Thyroid Gland

Question 1

Name the structures within the thyroid gland

Answer 1

Colloids surrounded by follicular cells
Follicles
Parafollicular cells

Question 2

Which gland embedded in thyroid

Answer 2

Parathyroid glands

Question 3

Which nerve runs close to the thyroid

Answer 3

Left recurrent laryngeal nerve (supples vocal chords)

Question 4

Explain the structure of the thyroid gland, with reference to its embryo logical origins

Answer 4

Midline out pouch from floor or pharynx (originates from base of tongue)
Development of thyroglossal duct 
Divides into 2 lobes
Duct disappears leaving for amen caecum
Final position by week 7
Thyroid gland then develops

Question 5

Summarise the process of TSH release to thyroid hormone synthesis

Answer 5

Iodine from blood transported into the thyroid by sodium iodine pumps in the follicular cells.
TSH attaches to TSH-R on the membrane causing the release of thyroglobulin TG from the nucleus and the release of the enzyme thyroid peroxidase TPO
Iodine then transported into the colloid via transport protein, undergoes iodination, then again with TG forming TG MIT + DIT. TG MIT DIT undergoes a coupling reaction to form T3 and T4 thyroid hormones .
All the reactions in the colloid (iodination and coupling reactions) are catalysed by TPO and mediated by hydrogen peroxide
Thyroid hormones are then transported back to the blood via the follicular cell .

Question 6

Full names of structure TG MIT DIT

Answer 6

Tyrosine attached to 3-monoiodotyrosine and 3,5-diiodotyrosine

Question 7

How is T3 formed using names of iodothyronines

Answer 7

3-monoiodotyrosine attaches to 3,5-diiodotyrosine to form 3,5,3’-tri-iodothyronine AKA T3

MIT+DIT= T3

Question 8

How is T4 formed using names of iodothyronines

Answer 8

3,5-diiodotyrosine attaches to another 3,5-diiodotyrosine to form 3,5,3’,5’-tetra-idothyronine AKA thyroxine AKA T4

DIT+DIT=T4

Question 9

When does deiodination take place?

Answer 9

Thyroxine T4 is the main hormone produced in the thyroid gland
T4 is a prohormone deiodinated by deiodinase enzyme into triiodothyronine T3, the bioactive form providing almost all the thyroid hormone activity in target tissues
(Also possible to deiodinate T4 to form reverse T3 the inactive version - depending on which iodine is removed)

Question 10

Ratio of T3 from thyroid gland directly vs deiodination of T4

Answer 10

20% made in thyroid directly vs 80% made by T4 to T3 conversion in peripheral tissue by deiodinase enzymes

Question 11

How are thyroid hormones transported in the blood

Answer 11

Mostly bound to plasma proteins:
thyroid-binding globulin TBG 70-80%
albumin 10-15%
prealbumin (transthyretin)
Only 0.05% T4 and 0.5% T3 is unbound (bioactive components)

Question 12

How do thyroid hormones affect gene expression?

Answer 12

Any T4 converted to T3 of which attach to thyroid hormone receptor enzymes in the nucleus (TR on TRE) which alters gene expression

Question 13

Purpose of thyroid hormones, lack of causes what and how to test levels

Answer 13

Essential for feral growth and development, particularly the CNS
untreated congenital hypothyroidism = cretinism
TSH measured in new borns via heel-prick test

Question 14

Actions of thyroid hormones

Answer 14

Increase basal metabolic rate
Protein, carb and fat metabolism
Potentiate actions of catecholamines (tachycardia, lipolysis)
Effects on GI, CNS, reproductive systems

Question 15

Half lives of T3 and T4

Answer 15

T3 - 2days

T4 - 7-9days

Question 16

How is thyroid hormone production controlled?

Answer 16

Using ideas of negative feedback when detecting certain levels of the
Controlling (increase/decrease) amount of Somatostatin (SS) or TRH released at hypothalamus, amount of TSH released at anterior pituitary or amount of iodide at the Thyroid which all in turn affects the amount of T3 and T4 produced

Question 17

Wolff-Chaikoff effect

Answer 17

The reduction in thyroid hormone synthesis caused by ingestion of a large amount of iodine in the follicular cells

Question 18

Hypothyroidism

Answer 18

Autoimmune damage to the thyroid causing thyroxine levels to decline and TSH levels to climb
Most common thyroid disease - Hashimoto’s thyroiditis

Question 19

Symptoms of hypothyroidism?

Answer 19

Deepening voice 
Depression and tiredness 
Hair loss
Enlarged thyroid gland
Swollen face 
Weight gain with reduced appetite
Constipation
Dry rough skin
Bradycardia
Muscle cramps

Question 20

Levothyroxine:

Mechanism/ Indication/ Dosing/ Potential Complications

Answer 20

Provides T4 instead of thyroid which can then be deiodinated into T3
Can be used for both hypo (increase T3 production) and hyperthyroidism (blocking and replacing)
Dosing adjusted according to TSH, common dose 100mg and commonly administered orally
Minor complications = weight loss and headache
Major complications = tachycardia and heart attack (all side effects rare)

Question 21

Patients adamant to have Liothyronine (T3 treatment), why not give that straight away?

Answer 21

No difference in action of T4 and T3
T4 converted to T3 in the body anyways
T3 drugs however are much more expensive

Question 22

Why not try combined thyroid hormone replacement?

Answer 22

T4 = prohormone, converted by deiodinase action to T3
Combination T4/T3 has some reported improvement in wellbeing
However complicated by symptoms of ‘toxicity’ - palpitations, tremors, anxiety and often suppresses TSH

Question 23

Hyperthyroidism

Answer 23

Thyroid makes too much thyroxine causing levels to rise and TSH levels to drop
Most common autoimmune disease: Graves’ disease where whole gland smoothly enlarged and overactive

Question 24

Grave’s Disease symptoms

Answer 24

Antibodies bind to and stimulate TSH receptor in the thyroid causing smooth goitre, diffuse enlargement of thyroid gland
Antibodies bind to muscles behind eye causing exophthalmos (being able to see iris all around eye)
Antibodies stimulate growth of soft tissue of shins aka pretibial myxoedema

Question 25

Symptoms of Hyperthyroidism

Answer 25

Heat intolerance 
Weight loss with increased appetite 
Hair loss
Nervousness, Irritability, Insomnia, Depression
Diarrhoea 
Warm moist skin
Arrhythmia 
Palpitations
Muscle cramps