Thyroid Drugs
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.
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
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.
Propylthiouracil (PUT)
This drug also inhibits the conversion of T4 to T3 by inhibiting 5’-deiodinase.
It can be used during pregnancy, but it is associated with hepatic toxicity.
Methimazole (Carbimazole)
This drug work only by inhibiting the peroxidase enzyme and is long-acting.
Not used in pregnancy as it causes aplasia cutis (congenital absence of skin).
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 iodide ions, and competitively inhibit the transport of iodide ions by the thyroid gland to inhibit thyroid hormone synthesis.
Side effects
Severely toxic causing aplastic anaemia – therefore discontinued
Iodide
In high concentrations, iodide ions inhibit many steps in thyroid hormone synthesis/release and reduce the size of the thyroid gland.
This is known as the Wolff-Chaikoff effect, a temporary inhibition of peroxidase enzyme which inhibits thyroid hormone synthesis.
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)
Radioactive iodine is transported and gets concentrated in the thyroid gland and emits toxic beta-particles, which damages thyroid follicular cells.
It is used to kill thyroid follicular cells non-surgically as a treatment for hyperthyroidism.
Side effects
Overdose can produce hypothyroidism
It can worsen eye symptoms in conditions like Graves’ disease
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