer 6

Follicle

Question 7

Follicular cells

Answer 7

Single layer of cuboidal columnar thyroid cells surrounding lumen
- lumen filled with colloid

Question 8

Parafollicular cells

Answer 8

Secrete calcitonin

- calcium regulation

Question 9

T3/T4 synthesis

Answer 9

Thyroglobulin synthesis

• iodide trapping in follicular cell
• iodide oxidation
• organification: adding I to tyrosine resides on TG
• coupling of iodinated tyrosine residues
• release of hormone

Question 10

Thyroglobulin

Answer 10

Precursor protein

• stored in colloid
• synthesized in follicular cells
• very large!!!
• thyroid hormone synthesized on tyrosyl residues of TG

Question 11

_____ is required for T3/T4 synthesis

Answer 11

Iodide

• absorbed from GIT
• excreted by kidney
• uptake by thyroid gland via iodide trapping and iodide pump (active)
• uptake by salivary gland

Question 12

Organification of iodine to tyrosine residues

Answer 12

Iodine added to tyrosine

• monoiodotyrosine (MIT)
• diiodotyrosine (DIT)

Question 13

Coupling of iodinated tyrosine residues

Answer 13

• DIT + DIT = T4

- MIT + DIT = T3

Question 14

Release of hormone

Answer 14

• endocytosis of colloid
• proteases break bonds between TG and T3/T4
• released into cirulation

Question 15

Thyroid peroxidase

Answer 15

• iodide oxidation
• organification
• coupling
• hyperthyroidism drugs inhibit thyroid peroxidase

Question 16

___ is more active than ____

Answer 16

T3; T4

• T3 enters cells more rapidly
• T3 is more potent

Question 17

How much T3 is made in thyroid?

Answer 17

20%

• rest from de-iodination of T4 in tissues
• deiodinases present in many cell types (liver, kidney, muscle)

Question 18

Deiodinase

Answer 18

• T4 is deiodinated to active T3 or inactive reverse T3

- rT3 formed during times of illness, starvation

Question 19

Protein-binding

Answer 19

Necessary for transport

• 99% T3 and T4 are protein bound
• binding affinity determines half life = more rapid degradation of unbound hormone

Question 20

Proteins

Answer 20

• thyroxine-binding globulin

- albumin

Question 21

Only the ______ is active

Answer 21

Free hormone

- lipophilic, enters cell

Question 22

Drugs alter protein-binding

Answer 22

Usually do not alter free T4 concentrations

• may bind to same proteins, lowering protein-bound fraction = decrease of total T4
• increase binding protiens = increased TBG, increase overall T4, not free T4

Question 23

Thyroid hormones in circulation

Answer 23

T4: 99% protein bound
- free form is biologically active
T3: 99% protein bound
- free form is biologically active
- reverse T3 has no biological activity

Question 24

Thyroid hormones do not affect which tissues?

Answer 24

• lungs
• testes/uterus
• retinas
• lymph nodes/spleen

Question 25

Metabolic effects of thyroid hormone

Answer 25

- increases metabolic rate via oxidative phosphorylation in mitochondria - catabolic effects: lipolysis, protein, carb utilization, GI absorption - increased GI motility

Question 26

Cardiovascular effects of thyroid hormone

Answer 26

- stimulation of cardiac beta adrenergic receptors = increased HR and cardiac output - increased GFR - stimulates erythropoiesis

Question 27

Neuromuscular effects of thyroid hormone

Answer 27

- increased mental alertness | - necessary for normal growth and neuronal development

Question 28

Hypothalamus

Answer 28

Input from higher centers in brain, body | - thyrotropin releasing hormone = increased TSH release

Question 29

Pituitary gland

Answer 29

Thyroid stimulating hormone - binds TSH receptors in thyroid - stimulates synthesis of T3, T4

Question 30

Thyroid axis has _______

Answer 30

Negative feedback - free thyroid hormone inhibits TRH and TSH secretion - increased T4 leads to decreased TRH and TSH

Question 31

TSH promotes

Answer 31

- iodine trapping - colloid endocytosis - thyroid cell hypertrophy

Question 32

Goiter

Answer 32

Decreased thyroid hormone secretion --> decreased negative feedback --> increased TSH = thyroid cell hypertrophy - rare in dogs, seen in cats

Question 33

If a normal cat is given a large amount of T3, what should happen to the TRH, TSH, and T4 concentrations?

Answer 33

Should decrease TRH, TSH, and T4