The most common hypothyroidism is due to insufficient secretions of thyroid hormones by the thyroid gland. The symptoms of hypothyroidism are a slowing down of the metabolism (constipation, muscle weakness, sensitivity to cold, weight gain, etc.) and of the accumulation of polysaccharides in some tissues (husky voice, puffy eyelids, etc.). Progression slowly results in myxoedema coma. The diagnosis of hypothyroidism is based on clinical signs and symptoms, and on blood tests, particularly to measure the level of TSH, a hormone that stimulates the thyroid.

In adults displaying symptoms that suggest hypothyroidism and who have blood levels of TSH over 10 mUI/l, confirmed by two successive tests and blood levels of free thyroxine (free T4), treatment with levothyroxine is required, in general over the long term. For these cases of established hypothyroidism, the main challenge is to identify temporary forms which either do not require treatment, or require only a short-term treatment. In other cases, levothroxine is continued, often for life.

In the case of mild or subclinical hypothyroidism, with few or no symptoms, a moderately high blood level of TSH with no detectable drop in the level of free thyroxine (T4) in the blood, the potential benefits of levothyroxine are uncertain and should be weighed up against the harms, especially in the event of overdose.

It is advisable to look for signs of overdose in patients taking levothyroxine: tachycardia, trembling, sweats, headaches, restlessness, insomnia, muscle weakness, heat intolerance, weight loss, diarrhoea, hyperglycaemia. It also exposes patients, the elderly especially, to fractures and atrial fibrillation.

©Prescrire 1 October 2015

"Hypothyroidism in adults" Prescrire Int 2015; 24 (164): 241-246. (Pdf, subscribers only).