Drugs affecting Thyroid

The levels of thyroid hormone must be carefully balanced within the body as an excess/shortage of thyroid hormone can have significant problems. There are drugs which both increase and decrease thyroid levels.

 

a) Pro-thyroid drugs
These drugs are synthetic analogues which aim to mimic the effects of normal thyroid hormone.

 

Levothyroxine
This is a synthetic sodium salt of T4 (thyroxine) that maintains normal T4 and T3 levels.
– It is the first drug of choice in hypothyroidism.

 

Liothyronine
This is a recombinant form of T3 which is used to treat hypothyroidism.

Side effects: Can produce hyperthyroidism –> nervousness, anxiety and headache
– Induce arrhythmias and angina in patients with underlying cardiovascular disease.

 

b) Anti-thyroid drugs
These drugs aim to reduce the effects of an excess of thyroid hormone in the body, either by inhibiting the synthesis of thyroid hormone, or blocking its release.

 

Thioamides
These interfere with the coupling of iodide ions to thyroglobulin by inhibiting the peroxidase enzyme, and so inhibit the synthesis of thyroid hormone.

o Propylthiouracil (PUT)
– Also inhibits the conversion of T4 –> T3 by inhibiting 5’- deiodinase.
– Can be used during pregnancy and is short acting
– Associated with hepatic toxicity

o Methimazole
– Works only by inhibiting the peroxidase enzyme and is long acting
– Cannot be used during pregnancy as it causes aplasia cutis (congenital absence of skin)
– Sometimes sold as a pro-drug called Carbamizole

Side effects: Associated with agranulocytosis and aplastic anaemia
– Therefore, these drugs need constant monitoring with blood tests to measure FBC

 

• Anion inhibitors – Thiocyanate, perchlorate, fluoborite (-ate)
These are small anions which are similar to iodide, and competitively inhibit the transport of iodide ions by the thyroid gland –> inhibit thyroid hormone synthesis

Side effects: Severely toxic causing aplastic anaemia – therefore discontinued

 

Iodide
In high concentrations, iodide inhibits many steps in thyroid hormone synthesis/release
– Also helps to reduce the size of the thyroid gland
– This is known as the Wolff-Chaikoff effect – temporary inhibition of peroxidase enzyme
– It is commonly used before thyroid surgery to decrease thyroid tissue size and decrease vascularity,

Side effects: Angioedema + metallic taste + hypersensitivity reactions

 

Radioactive iodine (131I)
This is transported and gets concentrated in the thyroid gland and emits toxic beta-particles.
-Used to kill thyroid follicular cells non-surgically as a treatment for hyperthyroidism

Side effects: Overdose can produce hypothyroidism

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