Thyroid Gland

Question 1

What does the thyroid gland produce?

Answer 1

T4, T3

Question 2

Functional unit of the thyroid gland

Lined by cuboidal cells

Lumen is filled with ____.

Contains C cells (parafollicular cells that secrete ____.)

Answer 2

Thyroid follicle

Colloid

Calcitonin

Question 3

Thyroid hormone contains ______.

What is the major secretory product of TH synthesis?

Colloid is composed of the newly synthesized thyroid hormones attached to ____.

Answer 3

Iodine

T4

Thyroglobulin

Question 4

Where is T4 converted to T3?

How?

What states cause this conversion?

Answer 4

In the thyroid and peripherally

Through the action of deiodinase

Fasting, medical and surgical stress, catabolic diseases

Question 5

Explain the conversion of T4 to T3

Answer 5

Dfdfd

Question 6

Sources of iodine

Answer 6

Soil, seawater
Cheese, cows milk, eggs, frozen yogurt, ice cream, multivitamins, table salt, saltwater fish, seaweed, shellfish, soy milk, yogurt

Question 7

TH synthesis:

Two transporters on the basolateral membrane that bring iodine into the cell.

Transporter on apical membrane that takes iodine out of cell.

What oxidizes iodide for combination with thyroglobulin?

What binds iodine to T4 and T3?

What then happens to the colloid droplet?

What cleaves T4 and T3 from thyroglobulin for release into circulation?

Answer 7

Na/K ATPase; Na/I symporter (NIS)

Pendrin (Cl/I counter-transporter)

Peroxidase

Thyroglobulin

Pincoytosis

Proteases

Question 8

Inhibitors of TH synthesis

What is the Wolff-Chaikoff effect?

Answer 8

Perchlorate and thiocynate: inhibit NIS

Propylthiouracil (PTU): inhibits peroxidase

Inhibits organification

Question 9

How is iodine stored?

Where?

For how long?

Answer 9

Iodinated as tyrosines of TG (more T3 than T4)

Follicular colloid

2-3 months

Question 10

How do you assess the activity of the thyroid gland?

Answer 10

Give a dose of radioactive iodine and measure uptake over a period of time

Hyperthyroidism: high uptake

Hypothyroidism: low uptake

Question 11

How is TH transported in the body?

Answer 11

Circulate in blood by being bound to plasma proteins or free

Binding proteins: thyroxin-binding protein (TBG), transthyreitin (TTR), albumin

Question 12

How do you assess circulating TBG?

Answer 12

Indirectly with T3 resin uptake test:

Wash TBG with unbound T3 and see how much binds to TBG, and how much T3 is free and absorbed.

Question 13

What causes changes in blood TBG?

Effects?

Answer 13

Hepatic failure: decreased blood TBG; increase level of free of T3 and T4; followed by inhibition of T3 and T4 synthesis through negative feedback

Pregnancy: increased blood TBG; increased bound (less free) T3 and T4; increases synthesis and secretion of T3 and T4; increase total levels of T3/T4 but free levels are normal

Question 14

Describe the hormonal control of thyroid hormone and its regulators

Answer 14

Hypothalamus-pituitary-thyroid axis:

TSH released from thyrotrophs of anterior pituitary; causes growth of thyroid gland (tropic effect) and secretion of TH

Regulated by TRH, free T3

Question 15

Explain thyroid hormone synthesis and secretion

Answer 15

?????

Question 16

Conversion of T to T3 occurs in ____ though the action of ____.

T3 binds to nuclear receptor/transcription factors of ____.

Answer 16

Thyroid and peripherally; deiodinase

TRalpha1
TRalpha1
TRbeta1
TRbeta2

Question 17

Actions of TH

Answer 17

Activates nuclear receptors and cAMP

Increased metabolic activity including BMR, CHO, lipid metabolism

Growth of fetus, neonate, adolescence

Increase cardiac output

GI motility

CNS development and excitation

Question 18

Intracellular actions of TH

Answer 18

Synthesis of new proteins:

Na/K ATPase
Transport proteins
Beta1-adrenergic receptors
Lysosomal enzymes
Proteolytic proteins
Structural proteins

Cardiac muscle: Myosin, beta1-adrenergic receptors, Ca ATPase

Question 19

Actions of TH on metabolism

Answer 19

Increased metabolic activity including BMR, CHO, lipid metabolism:

Increase O2 consumption.
Increase activity of Na/K ATPase.
Stimulates lipid mobilization, FA in blood, enhanced fat oxidation.
Cholesterol and TG in blood inversely correlated with thyroid hormone.
Conversion of carotene to vitamin A (hypothyroidism can lead to blindness).
Increase gluconeogenesis, glycogenolysis, insulin-dependent glucos uptake.

Question 20

Slow onset with lone duration action of TH (latent period)

Actions of T3 more rapid

Answer 20

Before T4 activity begins; 2-3 days before activity begins; 10-12 day to reach maximum; activity persists for 6-8 weeks

6-12 hours before activity begins; 2-3 days to reach maximum

Question 21

Cardiovascular actions of TH

Answer 21

Increase CO

Direct and indirect effects

Decreased TPR/systemic release

Increase inotropic effects

Increased blood volume/preload

Question 22

Indirect effects of TH on the cardiovascular system

Direct effects

Answer 22

Increase heat production and CO2 in tissues; decrease systemic vascular resistance; decrease diastolic BP; reflex increase adrenergic stimulation

Increase cardiac m of myosin heavy chain; increase ventricular contractility; decrease systemic vascular resistance

Question 23

Actions of TH one sympathetic sensitivity

Answer 23

Increased CO

High levels of thyroid hormone increases number of beta1-adrenergic receptors

More sensitive to stimulation by sympathetics

Question 24

Actions of TH on growth and CNS

Answer 24

Fetus, neonate, adolescence: ???

Question 25

What are the levels of hyperthyroidism?

Answer 25

Excessive TH production (thyrotoxicosis)

Primary: Graves’ disease

Secondary: TSH secreting pituitary tumor

Changes in TSH levels: decrease because negative feedback of T3 on anterior pituitary

Question 26

What is Graves Disease?

Answer 26

Hyperthyroidism

Produce antibody (thyroid-stimulating immunoglobulins, TSI) that binds to TSH receptor and constantly turns it on; doesn’t respond to negative feedback mechanisms

Question 27

Types of hypothyroidism

Answer 27

Gland destruction (Hashimoto’s thyroiditis)

Inhibition of hormone synthesis and release (iodine deficiency)

Hypothalamic disease, pituitary disease, resistance to TH

Question 28

Treatment of hypothyroidism

Answer 28

Replacement doses of T4

**

Question 29

Disease caused by thyroid hormone synthesis is impaired by antibodies agains thyroglobulin or TPO, leading to decreased T3 and T4 secretion

TSH levels high, trophic effect leads to goiter

Answer 29

Hashimoto’s thyroiditis

Question 30

Disease caused by iodide deficiency, maternal intake of anti-thyroid medications leading to impaired development of thyroid gland

Answer 30

Hypothyroidism Cretinism

Question 31

Hypothyroidism due to iodine deficiency

Answer 31

TSH levels elevated

Goiter

Question 32

When can a goiter develop?

Answer 32

Imbalances within the HPT axis:

Hyperthyroidism -> Graves’ disease, TSH-producing tumor

Primary hypothyroidism -> lack of iodine in diet, sporadic hypothyroidism, chronic thyroiditis (hashimoto’s disease/thyroiditis, autoimmune-induced deficiency in thyroid fx)