Introduced into medical practice in 1945, antithyroid drugs either decrease thyroid hormone synthesis (thionamides) or thyroid hormone release (iodides).
These drugs are used in children, adolescence, and even pregnant women but also older patients especially where surgery or radioactive iodine therapy may be contraindicated. They are increasingly used as adjuncts, in preparation for surgery or until radioactive iodine therapy takes full effect.
Indications for ablative therapy – i.e. indicators of poor chance of remission on drugs or drug intolerance
- large vascular goitre
- very severe toxicosis / high circulating TSI
- recurrent thyrotoxicosis (relapse following pharmacotherapy)
- poor compliance / follow-up
- drug toxicity
A younger (< 40 years old) patient with a very large coincidental, non-functioning, nodule is best suited to (sub-total) thyroidectomy.
An older patient (> 40 years old or elderly) with perhaps a history of previous thyroid surgery or recurrence and is too ill for surgery (i.e. poor surgical risk), is best suited by treatment with radioactive iodine (ablative therapy) and life-long supplemental thyroxine supplementation.
In general, anti-thyroid therapy is a 12-24 month course. Take a FBC prior to commencement of treatment to exclude, in Grave’s disease patients, any pre-existing leukopenia:
- FBC – WCC
Of the tow drug options, propylthiouracil (PTU) shows the more rapid response by reducing T4 and secondarily, also, T3. Both drugs interfere with thyroxine hormone synthesis and, therefore, take about 20 days (t1/2 x 5) to show effect.
- Propylthiouracil: 100-200 mg q8 hourly
- Carbimazole: 10-20 mg q8 hourly
- once euthyroid, reduce daily dose to smallest that controls thyrotoxicosis
- 20-60% remission (30% prolonged remission after 18 months)
- increased likelihood of remission if, during treatment: reduction in goitre size; reversion suppression test to normal; disappearance circulating immunoglobulins (auto-antibodies)
Complications of anti-thyroid therapy:
- hypothyroidism – enhancement ophthalmopathy, increased goitre
- leukopenia – cease treatment if neutrophils fall to < 1,500
- allergic rashes – consider changing drug
- agranulocytosis (< 0.2%) – patient and clinician must remain vigilant for opportunistic infection, even a sore throat or rash or fever warrants an urgent FBC ± cessation of therapy
If concern re: impending thyroid storm or severe thyro-cardiac disease, consider also other pharmacotherapies while awaiting radioactive iodine ablation therapy to achieve full effect.
- Potassium iodide (KI):
- ± large doses glucocorticoids: 2 mg dexamethasone q6 hourly
- adjunctive beta-blockers: propranolol 40-120 mg per day to alleviate sweating, tremor, tachycardia (but exercise caution if CCF)
These drugs are useful while awaiting response to the definitive treatment or conventional anti-thyroid drugs to take effect and help to manage a thyrotoxic crisis, to manage the hyperthyroid stage of subacute thyroiditis, or for symptom relief in severely ill patients.
References
Jastrzębska, Helena. “Antithyroid drugs.” Thyroid Research vol. 8,Suppl 1 A12. 22 Jun. 2015, doi:10.1186/1756-6614-8-S1-A12.
