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The prevalence of thyrotoxicosis is 2% in women, 0.2% in men. Over 80% are due to Graves' disease. Toxic nodular goitre, a single toxic adenoma and thyroiditis accounts for most of the other cases.
The preferred treatment varies with (1) whether this is an initial episode or relapse, (2) the patient's age and (3) the cause of thyrotoxicosis.
Diagnosis is based on the finding of an elevated free T4 or free T3 when accompanied by a suppressed TSH. Free T4 is elevated in around 75% of cases but in the remaining 25% of cases, the free T4 is normal, and only the free T3 elevated (T3 toxicosis). All patients with thyrotoxicosis (except rare cases of thyrotoxicosis due to TSH secreting pituitary adenoma) will have a suppressed TSH level. Measure FBC at start of therapy. Measurement of TSH receptor antibodies will help to identify if the aetiology of hyperthyroidism is Graves' disease.
The success of treatment is measured by a decrease in free T4 and/or T3 levels back to normal. TSH may remain suppressed for longer and should not be used acutely to measure response to treatment in thyrotoxicosis.
Oral medication is usually with carbimazole, which can either be in a block and replace schedule or reducing dose schedule.
The recommended duration of treatment is 18 months (range 6-24). Remission rate following oral medication is about 50%.
All patients must be warned about the danger of agranulocytosis (3/1000) and told to see their doctor if they develop a sore throat, ulcerated mouth or high fever. The white cell count and differential should be checked in these patients.
Allergic rashes are common with carbimazole and, often respond to antihistamines without needing to stop the drug. If troublesome, patient should be referred for consideration of alternative treatments (such as, radioiodine) for thyrotoxicosis.
Doctors are reminded of the importance of recognising bone marrow suppression induced by carbimazole and propylthiouracil, and the need to stop treatment promptly.
Local policy is usually to recommend block and replace treatment, for practical reasons of thyroid function remaining stable, especially in young people. Thus:
The alternative approach of reducing dose carbimazole is used when there is concern about compliance, in pregnant women, in elderly patients or those likely to remain on treatment long term (e.g. patients who have relapse) or patient preference. In this case, once the free T4 is normal, the dose of carbimazole should be halved and, as long as thyroid function remains normal, can often be halved every 6 weeks. In patients in whom the free T4 or free T3 is only just raised (less than 25% above normal) who are not markedly symptomatic, a starting dose of 10mg carbimazole may be appropriate.
Usually radioiodine and surgery are reserved for patients who relapse from medical treatment. Radioiodine should not be given to patients with active eye disease and only given to patients with inactive eye disease after careful assessment at the Joint Thyroid Eye Clinic. Patients who relapse after 18 months treatment should be started on carbimazole if symptomatic and referred urgently to the thyroid clinic to discuss the treatment options.
Hyperthyroidism in pregnancy should NOT be treated with block and replace.
Low dose propylthiouracil is probably the drug of choice for the first trimester and patients should be referred for specialist assessment and to combined antenatal clinic (every Tuesday AM) for close monitoring of prescription. Hyperthyroidism may improve in pregnancy with only very low doses or no anti-thyroid medication required. Thyroid stimulating antibodies should be checked in pregnancy to determine risk of neonatal hyperthyroidism.