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Laboratory Monitoring: Levothyroxine has a narrow therapeutic index. Appropriate levothyroxine dosage is based upon clinical assessment and laboratory monitoring of thyroid function tests. During the initial titration period, careful dosage titration and monitoring is necessary to avoid the consequences of under-or over-treatment. The symptoms of excessive levothyroxine dosage are the same as many features of endogenous thyrotoxicosis.
Levothyroxine should not be Used for the Treatment of Obesity or Weight Loss: In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for anorectic effects.
Weight loss drugs: Orlistat may decrease levothyroxine absorption which may result in hypothyroidism. To avoid this orlistat and levothyroxine should be administered at least 4 hours apart. Regular monitoring for changes in thyroid function is required.
Special Patient Populations: The initial dose and any dose increments should be carefully chosen in elderly and in patients with cardiac symptoms, diabetes mellitus or insipidus: too high initial dose or too rapid increase may cause or aggravate symptoms of angina, arrhythmias, myocardial infarction, cardiac failure or a sudden raise in blood pressure.
Treatment with levothyroxine in patients with panhypopituitarism or other causes predisposing to adrenal insufficiency may cause reactions, including dizziness, weakness, malaise, weight loss, hypotension and adrenal crisis. It is advisable to initiate corticosteroid therapy before giving levothyroxine sodium in these cases.
Patients with myxoedema have an increased sensitivity for thyroid hormones; in these patients the starting dose should be low with slow dosing increments.
Levothyroxine absorption is decreased in patients with malabsorption syndromes. It is advised to treat the malabsorption condition to ensure effective levothyroxine treatment with regular levothyroxine dose.
During pregnancy, serum thyroxine levels may decrease with a concomitant increase in serum TSH level to values outside the normal range. Patients taking levothyroxine should have their TSH measured during each trimester. An elevated serum TSH level should be corrected by an increase in the dose of levothyroxine. Since postpartum TSH serum levels are similar to preconception values, levothyroxine dosage can be reduced to the pre-pregnancy dose.
Haemodynamic parameters must be monitored when initiating levothyroxine treatment in preterm infants with a very low birth weight, as circulatory collapse due to immature adrenal function may occur.
In women, long-term levothyroxine therapy has been associated with increased bone resorption, thereby decreasing bone mineral density, especially in post-menopausal women on greater than replacement doses of in women who are receiving suppressive doses of levothyroxine. To minimise the risk of osteoporosis, dosage of levothyroxine should be titrated to the lowest possible levels.
Effects on Ability to Drive and Use Machines: From the pharmacokinetic and pharmacodynamics properties of levothyroxine, treatment with levothyroxine would not be expected to interfere with ability to drive or operate machinery.
