JOURNAL ARTICLE

Hypothyroidism in adults. Levothyroxine if warranted by clinical and laboratory findings, not for simple TSH elevation

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Prescrire International 2015, 24 (164): 241-4, 246
26594730
Hypothyroidism is a common disorder due to inadequate thyroid hormone secretion. When a patient has signs and symptoms suggestive of hypothyroidism, how is it determined whether thyroid hormone replacement therapy will have a favourable harm-benefit balance? How should treatment be managed? To answer these questions, we conducted a review of the literature using the standard Prescrire methodology. The symptoms of hypothyroidism are due to slow metabolism (constipation, fatigue, sensitivity to cold, weight gain, etc.) and to polysaccharide accumulation in certain tissues, leading to hoarseness, eyelid swelling, etc. A blood TSH concentration of less than 4 or 5 mlU/L rules out peripheral hypothyroidism. TSH levels increase with age. Between 30% and 60% of high TSH levels are not confirmed on a second blood test. In overt hypothyroidism, the TSH level is high and the free T4 (thyroxine) level is low. Most of these patients are symptomatic. So-called subclinical hypothyroidism, which is rarely symptomatic, is characterised by high blood TSH levels and normal free T4 levels. The natural history of hypothyroidism depends on its cause. In chronic autoimmune thyroiditis, the most common form seen in rich countries, hypothyroidism generally worsens over time. However, other situations can lead to transient hypothyroidism that may last several weeks or months. Subclinical hypothyroidism, as the name implies, is usually asymptomatic. The risk of progression to overt hypothyroidism is about 3% to 4% per year overall but increases with the initial TSH level. Treatment guidelines are mainly based on physiological and pharmacological considerations and generally recommend levothyroxine therapy. The adverse effects of levothyroxine are signs of thyrotoxicosis in case of overdose (tachycardia, tremor, sweating, etc.). Even a slight overdose carries a risk of osteoporotic fractures and atrial fibrillation, especially in the elderly. In young adults, levothyroxine is usually started at a dose of about 1.5 microg/kg per day, taken on an empty stomach. Elderly patients and those with coronary artery disease should start at a lower dose: 12.5 to 50 microg per day. Treatment monitoring is based mainly on blood TSH assay. Dose adjustment should only be considered after 6 to 12 weeks, given the long half-life of levothyroxine. Certain drugs, such as iron and calcium, reduce the gastrointestinal absorption of levothyroxine. Enzyme inducers reduce its efficacy. In 2015, there is no robust evidence that levothyroxine therapy has any tangible benefit in patients with subclinical hypothyroidism. Some practice guidelines recommend treatment when the TSH level is above 10 mIU/L, or sometimes trial treatment for a few months for patients with symptoms suggestive of hypothyroidism. In practice, replacement therapy is needed for patients with overt hypothyroidism and a blood TSH concentration above 10 mIU/L. The main challenge is to recognise transient hypothyroidism, which does not require life-long treatment. When the TSH is only slightly elevated, there is a risk of attributing non-specific symptoms to an abnormal laboratory result and prescribing unnecessary treatment. Watchful waiting is an alternative to routine levothyroxine prescription in case of TSH elevation.

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