Thyroid Drugs: The Key Points
- The thyroid gland contains two types of cells: follicular and parafollicular (C).
- Follicular cells secrete thyroid hormones (T3 and T4), while parafollicular (C) cells are responsible for the secretion of calcitonin.
- Iodine is taken up by follicular cells with the help of Na+: I– symporter (NIS), then oxidized to form iodinium (I+) ions, which combine with tyrosine residues of thyroglobulin to form mono-iodo tyrosine (MIT) and di-iodo-tyrosine (DIT), known as organification of iodine.
- DIT combines with DIT to form T4 and with MIT to form T3, known as coupling. These reactions are catalyzed by the thyroid peroxidase enzyme.
- T3 and T4 are transported to the follicles, where they remain stored as colloid until stimulated by TSH to be released into the circulation.
- In the liver and kidney, T4 is converted to T3 with the help of 5’-deiodinase and taken up by target tissues. Brain and pituitary take up T4 and conversion to T3 takes place in their own cells.
- The main indication for thyroid hormones is hypothyroidism (cretinism, myxedema and myxedema coma), for which levothyroxine (T4) is preferred due to its long half-life and less frequent dosing.
- Liothyronine can also be used for myxedema coma but should be used cautiously in patients with heart diseases like AF.
- Antithyroid drugs are used to treat hyperthyroidism by inhibiting various aspects of thyroid hormone synthesis and release.
- Inhibitors of Na+–I– Symporter: Iodine is transported into thyroid cells by Na+–I– symporter. Thiocyanate, fluoborate, perchlorate and nitrates can inhibit this transporter and, thus, thyroid hormone synthesis. These drugs are toxic and now obsolete.
- Thyroid Peroxidase Inhibitors: Carbimazole, methimazole, and propylthiouracil inhibit the thyroid peroxidase enzyme, which catalyzes the process of thyroid hormone synthesis. These drugs inhibit the formation of new thyroid hormones, but their action manifests only after 1-3 weeks. They are used for the control of thyrotoxicosis in patients with Graves’ disease and toxic nodular goitre.
- Inhibitors of Thyroid Hormone Release: Sodium iodide, potassium iodide, and Lugol’s solution can inhibit the release of T3 and T4. These drugs are used to shrink the thyroid gland before surgery or to treat thyroid storm.
- Drugs Causing the Destruction of Thyroid Gland: Radioactive iodine (I131) is used to destroy the thyroid gland from within by emitting x-rays and β-particles. It is indicated for patients older than 35 years, those with heart disease, and those with other contraindications to surgery. Thyroid peroxidase inhibitors are administered before I131 to make the patient euthyroid.
- Drugs inhibiting the peripheral conversion of T4 to T3: Propranolol and propylthiouracil inhibit the generation of active T3 from T4 by inhibiting 5’-deiodinase. Amiodarone also inhibits this enzyme and can result in hypothyroidism.
- Adjuvant Drugs: β-blockers, calcium channel blockers, and steroids are used as adjuvants to treat hyperthyroidism by antagonizing the sympathetic effects of thyrotoxicosis and treating Graves’ ophthalmopathy.
Disclaimer: This article is for informational purposes only and should not be taken as medical advice. Always consult with a healthcare professional before making any decisions related to medication or treatment.