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Flashcards in Thyroid Physiology Deck (33)
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1

T3

Triiodothyronine

2

T4

Thyroxine
- more abundant

3

Thyroid hormones are composed of ____ and _____

Tyrosine; iodine

4

Functions of thyroid hormones

- increased metabolic rate
- protein, lipid, CHO

5

Thyroid anatomy

Bi-lobed gland in cervical neck
- vascular!
- parathyroid glands (2 per lobe) = calcium metabolism

6

What is the functional unit of the thyroid

Follicle

7

Follicular cells

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

8

Parafollicular cells

Secrete calcitonin
- calcium regulation

9

T3/T4 synthesis

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

10

Thyroglobulin

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

11

_____ is required for T3/T4 synthesis

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

12

Organification of iodine to tyrosine residues

Iodine added to tyrosine
- monoiodotyrosine (MIT)
- diiodotyrosine (DIT)

13

Coupling of iodinated tyrosine residues

- DIT + DIT = T4
- MIT + DIT = T3

14

Release of hormone

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

15

Thyroid peroxidase

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

16

___ is more active than ____

T3; T4
- T3 enters cells more rapidly
- T3 is more potent

17

How much T3 is made in thyroid?

20%
- rest from de-iodination of T4 in tissues
- deiodinases present in many cell types (liver, kidney, muscle)

18

Deiodinase

- T4 is deiodinated to active T3 or inactive reverse T3
- rT3 formed during times of illness, starvation

19

Protein-binding

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

20

Proteins

- thyroxine-binding globulin
- albumin

21

Only the ______ is active

Free hormone
- lipophilic, enters cell

22

Drugs alter protein-binding

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

23

Thyroid hormones in circulation

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

24

Thyroid hormones do not affect which tissues?

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

25

Metabolic effects of thyroid hormone

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

26

Cardiovascular effects of thyroid hormone

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

27

Neuromuscular effects of thyroid hormone

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

28

Hypothalamus

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

29

Pituitary gland

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

30

Thyroid axis has _______

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