Thyroid Gland

Question 1

Location of gland

Answer 1

Anterior to upper trachea and lower lanynx

Question 2

Structure of gland

Type of cells found in it

Answer 2

Butterfly shaped with two lobes connected by an isthmus
2-3 cm across
follicular cells
parafollicular cells

Question 3

How are cells arranged in thyroid?

Answer 3

Spherical follicles surround central space and are lined ewith epithelial cells

Question 4

Hormones produced in thyroid?

Answer 4

T3
T4
Calcitonin

Question 5

How are T3 and T4 synthesised?

Answer 5

• • transport of iodide into the epithelial cells against a concentration gradient.
• • synthesis of a tyrosine rich protein (thyroglobulin) in the epithelial cells.
• • secretion (exocytosis) of thyroglobulin into the lumen of the follicle
• • oxidation of iodide to produce an iodinating species.
• • iodination of the side chains of tyrosine residues in thyroglobulin to form MIT (mono-
    iodotyrosine) and DIT (di-iodotyrosine).
• • coupling of DIT with MIT or DIT to form T3 & T4 respectively within the thyroglobulin. T3 & T4 residues are produced in the ratio of ~1:10

Question 6

Storage of T3 and T4?

Answer 6

Lumen of follicles in vesicles

Question 7

How is release of T3 and T4 controlled?

Answer 7

TRH from hypothalamus which stimulates …
TSH from pituitary which stimulates …
T3 and T4 from thyroid

Question 8

What increased release of TRH?

How does it travel to stimulate release of TSH?

Answer 8

Stress and decreasing temperature

Hypothalamic pituitary portal system

Question 9

Where is TSH released from?

When is it usually released?

Answer 9

Thyrotrophs in pituitary gland

Diurnal rhythm – increase release at night and decreased in morning

Question 10

What does TSH do?

Answer 10

Bind to receptors on follicular cells stimulating synthesis and secretion of T3 and T4.
Also has trophic effect on follicular cells which can lead to goitre

Question 11

How are thryroid hormones transported in the blood?

Answer 11

Transported in the blood bound to proteins (thyronine binding globulin, pre-albumin and albumin)
— only small amount is free and therefore able to exert its effects on the body

Question 12

Describe changes that take place in pregnancy to thyroid hormone production

Answer 12

Oestrogens increase the synthesis of TBG during pregnancy and this produces a fall in the amount of T3 & T4 in the circulation as more is bound. The fall in free T3 &T4 removes the inhibitory feedback on the pituitary and hypothalamus. More TRH and TSH are produced and the thyroid gland secretes more T3 & T4. As a result the amount of free T3 & T4 returns to normal but the total amount in the blood is increased.

Question 13

What are the physiological actions of T3 and T4?

Answer 13

1. Increased metabolic activity – increased glucose uptake & metabolism, increased protein metabolism. Lead to increased BMR, O2 consumption & heat production
2. Normal growth and development – bone mineralisation
3. CNS – development of nerve cells, hyperplasia of cortical neurons and myelination of nerve fibres.
4. Stimulate hormone and neurotransmitter receptor synthesis in a variety of tissues (eg tachycardia in heart muscle, increased motility in GI tract). This increases responsiveness in tissues
5. Role in LH and FSH actions – ovulation doesnt occur in absence of

Question 14

What is cretinism?

Answer 14

In the absence of thyroid hormones from birth to puberty the child remains mentally and physically retarded (cretinism).

Question 15

Describe how T3 and T4 have their effect on cells

Answer 15

T3 & T4 cross the plasma membrane of target cells and interact with specific high affinity receptors located in the nucleus and possibly mitochondria.
Binding of T3 to the hormone-binding domain is thought to produce a conformational change in the receptor that unmasks the DNA-binding domain.
Interaction with DNA binding domain increases the rate of transcription of specific genes that are then translated into protein.
Due to increased protein production, there is an increased demand for energy.

Question 16

How can T4 be converted into T3?

Answer 16

T4 can be converted to T3 in tissues by removal of the 5’-iodide.

Question 17

How can reverse T3 be produced?

Answer 17

Removal of the 3’-iodide produces inactive reverse T3 (rT3). rT3 can bind to thyroid hormone receptors without stimulating them, but it blocks the effect of T3.

Question 18

Define hyperthyroidism

Answer 18

Too much thyroid hormones

Question 19

Define hypothyroidism

Answer 19

Too little thyroid hormones

Question 20

Describe hashimotos disease

Answer 20

most common form of hypothyroidism
It is an autoimmune disease caused by:
– destruction of the thyroid follicles (no synthesis of T3 and T4)
– production of an antibody that blocks the TSH receptor on follicle cells preventing them from responding to TSH (no T3 and T4 production)

Question 21

How is hashimotos treated?

Answer 21

Oral thyroxine (T4)

Question 22

Describe Graves disease

Answer 22

most common form of hyperthyroidism
autoimmune disease in which antibodies are produced that stimulate the TSH receptors on follicle cells resulting in increased production and release of T3 & T4.

Question 23

How is graves disease treated?

Answer 23

Treated with carbimazole

• • drug that inhibits the incorporation of iodine into thyroglobulin
• • Inhibits the enzyme thyroid peroxidase and therefore prevents coupling and iodination

Question 24

Hypothyroidism symptoms and why they occur

Answer 24

• • Cold intolerance (reduced BMR)
• • Weight gain (reduced BMR)
• • Tiredness and lethargy (reduced nutrient uptake)
• • Bradycardia (reduces responsiveness to catecholamines and reduced synthesis of heart muscle proteins)
• • Neuromuscular system - weakness, muscle cramps and cerebellar ataxia (clumsiness of
    movement) .
• • Skin dry and flaky (reduced protein synthesis)
• • Alopecia (reduced protein synthesis)
• • Voice is deep and husky
• • Constipation (reduced responsiveness of GI tract)

Question 25

Hyperthyroidism symptoms and why they occur

Answer 25

- - Weight loss (increased metabolic rate) - - Heat intolerance, sweating, warm vasodilated hands - - Irritability, emotional lability - - Tachycardia (noticeable heart beat) often irregular (increased catecholamine synthesis) - - Fatigue and weakness - - Increased bowel movements - increased appetite (increased responsiveness of GI tract) - - Menstrual dysfunction (has effect on LH and FSH) - - Hyper-reflexive - - Possible tremor of outstretched hands