Wisdom Wednesday: Levothyroxine

Up to 7% of the general population has hypothyroidism, which is corrected with thyroid hormone treatment. The general goals of thyroid hormone replacement are to provide resolution of patient symptoms and hypothyroid signs, including biological and physiologic markers of hypothyroidism; achieve normalization of serum thyroid-stimulating hormone (TSH) concentrations with improvement in circulating thyroid hormone concentrations; and avoid overtreatment (eg, iatrogenic thyrotoxicosis), especially in elderly persons.

Levothyroxine, a synthetically made thyroxine (T4), is the predominant form of thyroid hormone replacement used on patients with hypothyroidism. In healthy and iodine-sufficient individuals, the majority of thyroid hormone produced is T4, synthesized exclusively by the thyroid gland, with a smaller amount of T3, which is produced by the thyroid and in peripheral tissues via diodination of the circulating T4.

In the setting of fluctuating T4 levels, deiodinase activity is tightly regulated to maintain normal T3 levels at the various target tissues. In hypothyroidism, the 5’ deiodinase is activated to allow greater conversion of T4 to the bioactive form of thyroid hormone, T3.
Given the high prevalence of hypothyroidism in the general population, levothyroxine has consistently been the most frequently prescribed medication in the United States over the past several years. In 2016, approximately 123 million prescriptions for levothyroxine were dispensed.

Standard Treatment for Underactive Thyroid Gland Still Best

A expert panel reviewing treatments for hypothyroidism has concluded that the drug levothyroxine (L-T4) should remain the standard of care.

The updated guidelines from the American Thyroid Association were published recently in the journal Thyroid.

The thyroid gland produces hormones that control the way every cell in the body uses energy. For example, thyroid hormones control how fast you burn calories and how fast your heart beats. If the thyroid gland produces too little thyroid hormone to control these activities the condition is called hypothyroidism.

A task force convened by the thyroid association reviewed available medical literature and found no evidence that other types of thyroid hormone replacements are better than L-T4, which has long been the standard of care for patients with hypothyroidism. Brand names for L-T4 include Synthroid and Levoxyl.

However, while L-T4 is effective in most patients, some do not regain ideal health when taking L-T4 alone, the authors noted.

The guidelines “provide useful, up-to-date information on why to treat, including subclinical disease, who to treat, and how to treat hypothyroidism,” Dr. Hossein Gharib, president of the thyroid association, said in a journal news release.

“Information is evidence-based and recommendations are graded. I think they will be used extensively by all clinical endocrinologists, especially by our members,” added Gharib, a professor of medicine at the Mayo Clinic College of Medicine in Rochester, Minn.