Investigating Thyroid Function
Monitoring patients on thyroxine
TSH is the most appropriate test when monitoring patients receiving thyroxine for the treatment of hypothyroidism. It should be measured no sooner than 6-8 weeks after the start of treatment. In the unusual situation where thyroid function needs to be assessed before this time, FT4 should be used, as the TSH will not have plateaued at this stage.
Once the target TSH has been reached, a further TSH test in 3-4 months is often helpful to ensure the TSH is stable. Patients on long-term stable replacement therapy usually require only an annual TSH, unless pregnant. The usual goal of treatment for primary hypothyroidism is for the TSH to be within the reference range. Occasionally drugs such as iron, antacids, or HRT may increase the required dose of thyroxine. Therefore drug doses should be separated and if there is doubt, TSH should be rechecked after several weeks.
Biological and assay variability means that minor variations in TSH (e.g. 1-2 mIU/L) are not usually clinically significant.
Monitoring untreated subclinical thyroid disease
Subclinical hypothyroidism - Is defined as an asymptomatic patient with raised TSH levels but normal FT4 concentration. A common cause is Hashimoto’s thyroiditis and, many of these patients subsequently develop overt hypothyroidism, especially if thyroid antibodies are positive. The decision to initiate thyroid replacement therapy should be made based on the presence of symptoms; patients with TSH between 5-6 mIU/L usually have no symptoms, while as the TSH approaches 10 mIU/L more symptoms are probable. In the remainder of patients thyroxine should be considered for those with a TSH persistently >10mIU/L. Patients not treated with thyroxine should be monitored using TSH every 6-12 months.
Subclinical hyperthyroidism - Is defined as an asymptomatic patient with a suppressed TSH level and normal FT4 and FT3. Common causes include excessive thyroxine replacement, autonomously functioning multinodular goitre and subclinical Graves disease. These patients are at increased risk of developing atrial fibrillation and possibly osteoporosis. Further investigation and treatment should be considered for patients with an undetectable TSH on repeated testing.
Monitoring patients on anti-thyroid drugs
Following initiation of anti-thyroid medication, the TSH may remain suppressed for 3-6 months. Therefore, it is recommended that thyroid function be monitored every 4 weeks using FT4 and TSH to adjust the dose until the TSH normalises and clinical symptoms have improved. Then the patient can be monitored every 2 months using TSH only.
All patients on anti-thyroid medication should be warned about the rare but serious complication of agranulocytosis. Patients should be instructed to stop treatment if fever, sore throat or other infection develops. Because the onset of agranulocytosis is abrupt, and the occurrence is rare, routine full blood counts are not recommended,1 instead, patients should be advised to report fever, sore throat or infection.
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