Today I would like to write something about levothyroxine. My hospital uses levothyroxine often. Everyday there are lots of patients prescribed with levothyroxine. I do believe it is necessary to write below for education, which is not only for patients but also providers. The reference I use comes from U.S. FDA’s official drug information database.

Indications and Usage

Levothyroxine sodium is used for the following indications:

Hypothyroidism – As replacement or supplemental therapy in congenital or acquired hypothyroidism of any etiology, except transient hypothyroidism during the recovery phase of subacute thyroiditis. Specific indications include: primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) hypothyroidism. Primary hypothyroidism may result from functional deficiency, primary atrophy, partial or total congenital absence of the thyroid gland, or from the effects of surgery, radiation, or drugs, with or without the presence of goiter.

Pituitary TSH Suppression – In the treatment or prevention of various types of euthyroid goiters, including thyroid nodules, subacute or chronic lymphocytic thyroiditis, multinodular goiter and, as an adjunct to surgery and radioiodine therapy in the management of thyrotropin-dependent well-differentiated thyroid cancer.

Contraindications

Levothyroxine is contraindicated in patients with untreated subclinical (suppressed serum TSH level with normal T3 and T4 levels) or overt thyrotoxicosis of any etiology and in patients with acute myocardial infarction. Levothyroxine is contraindicated in patients with uncorrected adrenal insufficiency since thyroid hormones may precipitate an acute adrenal crisis by increasing the metabolic clearance of glucocorticoids. Finally, levothyroxine is contraindicated in patients with hypersensitivity to any of the inactive ingredients in levothyroxine.

Dosage and Administration

  • The goal of replacement therapy is to achieve and maintain a clinical and biochemical euthyroid state.
  • The goal of suppressive therapy is to inhibit growth and/or function of abnormal thyroid tissue.

To acheive the two goals above, it depends on variety of factors including the patient’s age, body weight, cardiovascular status, concomitant medical conditions (e.g., pregnancy, concomitant medications, and the specific nature of the condition being treated). As a result Dosing must be individualized and adjustments made based on periodic assessment of the patient’s clinical response and laboratory parameters.

Levothyroxine sodium tablets are administered as a single daily dose.

Table 1 The Indication and Dosages of Levothyroxine

Indication and UsageDosage
1HypothyroidismIndividuals who are at low risk of coronary artery diseaseStarting at 1.7 mcg/kg/day (Full dose). Adjusting dosage in 12.5-25 mcg increments until clinically euthyroid and serum TSH has normalized.
If myxedema coma, administer intravenously rather than orallyIndividuals older than 50 yrs or under 50 yrs with underlying cardiac diseaseStarting from 25-50 mcg/day, with increments of 12.5-25 mcg/day at 6-8 week intervals until clinical euthyroid and the serum TSH has normalized
Elderly individualsStarting from 12.5-25 mcg/day, with increments of 12.5-25 mcg/day at 4-6 week intervals until clinical euthyroid and the serum TSH has normalized
Individuals with severe hypothyroidismStarting from 12.5-25 mcg/day, with increments of 25 mcg/day at 2-4 week intervals until clinical euthyroid and the serum TSH has normalized
Secondary or tertiary hypothyroidismDosage as above but titrated until clinically euthyroid and serum free-T4 level is restored to the upper half of the normal range
2TSH Suppression – various types of euthyroid goiters and thyroid cancerWell-differentiated thyroid cancer> 2 mcg/kg/day (Target: TSH suppressed to <0.1 mU/L)
Contraindicated if the serum TSH is already suppressedWell-differentiated thyroid cancer (high risk)Target: TSH suppressed to <0.01 mU/L
Benign nodules and nontoxic multinodular goiter (controversial)Target: TSH suppressed to between 0.1 to either 0.5 or 1.0 mU/L

 

The adequacy of therapy is determined by periodic assessment of appropriate laboratory tests and clinical evaluation. In adult patients with primary (thyroidal) hypothyroidism, serum TSH levels alone may be used to monitor therapy. The frequence of TSH monitoring during levothyroxine dose titration depends on the clinical situation but it is generally recommended at 6-8 week intervals until normalization.

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