Thyroid Hormones

Question 1

Levothyroxine

Answer 1

Pure T4 -> converted to T3 in body
Slow onset, long duration x1/day 
low potency, increase dose needed
Primary drug because longer duration
Preferred IV for myxedema coma

Question 2

Liothyronine

Answer 2

Pure T3
Rapid onset, short half life, dramatic changes
More potent, less dose
Rarely used
Uses: thyroid cancer surgery (taper off T4), following radioiodine, depression

Question 3

Hyperthyroid treatment

Answer 3

1. Symptomatic reliefs (propanolol, sympathomimetics)
2. Anti-thyroid drugs - inhibit TH synthesis
3. Therapeutic iodine - non-surgical destruction of thyroid gland
4. Partial thyroidectomy

Question 4

Thioamides:
Propylthiouracil (PTU)
Methimazole (MMI)

Answer 4

Inhibit peroxidase enzyme
Inhibit iodination and coupling steps
Do NOT inhibit release of preformed TH
= Latent period x3 weeks
Adverse: *Agranulocytosis*, hypothyroidism, goiter

Question 5

Propylthiouracil (PTU)

Answer 5

Prevents conversion of T4 to T3, MMI does not
Less potent than MMI
Shorter half life 2-4x/day
Adverse: Liver toxicity/failure (not seen in MMI)
Uses: Pregnancy 1st trimester, Thyroid storm

Question 6

Methimazole (MMI)

Answer 6

More potent 10x (not a significant advantage)
Longer half life, x1/day
better for patient compliance = preferred drug
Does NOT cause liver toxicity!!
Used during last 2 trimesters of pregnancy and breast feeding

Question 7

Iodides

Answer 7

Acute inhibition of synthesis and release of THs
Decrease size and vascularity of thyroid gland
Uses: thyroid storm (decrease synthesis and release)
during last ten days before surgery
Radiation emergencies
DO NOT use with radioiodine therapy

Question 8

Radioactive iodine

Answer 8

Iosotope I135 = TX OF CHOICE
(easy, effective, cheap, safe)
85% weak beta radiation = localized tissue destruction
Minimal radiation danger, but contraindicated in pregnancy

Question 9

Partial thyroidectomy

Answer 9

Alternative to radioiodine
Associated with higher morbidity
Used when Thyroid CA present or if anti-thyroid drug fails during pregnancy
**Best tx for Graves*

Question 10

Strategy for surgical removal

Answer 10

1. Try to achieve euthyroid w/ thioamides 6 weeks prior to surgery
2. Iodide therapy 2 weeks before surgery (decrease size and vascularity)
3. Surgical removal
4. Replace w/ levothyroxine

Question 11

Treating thyroid storm

Answer 11

1. PTU to prevent T4->T3
2. Iodides to decrease release and synthesis
3. GCs to prevent shock
4. Propanolol and sympatholytics symptomatic relief

Question 12

Choice drug for young patient

Answer 12

Methimazole

Question 13

Older patient

Answer 13

Radioactive iodine

Question 14

Pregnancy

Answer 14

Propylthiouracil

Question 15

Graves Disease

Answer 15

Surgery