Propanolol Pharmacology: Non-selective beta-1 and beta-2 blocker

1. Thyroid Storm Physiology: The extreme amounts of T4 is released from the thyroid and converted to T3 (more potent hormone) by two enzymes: monodeiodinase type I and monoiodinase type II.
1. How Propranolol Works: Propranolol blocks the activity to monodeiodinase type I which decreases peripheral conversion of T4 to T3.

2. Thyroid Storm Physiology: Circulating thyroid hormone inreases cardiac inotropy and chronotropy (cardiac contractility and heart rate), thereby increasing cardiac output.
2. How Propranolol Works: Beta-1 blockade limits these effects on the heart.

3. Thyroid Storm Physiology: Circulating thyroid hormone decreases systemic vascular resistance. In the setting of increased cardiac output and decreased peripheral vascular resistance, high-output failure can occur.
3. How Propranolol Works: Beta-2 blockade increases systemic vascular resistance.

Read more about propranolol use in thyrotoxicosis and thyroid storm thanks to EBM Consult: Why Propranolol is Preferred to Other Beta-Blockers in Thyrotoxicosis or Thyroid Storm.