The Thyroid Gland

Question 0

What is the thyroid gland called if it enlarged?

Answer 0

A goitre

Question 1

Where is the thyroid gland located?

Answer 1

In the upper neck

In front of the larynx and upper trachea

Question 2

Structure of thyroid?

Answer 2

Two lateral lobes joined by a central isthmus - butterfly shaped
2-3cm across and weighs ~20g
Follicular cells arranged in functional units called follicles.
Para follicular cells found in connective tissue.

Question 3

Structure of follicles in thyroid gland?

Answer 3

Spherical follicle cells lined with epithelium surrounding a lumen containing the protein

Question 4

Which hormones are produced in the thyroid and from which cells?

Answer 4

T3 (thyroxine) and T4 in the follicular cells

Calcitonin in the parafollicular cells

Question 5

Synthesis of T3 and T4?

Answer 5

Transport of iodide ions into epithelial cells against a concentration gradient
Synthesis of a tyrosine rich protein, thyroglobulin in the epithelial cells
Secretion of thyroglobulin into the lumen
Oxidation of iodide to give an iodinating species
Iodination of tyrosine residues in thyroglobulin to form MIT and DIT
Coupling of DIT with MIT (T3) or with DIT (T4) in the thyroglobulin.

Question 6

What are T3 and T4 produced in the ratio of?

Answer 6

1:10

Question 7

Where are T3 and T4 stored?

Answer 7

Extracellularly in the lumen of the follicles as part of the thyroglobulin molecules.

Question 8

How are T3 and T4 secreted?

Answer 8

Thyroglobulin is taken into epithelial cells from the lumen by endocytosis
Proteolytic cleavage to release T3&4
They diffuse from cells into the circulation

Question 9

How is thyroid hormone secretion controlled?

Answer 9

TRH released from dorsomedial nucleus of hypothalamus under the influence of circulating T3&4 (negative feedback)
This stimulates release of TSH from thyrotrophs in the anterior pituitary
TSH travels in blood to affect follicular cells of the thyroid

Question 10

What stimulates the release of TRH?

Answer 10

Low levels of T3&4
Low temperature
Stress

Question 11

What does TSH do?

Answer 11

Interacts with receptors on the surface of follicle cells and stimulates synthesis and secretion of T3&4.
Has trophic effects on the thyroid causing increased vascularity, size and number of follicle cells.

Question 12

Which has a lower affinity for binding to transport proteins in the blood? T3 or 4

Answer 12

T3 has a lower affinity so it has a greater percentage free in the blood (less T3 is produced that T4)

Question 13

What happens during pregnancy to the levels of hormones concerning the thyroid gland?

Answer 13

Oestrogen increases the synthesis of Thyroid Binding Globulin
Fall in the amount of free T3&4 in circulation
Removed inhibitory feedback so more TRH and TSH are produced
More T3&4 secreted
Same amount of free T3&4 in blood but there is more T3&4 over all

Question 14

What effect do the thyroid hormones have?

Answer 14

Increase metabolic rate
Stimulate glucose uptake
Stimulate mobilisation and oxidation of fatty acids
Stimulate protein metabolism
Increase in BMR, heat production and oxygen consumption
Affect bone mineralisation
Development of nervous system

Question 15

How do T3&4 act on a cell

Answer 15

Pass through plasma membrane
Bind to high affinity receptors in the nucleus and mitochondria (higher affinity for T3 than T4)
Binding of T3 to hormone-binding domain unmasks the DNA binding domain
This increases the rate of transcription of specific genes

Question 16

How can T4 be converted to T3 and what effect does this have?

Answer 16

Removal of 5’ iodide from T4 producing rT3
rT3 can bind to hormone receptors without having any effect - inactive
Blocks the effect of T3
This is important in regulating the amount of active hormone

Question 17

What is the most common form of hypothyroidism and what does this disease do?

Answer 17

Hashimoto’s disease

Autoimmune destruction of the thyroid follicles or production of antibodies that block the TSH receptor.

Question 18

How is hypothyroidism treated?

Answer 18

Oral thyroxine

Question 19

What a some signs and symptoms of hypothyroidism?

Answer 19

Cold intolerance and reduced BMR
Bradycardia
Weight gain
Tiredness and lethargy
Dry and flaky skin
Alopecia
Deep and husky voice
Affects neuromuscular system - weakness, muscle cramps, and cerebellar ataxia

Question 20

What is the main form of hyperthyroidism and what does it do?

Answer 20

Grave’s disease

Antibodies produced which stimulate TSH receptors on the follicle cells, increasing synthesis and secretion of T3&4

Question 21

How can hyperthyroidism be treated and how does this drug work?

Answer 21

Carbimazole

Inhibits the enzyme thyroid peroxidase, preventing coupling of iodidination of tyrosine residues on thyroglobulin

Question 22

What are signs and symptoms of hyperthyroidism?

Answer 22

Weight loss
Tachycardia
Heat intolerance, increased oxygen consumption and BMR
Physical and mental hyperactivity
Intestinal hyper-mobility
Skeletal and cardiac myopathy causing tiredness, weakness and breathlessness
Osteoporosis due to increased bone turnover and preferential resorption