Thyroid Gland physiology

Question 1

What hormones does the thyroid gland produce?

Answer 1

Active T3 (triiodothyronine) and inactive T4 (tetraiodothyronine)

Question 2

Describe the structure of the thyroid gland:

Answer 2

Thyroid follicle are surrounded by epithelial cells. Lumen in the center where thyroid hormones are stored in the colloid.
Also has parafollicular C cells that secrete calcitonin

Question 3

What is the back bone of the thyroid hormone?

Answer 3

Thyroglobulin

Question 4

What molecule is important for thyroid hormone synthesis?

Answer 4

Iodine (I2) from diet converted into I- and brought into thyroid gland via the iodide trap. Attached to the thyroid hormone

Question 5

Draw out the synthesis equation of T3 and T4. What is the major product of this pathway?

Answer 5

Inactive T4. Later converted to T3 when needed via peripheral conversion

Question 6

Draw the conversion pathway of T4 to T3

Answer 6

Ok

Question 7

Peripheral conversion of T4 to active T3 can be hindered by …

Answer 7

Fasting
Physical stress
Catabolic disease

Question 8

Draw the synthesis pathway of T3 and T4 inside the thyroid follicle

Answer 8

Ok

Question 9

What enzyme recycles Thyroglobulin and Iodine? What happens if this is deficient?

Answer 9

Intrathyroidal deiodinase. Deficiency mimics Iodine dietary deficiency.

Question 10

If there is low Iodine available, what is the favored product of thyroid synthesis?

Answer 10

T3 (requires less iodine)

Question 11

What transports Iodine into the lumen of the thyroid follicle?
What happens if there is a mutation in this transporter?

Answer 11

Pendrin (a chloride iodide pump). Mutation = hypothyroidism = goiter and hearing loss since cochlear pendrin is also mutated.

Question 12

How does hypothyroidism cause goiter?

Answer 12

TSH keeps stimulating the thyroid trying to make it make thyroid hormones = hyperplasia

Question 13

What is perchlorate and thiocyanate?

Answer 13

Anions that compete with iodine binding to transporter. Decreases synthesis of thyroid hormones

Question 14

How does PTU disrupt thyroid hormone synthesis?

What effect does this cause?

Answer 14

Disrupts peroxidase (which combines Iodine with TG)
Causes buildup of Iodine which inhibits further synthesis of hormoness (Wolff-Chalkoff effect)

Question 15

How does radioactive iodine uptake relate to thyroid gland activity?

Answer 15

Hyperthyroidism = more iodine uptake
Hypothyroidism = less iodine uptake

Question 16

What are the main binding proteins for thyroid hormones in the bloodstream?

Answer 16

Thyroxine binding globulin (TBG, higher affinity for T4)
Transthyretin (in brain tissue)
Albumin (everything else)

All of these are formed in the liver

Question 17

Predict the T4 and resin uptake in these conditions (remember the pattern)
Hyperthyroidism
Hypothyroidism
High TBG
Low TBG
Hepatic failure
Pregnancy

Answer 17

Ok

Question 18

Draw the HPT axis

Answer 18

OK

Question 19

What is the pathway of TSH action on the thyroid gland?

Answer 19

TSH > GPCR > Adenlyl Cyclase > cAMP > mediate lots of things that help with T3/T4 production as well as thyroid gland growth

Question 20

What is the mechanism of action of thyroid hormones?

Answer 20

-Act like steroid hormones (nuclear receptor)

Question 21

Functions of thyroid hormones:

Answer 21

Growth
Synapse plasticity
BMR increase
Increase in cardiac output and B-adrenergic receptors

Question 22

How exactly do thyroid hormones increase basal metabolic rate?

Answer 22

Thyroid hormone increases activity of Na/K ATPase leading to increase of O2 consumption and heat production]
Effect of T4 (thyroxine) is usually long lasting

Question 23

Effects of thyroid hormone on lipid metabolism:

Answer 23

• Increase fat mobilization and FA oxidation
• Blood cholesterol level inversely correlated with thyroid hormone levels
• needed for carotene conversion to Vitamin A (eye problems and jaundice in hypothyroid patients)

Question 24

Describe the effect of thyroid hormones on cardiovascular system:

Answer 24

Thyroid > T4 > T3 > Na/K ATPase > increase tissue thermogenesis > decrease systemic vascular resistance > increase speed and force of cardiac contraction > increase cardiac output > increase blood volume

Question 25

Describe how thyroid hormones directly stimulate cardiac muscle:

Answer 25

Bind to B1 adrenergic receptors during sympathetic stimulation > increase in cardiac rate

Question 26

What are the other effects of thyroid hormones?

Answer 26

``` Growth (working with growth hormone) CNS maturation (synaptic plasticity ```

Question 27

What happens to neurons when there is thyroid deficiency?

Answer 27

Abnormal development of synapse, decreased dendrites or myelination If in prenatal period = congenital hypothyroidism

Question 28

What happens to TSH levels in primary hyperthyroidism? | How about ins secondary hyperthyroidism?

Answer 28

Lots of Thyroid hormones feedback and stop production of TSH | TSH levels are elevated since the defect is in the AP

Question 29

What causes grave's disease?

Answer 29

Thyroid stimulating immunoglobulins act as agonist for TSH which causes release of thyroid hormones that feedback to TSH

Question 30

Symptoms of Grave's disease:

Answer 30

Exophthalmos (eyeball protrusion) and periorbital edema | Elevated thyroid hormones, goiter. Presence of TSI in the blood (defining feature of Grave's)

Question 31

What are the primary causes of hypothyroid?

Answer 31

Destruction of gland (as in Hashimoto's thyroiditis) Thyroid hormone synthesis inhibition Agenesis of thyroid Short term reasons (after surgery/postpartum etc.)

Question 32

How do you treat hypothyroidism?

Answer 32

Replacement doses of T4 (half life increases with age so younger patients need more doses) Overprescribing T4 can lead to osteoporosis in menopausal women

Question 33

Hashimoto's thyroiditis: Cause Clinical

Answer 33

-Thyroid gland defective even if I2 is sufficient Caused by thyroglobulin or TPO antibodies which affects synthesis and secretion -TSH levels are high because TSH keeps poking the thyroid gland to make hormones (trophic effect = goiter)

Question 34

Congenital hypothyroidism: Cause Clinical

Answer 34

- Iodide deficiency or taking anti-thyroid meds during pregnancy leading to impaired development of thyroid gland - protruding tongue, curse facial features, growth and mental retardation, hypotonia and jaundice

Question 35

Iodide deficiency

Answer 35

- No iodine = can't make proper hormones | - TSH will be elevated since it keeps poking the thyroid gland to make hormones, can lead to goiter

Question 36

Sheehan syndrome: Cause Clinical

Answer 36

- Necrosis of pituitary gland usually postpartum | - difficulties with lactation, no menses ( amenorrhea), hypothyroidism, other endocrine dysfunction also present

Question 37

Generally what are the symptoms of hyper and hypothyroidism?

Answer 37

Hyper: weight loss, sweating, rapid HR and high BP because there's too much thyroid in the body and increases body functions Hypo: weight gain, cold intolerance, fatigue because there's too little thyroid in the body and decreases body functions

Question 38

Goiter development in hyperthyroidism:

Answer 38

TSH or something else stimulates/attacks the thyroid gland too much that it swells

Question 39

Goiter development in hypothyroidism:

Answer 39

I2 deficiency or thyroiditis (TSH pokes the thyroid gland too much in causes it to hypertrophy)