The Thyroid Gland

Question 1

Describe the location of the thyroid gland

Answer 1

Lies against and around front larynx and trachea,

below thyroid cartilage,

above suprasternal notch,

isthmus extends from 2nd to 3rd rings of trachea

Question 2

Outline the structure of the thyroid gland

Answer 2

2 lobes L + R,
joined by isthmus,
bow tie shape,
each lobe the size of 50p

Question 3

Briefly out the embryological development of the thyroid gland

Answer 3

First endocrine gland to devel,
starts as prolif of epithelial cells at the base of the tongue,
over several weeks it descends,
through thyroglossal duct which then degenerates

Question 4

Describe the histology of the thyroid tissue

Answer 4

Follicular cells arranged in spheres = thyroid follicles filled with colloid (stores of thyroglobulin) = extracellular

Question 5

What cells prod thyroid hormone?

Answer 5

Thyroid follicular cells

Question 6

How are thyroid hormones synthesised?

Answer 6

Small intestine: Dietary iodine reduced to iodide before absorption

Iodide oxidation

Inside thyroglobulin: tyrosine iodinated, then coupled with others: MIT/DIT

when needed the protein is degraded by fusing with lysosome and hormone released

Question 7

Describe the chemical structure of the thyroid hormones

Answer 7

MIT = mono
DIT = diiodo

coupling reaction:
MIT + DIT = T3 (tri)
DIT + DIT = T4

Question 8

What is the function of thyroid peroxidase?

Answer 8

Facilitates thyroid hormone production = oxidation, addition of iodine, coupling reaction

Question 9

How are thyroid hormones secreted?

Answer 9

90% secreted as T4

then converted to T3 (4x more biologically active) by the liver/kidneys

transported in blood bound to thyroxine-binding protein

Question 10

Describe how the activity of the thyroid gland is controlled

Answer 10

Hypothalamus released TRH to anterior pit,

AP releases TSH to thyroid gland,

TG releases thyroid hormone to target tissues

Question 11

Describe the effects of thyroid hormones on cells and the body as a whole

Answer 11

General actions = increased basal met rate and heat prod (increase mitochondria, enzymes in respiratory chain), stim met pathways (lipid/carb), sympathomimetic effects (increasing target cell receptor number).

Tissue specific = CVS: increase CO, HR, ionotropy.

Nervous = increased myelination

Question 12

Where are thyroid hormone receptors found?

Answer 12

Nuclear – act by modulating gene expression by modulating transcription factors.

Hormone binding = relieves repression of gene = increased expression

Question 13

Describe the consequences of over- and under-secretion of thyroid hormones

Answer 13

Goitre = hypo/hyper.

Hypo = TSH or TRH def, dietary iodine def,

Hashimotos (autoimmune)= obesity, lethargy, intolerance to cold, bradycardia, constipation.

Hyper = Graves (autoimmune), multi-nodular goitre, toxic,

drugs = weight loss, irritability, increased bowel movements, breathlessness

Question 14

What antithyroid drug is used in the UK?

Answer 14

Carbimazole = pro drug – prevents thyroid peroxidase from coupling and iodonating tyrosines on thyroglobin

Question 15

What is thyroglobulin?

Answer 15

acts as a scaffold on which thyroid hormones are formed

Question 16

What do parafollicular cells secrete?

Answer 16

Calcitonin

Question 17

What are T3 and T4 called?

Answer 17

T3 = triiodothyronine

T4 = thyroxine

Question 18

Which form of thyroid hormone has the longest half-life?

Answer 18

T4 (~5-7 days)

T3 ~1 day

Question 19

Which protein transports thyroid hormone T3 and T4?

Answer 19

Tyroxine-binding globulin TBG (majority)

Transthyretin
Albumin

Question 20

What effect does thyroid hormone have on mitochondria?

Answer 20

Increases number and size of mitochondria

Question 21

What type of hormone is TSH?

Answer 21

Glycoprotein