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Flashcards in Thyroid Physiology Deck (28)
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1
Q

What is the arterial supply to the thyroid?

A

Superior thyroid artery
-From external carotid

Inferior thyroid artery
-From thyrocervical trunk

Occasional supply from IMA (10% of population)

2
Q

What is the venous drainage of the thyroid?

A

Superior, middle and inferior thyroid veins

3
Q

What 3 hormones are produced by the thyroid gland and what are they involved with?

A

Thyroxine (T4) and
Tri-iodothyronine (T3) (basal metabolic rate)
Calcitonin (calcium homeostasis)

4
Q

In what part of the thyroid gland are the thyroid hormones made and stored?

A

Follicles

5
Q

What cells of the thyroid gland produce calcitonin?

A

Parafollicular C cells

6
Q

What are the 6 steps of thyroid hormone synthesis?

A

Occurs in follicular cells

1) Thyroglobulin synthesised- tyrosines on surface
2) Uptake and concentration of iodide
3) Oxidation of iodide into iodine catalysed by TPO
4) Iodine is conjugated to 1 or 2 tyrosines on the thyroglobulin molecules. Forms either Mono or Di-iodotyrosine (MIT or DIT)
5) MIT and DIT join together to form either T3 or T4.
6) When needed, T3 and T4 taken up into the follicle cell into vesicle attached to lysosome which secretes protease. Protease separate the thyroglobulin ( via hydrolysis) which re-enters the colloid via pendrin to be recycled. T3 and T4 are secreted into the blood.

7
Q

Describe the regulatory process of T3 and T4 secretion

A

Hypothalamus releases TRH which acts on anterior pituitary which then releases TSH which stimualates thryoid to produce T3 and 4.
Via a negative feedback loop, T3 and 4 act on hypothalamus and anterior pituitary to decrease stimulation of T3 and T4 secretion.

8
Q

How does TSH increase production of T3 and T4?

A

Increases action at every stage of T3 and T4 production in the follicles.

9
Q

How are thyroid hormones transported in the blood?

A

Lipophilic so can’t dissolve in blood.
70% bound to thyroxin binding globulin (TBG)
30% bound to albumin
(TBG has a higher affinity for T4)

10
Q

Which is the active thyroid hormone? Which is produced in higher supply?

A

T3 is active

T4 is inactive and produced in large quantities, must be converted to T3 to be used.

11
Q

How do T3 and T4 enter the cell?

A

Only free T3 and 4 can enter the cell. Binding proteins release the hormones when required.

12
Q

Which receptor receives thyroid hormone?

A

T3r- receives T3

13
Q

What converts T4 to T3?

A

Deiodinases (enzymes)

Can activate or inactivate thyroid hormone.

14
Q

What are the different types of deiodinases and their roles?

A

Type 1: activate T4 by converting it to T3 by inner and outer ring deiodination. (exist on cell surface of most cells)

Type 2: Activates T4 by turning it to T3 (intracellular, CNS, brown fat, placenta, skeletal and cardiac muscle)

Type 3: Inactivating, turns T4 into reverse T3 (rT3) or T3 into T2. (placenta and CNS)

15
Q

How do T3 and T4 enter the and act on the cell?

A

Via diffusion or by MCT8(10) transporter.

T3 (T4 converted first by deiodinases) binds to receptor and is then taken to the nucleus where it binds to DNA and alters protein synthesis.

16
Q

What are the physiological actions of thyroid hormone?

A

Metabolism- affects basal metabolic rate
Maturation and differentiation- bone, lung and brain tissue
Neurological function- synapse formation, myelinogenesis, neuronal outgrowth
Growth- Regulated by GH

17
Q

What are the metabolic actions of thyroid hormone?

A

Increases basal metabolic rate by increasing O2 consumption.
Increases O2 consumption by increasing ventilation, heart rate, myocardial activity (therefore CO) and increased metabolism of substrates.

18
Q

What are the effects of increased metabolism?

A

Increased CO2 therefore increased increased ventilation, increased urea, renal function. Decreased muscle and adipose tissue.

19
Q

What thyroid function test results would indicate hyperthyroidism?

A

High T4, low TSH (low TSH should normally cause low T4)

20
Q

What thyroid function test result would indicate hypothyroidism?

A

Low T4, High TSH (normally high TSH should cause high T4)

21
Q

What would low T4 and low TSH (or high T4 and high TSH) indicate and why?

A

Anterior pituitary gland problem (low T4 should normally stimulate secretion of TSH and high T4 should cause low TSH)

22
Q

How does hypothyroidism from iodine deficiency produce a goitre?

A

Reduced T4 stimulates anterior pituitary to produce TSH which stimulates thyroid gland to produce T4, however it is unable due to iodine deficiency. This results in thyroid gland hypertrophy and a goitre.

23
Q

How does hyperthyroidism produce a goitre?

A

(Grave’s disease only) Thyroid Stimulating Immunoglobulin (TSI) produced which mimics TSH so stimulates thyroid gland to produce T4. Increased T4 causes anterior pituitary to release less TSH however TSI is still continually stimulating thyroid gland to produce T4. Over stimulation results in hypertrophy and goitre.

24
Q

What are the signs and symptoms of hyperthyroidism?

A

Heat intolerance, sweating, weight loss despite increased appetite, diarrhoea, palpitations, fatigue and weak muscles, nervousness, irritability, thirst, polyuria, goitre (Grave’s disease only) protruding eyes (Grave’s only)
Symptoms due to increased metabolic rate.

25
Q

What are the signs and symptoms of hypothyroidism?

A

Fatigue, cold sensitivity, constipation, dry skin and hair (alopecia), low mood, mental slowness, goitre, weight gain, heavy periods and fertility problems.
Symptoms due to low metabolic rate.

26
Q

What are the causes of hyperthyroidism?

A

Grave’s disease

27
Q

What are the causes of hypothyroidism?

A

Iodine deficiency, hasimoto’s thyroiditis (autoimmune)

28
Q

What is used to treat hypothyroidism?

A

Levothyroxine, synthetic T4. Metabolised to T3 in body as required.