7. Thyroid hormone physiology

Question 1

Are thyroid hormones essential for life?

Answer 1

• not essential for life
• essential for normal development
• essential for physical and mental wellbeing

Question 2

Where is the thyroid gland located?

Answer 2

• neck
• around larynx and trachea
• 4 parathyroid glands

Question 3

Describe anatomy of thyroid gland

Answer 3

• 2 lobes

- isthmus in middle

Question 4

What is thyroid gland composed of?

Answer 4

• thyroid gland is composed of follicles
• follicles made of a monolayer of epithelial cells enclosing a large core of viscous, homogeneous colloid
• Colloid acts as a reservoir of thyroid hormone

Question 5

What does colloid act as?

Answer 5

reservoir of thyroid hormones

Question 6

What are the primary molecules secreted by the thyroid gland?

Answer 6

• tri-iodothyronine (T3)
• tetraiodothyronine(T4, thyroxine)
• calcitonin

Question 7

What is calcitonin responsible for?

Answer 7

calcium homeostasis, and is independent of other thyroid hormones

Question 8

What is the thyroid hormone pathway?

Answer 8

iodine & tyrosine-> monoiodine rosine (MIT) and dioiodotyrosine (DIT)

DIT + DIT -> tetraiodothyronine (T4)

MIT+ DIT -> triiodothyronine (T3)

In tissues, T4 made into T3 by iodothyronine deiodinase

Question 9

How is T4 made into T3

Answer 9

by iodothyronine deiodinase in tissues

Question 10

How is iodine freed from MIT and DIT?

Answer 10

• MIT and DIT are rapidly degraded by halogenases to free the iodide
• iodide reutilised by combining with thyroglobulin

Question 11

How do T3 and T4 get to target tissues?

Answer 11

• T3 and T4 leave the follicular cells and enter bloodstream for distribution to target tissues
• 95% of thyroid hormone leaving thyroid gland is in the form of T4 (thyroxine)

Question 12

What is the difference in effect between T3 and T4?

Answer 12

• deiodinase enzymes convert the T4 to T3 (80%) or reverse T3(20%) in target tissues
• T3 has a biological activity approx. 40 times greater than that of T4, whilst reverse T3 is biologically inactive
• up to 90% of the biologically active thyroid hormone within the cell is in form of T3
• Plasma half-life of T4 is 6-8 days whilst T3’s is one day

Question 13

What does iodine deficiency cause?

Answer 13

higher TSH

Derbyshire neck

Question 14

How does thyroid hormone travel around the body?

Answer 14

• insoluble in water, transported in blood bound to plasma proteins
• over 99% of circulating thyroid hormones protein bound
• most of T4 (75%) is bound to thyronine-binding globulin (TBG)
• 15-20% of T4 bound to thyroxine-binding prealbumin (TBPA)
• 5% bound to albumin

Question 15

Describe thyroid hormone receptors

Answer 15

• Intracellular, more specifically nuclear

- effect of interaction of thyroid hormones with receptors is to influence gene transcription and protein synthesis

Question 16

What is the main effect of thyroid hormones?

Answer 16

to increase basal metabolic rate:
- increase carbohydrate metabolism
-increase in synthesis, mobilisation and degradation of lipids
-increased protein synthesis
thyroid hormones essential for normal development of the CNS, especially myelination of nerve fibres

Question 17

What is the primary mechanism of thyroid hormone?

Answer 17

Increase number and size of mitochondria and increased activity of metabolically important enzymes

All aspects of carbohydrate metabolism are increased:

• glycogenesis and glucose uptake by muscle cells and adipose cells
• potentiation of effects of insulin
• potentiation of effects of catecholamines
• increase in glucose absorption by gastrointestinal tract

Question 18

thyroid hormones increase basal metabolic rate and therefore oxygen consumption in nearly every organ except:

Answer 18

• brain
• uterus
• testes
• spleen
• thyroid gland
• anterior pituitary gland

Question 19

What are some anti-thyroid drugs and how do they work?

Answer 19

carbimazole
methimazole
inhibition of synthesis of thyroid hormones by preventing the incorporation of the iodide into the thyroglobulin

Question 20

What does propylthiouricil do?

Answer 20

propylthiouricil prevents peripheral conversion of T4 to T3

Question 21

How does potassium perchlorate work?

Answer 21

acts by competing with iodide for the active iodide uptake pump

Question 22

What can radioactive (131I) iodine do?

Answer 22

• alternative to surgery
• selectively concentrated in thyroid gland where it causes tissue damage and therefore reduced thyroid hormone secretion

Question 23

What can prophylactic administration of iodine cause?

Answer 23

• prophylactic administration of iodine through either injection or incorporation into table salt/flour has reduced incidence of endemic goitre worldwide
• iodine carries risk of Jod-Basediw phemonon in which iodine administration precipitates hyperthyroidism

Question 24

What drugs can induce goitre?

Answer 24

• lithium, used in treatment of bipolar depression
• iodides contained in vitamin preparations and some cough remedies
• these ions are selectively concentrated within the thyroid gland where they interfere with iodide incorporation and hormone release