Thyroid hormone and antithyroid drugs

Thyroid hormones, including thyroxin (T4) and triiodothyronine (T3), are essential to maintain the body's normal growth, development and metabolism. Its lack of secretion or being excessive can both cause disease with being lack capable of affecting the physical and mental development, causing cretinism and myxedema while being excessive capable of leading to hyperthyroidism.

It is clinically majorly used for the substituting and complementary therapy for the thyroid hypofunction. Its indications include cretinism, myxedema, and simple goitre. Being excessive can cause symptoms of hyperthyroidism and can lead to angina and myocardial infarction for the elderly and patients with heart disease in which case the drugs to be discontinued and patients should be subject to β-blockers treatment for antagonism.

Anti-thyroid drugs are mainly used for the treatment of hyperthyroidism with clinical types including thiourea, iodide, radioactive iodine and β-blockers.

Thiourea
Thiourea can be divided into two categories: 1: thiouracil type: methylthiouracil, propylthiouracil; 2 imidazole type: thiamazole, carbimazole. Pharmacological effects:
1. Inhibition of thyroid hormone synthesis: the basic effect of the thiourea is inhibition of thyroid peroxidase and further prevention of the tyrosine iodination and coupling of the iodinated tyrosine, thereby inhibiting the biosynthesis of the thyroid hormone. It has no antagonism on the synthesized thyroid hormone and can play efficacy only when the hormone already synthesized in vivo have been consumed. Therefore, this class of drug has a slow onset rate with generally 2 to 3 weeks of medication before the symptoms of hyperthyroidism begins to reduce and 1 to 3 months for the basal metabolic rate to return to the normal value.
2. Inhibition of T4 transformation: propylthiouracil can inhibit the transformation of peripheral T4 into T3, achieving rapid control of the blood T3 level of relatively strong biological activity and rapid efficacy.
3. Immunosuppressive effect: thiourea can inhibit the production of immunoglobulin, leading to decrease of the thyroid stimulating immunoglobulin in the blood. For the treatment of hyperthyroidism, in addition to control symptoms, there are also certain etiological treatment effects.

Iodine and iodide
Iodine is an essential raw material for the synthesis of thyroid hormones with different doses of iodine agents being able to produce different effects on thyroid function.
1. Low-dose of iodine can treat the simple goiter caused by iodine deficiency.
2. Large doses of iodine can produce anti-thyroid effects with the main effect of inhibition of thyroid hormone release as well as inhibiting the synthesis of thyroid hormone. Its anti-thyroid effect is strong and rapid with 1 to 2 days after medication for onset and 10 to 15 days to achieve maximum effect. Further continuing the drugs will lose anti-thyroid effects and lead to the relapse of the thyroid symptoms. In clinical practice, it is mainly used for:
1 the preparation before hyperthyroidism surgery: it is generally administrated of compound iodine solution at two weeks before the surgery so that the thyroid tissue get degraded; blood vessels number is reduced; glands shrink and become tough, being conducive to surgery and reduce bleeding;
2 thyroid crisis treatment.

β-receptor blockers
Β receptor blocker propranolol, etc., can quickly counteract the symptoms of increased sympathetic nerve activity in patients with hyperthyroidism, and can inhibit the transformation from peripheral T4 into T3. It is a kind of valuable adjuvant therapy for the treatment of hyperthyroidism and thyroid crisis.

---