Curriculum · Metabolic and Endocrine and Diabetes Mellitus
Interpreting thyroid function tests on and off treatment
What it is
Thyroid function tests are read in a fixed order, and the order is the point. TSH is measured first, and what is measured after it depends on the TSH: free T4 when TSH is high, free T4 and total T3 when TSH is low, and TPO antibodies straight away when TSH is normal. The tests give only one of the three diagnoses you should have after a thyroid swelling is seen. After history and examination you should have an anatomical diagnosis (diffuse, multinodular or single nodule goiter), a physiological one (hyper, hypo or euthyroid), and a pathological one (benign or malignant). The function tests give the physiological one; the ultrasound and the needle give the pathological one.
How it presents
What sends the test is the thyroid function symptoms asked for in the history: heat or cold intolerance, palpitations, weight or appetite changes, behavioural changes (irritability, restlessness or depression), and constipation or diarrhoea. A swelling adds two more lines to the history: compressive symptoms, which are dysphagia, difficulty breathing and voice changes, and symptoms more suggestive of malignancy, which are a rapidly enlarging swelling and hoarseness of voice. On auscultation, a bruit means increased vascularity and hyperdynamic circulation in hyperthyroidism.