Thyroid and Antithyroid Drugs

Question 1

Explain the feedback loop that determine release from the thyroid gland

Answer 1

T3 and T4 release is dependent on Iodine (from diet). Once released, negative feedback loop such that when levels of T3 and 4 fall,it is detected by the Hypothalamus and TRH is released which stimulates the anterior pituitaty to release TSH. The anterior pituitary also has receptors for this negative feedback loop and TSH acts on the thyroid increasing T3 and 3 secretion.
T4 is comverted to T3 peripherally

Both are released bound to plasma protins.

Question 2

When does goitre occur and why?

Answer 2

Hypothyroid due to iodine deficiency as excess TSH is produced and acts on the thyroid enlarging it.

Question 3

What happens in Grave’s disease?

Answer 3

IgG causes prolonged TSH receptor activation causing excessive T3 and 4 secretion= Thyrotoxicosis

Question 4

What are the treatments for thyrotoxicosis

Answer 4

Thionamides- prevent T3 and 4 synthesis by preventing iodine bonding. eg Carbimazole
131 Iodine- Radioactive iodine destroys thyroid cells preventing T3 and 4 release
Subtotal thyroidectomy
BBs symptom releif

Question 5

How long does Carbimazole take to be effective?

Answer 5

2-3 weeks and symptoms worsen in mean time.

Question 6

What is the commonest cause of hypothyroid?

Answer 6

Hashimotos Thyroiditis- autoimmune inflammation

Question 7

How is hypothyroid treated?

Answer 7

Thyroid replacement- Levothyroxine (Oral T4)