Levothyroxine: Classification, Uses & Dosage

Instructor: Anne Vaz

Anne has worked as a nurse for 14 years and is a Family Nurse Practitioner.

Levothyroxine is synthetic T4 used in the treatment of hypothyroidism. This lesson discuses the classification, uses and dosage of levothyroxine. This lesson also discusses how to monitor and adjust dose for patient who is started on levothyroxine.

What Is Levothyroxine?

Cars require fuel to run. In the body, the thyroid hormone regulates the metabolism in cells, which provides fuel for body functions. Let's briefly review how the thyroid hormone is secreted in the body.

The hypothalamus produces TRH (TSH releasing hormone), which stimulates the pituitary gland to produce TSH (thyroid stimulating hormone). TSH then stimulates the thyroid gland to produce the active thyroid hormones T3 and T4. Levothyroxine is synthetic hormone, or synthetic T4.

The Thyroid System
Thyroid system

The Classification and Indications for Use of Levothyroxine

Levothyroxine is used for treatment of hypothyroidism, or low thyroid hormone levels in the body. Hypothyroidism is identified through laboratory testing for TSH levels. A high TSH level indicates decreased thyroid hormone production in the body.

Hypothyroidism may be classified as primary or secondary. Primary hypothyroidism occurs when the thyroid gland is unable to produce enough thyroid hormone to meet the body's needs, after it is stimulated by the pituitary gland. In secondary hypothyroidism, there is decreased thyroid hormone production because the thyroid gland is not being stimulated to produce thyroid hormone, indicating a disorder in hypothalamus or pituitary gland.

Examples of primary hypothyroidism are idiopathic hypothyroidism, Hashimoto's thyroiditis, irradiation of thyroid gland secondary to Graves' disease, surgical removal of the thyroid gland, iodine deficiency, side effects of certain drug therapies such as lithium or interferon and infiltrative disorders such as sarcoidosis or scleroderma. Examples of secondary hypothyroidism are pituitary or hypothalamic neoplasm, congenital hypopituitarism and pituitary necrosis. Primary hypothyroidism makes up 95% of cases of hypothyroidism, and 5% of hypothyroidism occurs due to secondary causes.

In rare cases, a severe complication of hypothyroidism called myxedema coma occurs. This is a life threatening condition that results from physiological decompensation from prolonged severe hypothyroidism. This is an emergency and patients are treated in intensive care units with intravenous levothyroxine.

Dosing for Levothyroxine

Once a patient is identified as being hypothyroid, the next step is to start thyroid replacement therapy. The goal of treatment is to achieve a normal TSH level, or a euthyroid state. Most otherwise healthy adults require a replacement of 1.7mcg/kg body weight per day. The dose for elderly patients is 1mcg/kg body weight per day, an in children the dose required for full replacement is higher, at 4mcg/kg/ day. Levothyroxine is given as a once daily dose.

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