There is an interesting phenomenon I don’t know if you have noticed: the indications for Eugenol include both replacement therapy for hypothyroidism and adjuvant therapy for hyperthyroidism with anti-thyroid drugs. Leaving aside the cause, hypothyroidism is less thyroxine and hyperthyroidism is more thyroxine. Why does Eugenol even work? 1. What is Eugenol? Before talking about what Eugenol is, it is still important to review the thyroid hormone in a few words. In most contexts, thyroid hormones refer to T4 and T3, and it is mainly T3 that plays a physiological role, and most T4 needs to be deiodinated into T3 in order to function. Naturally, both T3 and T4 can inhibit thyroid stimulating hormone (TSH) secretion through negative feedback. The main component of Eugenol is T4, and taking Eugenol is equivalent to supplementing T4. 2. Why can Eugenol cure hypothyroidism? It is easy to understand why Eugenol is used for hypothyroidism, because hypothyroid patients do not synthesize enough thyroid hormone by themselves, and giving Eugenol is a replacement therapy. T4 is supplemented, T4 is sufficient, T3 converted from T4 is also sufficient, then the thyroid hormone that can perform physiological functions is also OK. 3. Why can Eugenol be used as an adjunct to anti-thyroid drugs? Please note that eugenol is used as an adjunct to anti-thyroid medication for hyperthyroidism treatment, not as a direct treatment. There are currently two options for hyperthyroidism treatment: (1) an antithyroid drug (ATD) alone, such as tapazole or propylthiol. (2) Anti-thyroid medication + Eugenol. As to why Eugenol can be used as an adjunct to the treatment of hyperthyroidism, it is important to mention the advantages and disadvantages of each of these two treatment options. The first option is better because the dosage of antithyroid drugs is less, but the possibility of hypothyroidism is higher. The second option can better avoid hypothyroidism and reduce the fluctuation of hormone levels, but the disadvantage is that the dosage of anti-thyroid drugs will be greater, and the corresponding side effects will be more. 4. What exactly should I do with Eugenol? As you may have already thought, there are clinical situations in which patients with hyperthyroidism may experience hypothyroidism when they take a little more anti-thyroid medication and hyperthyroidism when they take a little less anti-thyroid medication during treatment. (1) In cases like this, which are more difficult to regulate, the combination of antithyroid medication with eugenol can better balance the levels of thyroid hormones. (2) Alternatively, a small amount of Eugenol can be given to a hyperthyroid patient with proptosis or goiter, if appropriate. The purpose of this is to suppress TSH, as the patient’s thyroxine will gradually decrease when given anti-thyroid medication for hyperthyroidism, and then the suppression of TSH will gradually weaken, and an increase in TSH will aggravate proptosis or goiter. (3) Of course, it is logical to use eugenol for hypothyroidism. (4) Don’t forget that there are other indications for the use of euthyroxine: non-toxic goiter, prevention of goiter recurrence after goiter removal, etc.