Thyroid drugs

Question 1

Why is dessicated thyroid not the best?

Answer 1

It has variable T4 and T3 content and it is made from animal thyroid glands which some vegans may hate
- Used to treat hypothyroidism

Question 2

Levothyroxine

Answer 2

MOA: Activation of the nuclear thyroid hormone receptor (T3 is more activating here)
Indications: hypothyroidism

Question 3

T/F: Liothyronine (T3) is less bioavailable that levothyroxine (T4)

Answer 3

False, T3 has bioavailability of 95% AND has a faster onset than T4 which is only 50-70% bioavailable with a gradual onset (3-5 days)
Note: T4 also has a much longer half life of 7 days compared to T3 that is less than a day

Question 4

T/F: It is best to take levothyroxine before breakfast

Answer 4

True, make sure to take it on an empty stomach

Question 5

T/F: Levothyroxine is more preferred for stable levels

Answer 5

True

Question 6

Potassium iodide

Answer 6

• Example of an anti-thyroid drug
  MOA: Inhibits NIS, TPO, and thyroid hormone release
  Formulation: SSKI (1000 mg/ml of KI) or lugols solution (100 mg/ml of KI + 50 mg/ml of elemental iodine)

Question 7

T/F: Anti-thyroid drugs increase TSH

Answer 7

True, monitor this level in drug therapy

Question 8

What are some clinical effects of potassium iodide?

Answer 8

• It is only effective for short term use and usually used before thyroidectomy to shrink the thyroid gland
• Used in combination with another anti-thyroid medication
• Based on thyroid autoregulation and the wolff-chaikoff effect

Question 9

Methimazole

Answer 9

• Anti-thyroid drug
• MOA: inhibits TPO

Question 10

Propylthiouracil

Answer 10

• Anti-thyroid drug
  MOA: inhibits TPO and peripheral T4–>T3 conversion by 5’ deiodinase

Question 11

How are thyroid hormones made?

Answer 11

1. Iodide uptake by NIS
2. Hormone synthesis by TPO
   - Oxidation of iodide–>iodine
   - Organification: iodination of TG and tyrosine converted to MIT + DIT
   - Coupling: forms hormone precursors still bound to Tg (two DITs= T4 precursor and DIT+MIT= T3 precursor
3. Hormone endocytosis, activation, and release
4. Peripheral conversion by 5’ deiodinase

Question 12

What type of patient is methimazole better for?

Answer 12

Someone who only wants to take it once a day since it lasts longer

Question 13

What type of patient is PTU better for?

Answer 13

A patient who is pregnant/lactating since it is highly protein bound and less can be transferred to the baby