Hormones

Question 1

Where is the thyroid gland found?

Answer 1

Discrete organ

Adheres to the trachea

Question 2

How many lobes make up the thyroid gland?

Answer 2

2 lobes

Connected by isthmus

Question 3

At what week of gestation is the thyroid gland fully developed and responsive to thyroid stimulating hormone?

Answer 3

Week 12

This highlights the critical role of thyroid hormones in development

Question 4

What is the functional unit of the thyroid gland?

Answer 4

The follicle

There are thousands of follicles in each gland

Question 5

What are the two hormones secreted by the thyroid gland?

Answer 5

Triiodothyronine (T3)

Thyroxine (T4)

Question 6

Does the thyroid gland secrete more T3 or T4?

Answer 6

The thyroid secretes more T4 (thyroxine) than T3 (triiodothyronine)

HOWEVER

triiodothyronine is more biologically active

Question 7

What are the main features of a follicle in the thyroid gland?

Answer 7

Follicular cells (simple cuboidal epithelial) surrounding a colloid (glycoprotein) filled cavity.

Question 8

What is the function of C cells in a thyroid follicle?

Answer 8

C cells secrete calcitonin

Question 9

In a typical day, how much triiodothryonine is secreted?

Answer 9

5 micro grams

Question 10

In a typical day, how much thyroxine is secreted?

Answer 10

80 micro grams

Question 11

What amino acid makes up T3 and T4?

Answer 11

Tyrosine

Question 12

What class of hormones do thyroid hormones belong to?

Answer 12

Amino acid derived hormones

Question 13

How can the half life of thyroid hormones be prolonged in the body?

Answer 13

The half life of hormones is extended if they are bound to a carrier protein.

Thyroxine and triiodothyronine can be bound to Albumin and thyroid binding globulin.

Question 14

What is the half life of T3 (triiodothyronine) in the body?

Answer 14

T3 = 1-3 days

Question 15

What is the half life of T4 (thyroxine) in the body?

Answer 15

T4 = 5-7 days

Question 16

What is the half life of reverse T3 (rT3)?

Answer 16

5 hours

Question 17

What are the two raw ingredients needed to make thyroxine (T4) and triiodothyronine (T3)?

Answer 17

Tyrosine (provided by thyroglobulin)

Iodine

Question 18

Discuss thyroid hormone synthesis…

Answer 18

1) Thyroglobulin is produced in the follicular cells
2) It is then packaged into vesicles and released into the colloid
3) Iodide from diet is actively transported into follicular cells from blood
4) As iodide leaves the follicular cells and enters the colloid it is oxidised to iodine (by TPO - thyroid peroxidase
5) Tyrosine residues in thyroglobulin are iodinated forming mono and di iodotyrosine.
6) MIT and DIT are then linked together to form T3 and T4 (still attached to thyroglobulin)
7) T3 and T4 are endocytosed back into follicular cells and you have enzymes which cleave rest of thyroglobulin.
8) Hormones diffuse into bloodstream

Question 19

What medication can be used as a synthetic thyroid hormone and which form is it T3 or T4?

Answer 19

Levothyroxine
T4

T4 will then be converted to the more biologically active T3 in the peripheral tissues

Question 20

Thyroid hormone effects

Answer 20

Calorigenesis
Growth and maturation
Electrolyte balance
Vitamin metabolism
Cardiovascular system
Pregnancy

Question 21

What is the principle metabolic action of thyroid hormone?

Answer 21

Increases mobilisation and utilisation of glucose, fat and protein

Question 22

How is thyroid hormone involved in heat production?

Answer 22

Important in temperature regulation and adaption to cold environments. Calorigeneis is the generation of heat

Question 23

What are the principal cardiac effects of thyroid hormone?

Answer 23

Increased cardiac output

Increased ventilation rate

Question 24

In order to release T3 and T4 from the thyroid gland, what is released from the HYPOTHALAMUS and PITUITARY gland?

Answer 24

Hypothalamus secretes thyrotropin releasing hormone

Anterior pituitary secretes thyroid stimulating hormone

Question 25

What are the two principal effects of thyroid stimulating hormone released from the anterior pituitary?

Answer 25

Promoting the release of T3 and T4 into the blood

Increasing the activity of the iodide pump so more can be actively transported into the follicular cells from the diet

Question 26

The hypothalamus secretes thyrotropin releasing hormone, what can trigger this release and what can inhibit this?

Answer 26

Being in a very cold environment can stimulate the hypothalamus to release thyrotropin releasing hormone

Being stressed can have a negative impact on thyroid hormone production

Question 27

What are the symptoms of hypothyroidism?

Answer 27

Dry cold skin
Sensitivity to cold - no calorigenesis
Weight gain 
Impaired memory
Mental dullness
Lethargy

Question 28

What is the principal action of thyroid hormone on growth and development?

Answer 28

Essential for normal growth of tissues - CNS
Synergy with growth hormone
Thyroid hormone deficiencies can result in mental impairment and short stature

Question 29

Upon clinical exam, what would be the main features of hypothyroidism?

Answer 29

Reduced metabolic rate
Slow pulse due to reduced cardiac output

Diagnosis would often be based on low plasma levels of T3 and T4

Question 30

What happens to TSH levels if you have low circulating levels of T3 and T4?

Answer 30

TSH levels would increase because there is no negative feedback stopping TSH production by the anterior pituitary

Question 31

What are the main causes of goitre?

Answer 31

Iodine deficiency

Cannot make thyroid hormones as you are lacking one of the key ingredients. Therefore you have insufficient amounts of T3 and T4 but the anterior pituitary doesn’t know that and it keep chucking out TSH to try to stimulate the thyroid gland to make more T3 and T4. TSH has trophic effects and makes the thyroid grow.