Dangerous side-effects (angina pectoris, arrhythmias, cardiac infarction) are to be expected mainly in cardiac patients when the levothyroxine dose has been increased too rapidly or when it is too high.

Occasionally the prescribed levothyroxine doses are too high and cause symptoms such as hyperthyroidism (tachycardia, dysrhythmias, tremor, nervousness, insomnia, diarrhea, weight loss, sweating, heat sensations, fever, decreased glucose tolerance).

Whether long-term levothyroxine treatment increases the risk of osteoporosis in pre- and postmenopausal women has not yet been safely established.

Levothyroxine: Interactions

Levothyroxine can significantly increase the effect of anticoagulants. Insulin or oral antidiabetic requirements may be increased under levothyroxine. The resorption of levothyroxine is impaired by anion exchange resins (colestipol, cholestyramine, divistyramine), and possibly also by antacids, iron salts, and sucralfate. Estrogens may involve an increase of the levothyroxine dose.

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