Hypothyroidism is one of the most common endocrine conditions with a UK prevalence of 1.4% in women, but under 0.1% in men. Underactivity of the thyroid is usually primary, from disease of the thyroid, but may be secondary to hypothalamic-pituitary disease (reduced TSH drive.
Losing weight with hypothyroidism
Treatment of Hypothyroidism
Replacement therapy with thyroxine (i.e. T 4 ) is given for life. The starting dose will depend upon the severity of the deficiency and on the age and fitness of the patient, especially cardiac performance. In the young and fit, 100 µg daily is suitable, while 50 µg daily (increased to 100 µg after 2-4 weeks) is more appropriate for the small, old or frail. Patients with ischaemic heart disease require even lower initial doses, especially if the hypothyroidism is severe and long-standing. Most physicians would then begin with 25 µg daily and perform serial ECGs, increasing the dose at 3- to 4-week intervals if angina does not occur or worsen and the ECG does not deteriorate. Some, however, would use T 3 beginning with 2.5 µg 8-hourly, doubling the dose every 48 hours up to 10 µg three times daily. If progress is satisfactory, T 4 (100 µg daily) is then started and T 3 is discontinued 5 days later.
Adequacy of replacement should be assessed clinically and by thyroid function tests after at least 6 weeks on a steady dose; the aim is to restore T 4 and TSH to well within the normal range. If serum TSH remains high, the dose of T 4 should be increased in increments of 25-50 µg and the tests repeated 6 weeks later. This stepwise progression should be continued until TSH becomes normal, though some physicians believe that complete well-being is only restored in some patients when the T 4 is high-normal and the TSH is slightly suppressed. The usual maintenance dose is 100-150 µg given as a single daily dose; over-replacement may increase the risk of atrial fibrillation in those aged over 60. An annual thyroid function test is recommended - this is usually performed in the primary care setting, often assisted and prompted by district 'thyroid registers'.
Clinical improvement on T 4 may not begin for 2 weeks or more and full resolution of symptoms may take 6 months. The importance of lifelong therapy must be emphasized and the possibility of other autoimmune endocrine disease developing, especially Addison's disease or pernicious anaemia, should be considered. During pregnancy, an increase in T 4 dosage of about 25-50 µg is often needed to maintain normal TSH levels, and the necessity of optimal replacement during pregnancy is emphasized by the finding of reductions in cognitive function in children of mothers with elevated TSH during pregnancy.