Hypothyroidism

Abstract

<h2>Abstract</h2> Hypothyroidism is common, affecting 1–2% of women. The onset is often insidious and symptoms and signs are vague. It is essential to confirm the diagnosis biochemically before initiating treatment. Measurement of serum thyroid-stimulating hormone (TSH) is a highly sensitive and specific test for primary thyroid failure. Elevation of TSH with normal serum T4 is termed ‘subclinical' hypothyroidism. If free T4 is low, with raised serum TSH, the biochemical diagnosis is overt hypothyroidism. In the UK, hypothyroidism is usually autoimmune (characterized by positive antithyroid antibodies) or iatrogenic (secondary to treatment for hyperthyroidism). Worldwide, iodine deficiency remains a major cause. Overt hypothyroidism is treated with T4 replacement; the aim is to restore serum TSH and free T4 to the normal range and hence abolish relevant symptoms and signs. The starting T4 dose is 100 <i>µ</i>g/day, though lower doses should be used in those with a history of ischaemic heart disease. Serum TSH should be checked no sooner than 6–8 weeks after initiating T4, to prompt dose adjustment. The T4 dose required to restore biochemical euthryoidism is usually 100–125 <i>µ</i>g per day. Over-treatment with T4 (indicated by suppression of serum TSH) should be avoided, because of potential long-term risks. After euthyroidism has been restored, TSH should be checked annually to ensure ongoing compliance. There is little evidence that mild, subclinical hypothyroidism is associated with symptoms or adverse events, so if TSH is less than 10 mU/litre conservative management is reasonable, with annual monitoring of TSH to identify biochemical progression.