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Levothyroxine (Synthroid), Thyrax (Europe)

Updated: Jun 20, 2021

Typically supplied as the following:

  • Immediate Release Oral Tablet: 25 MCG, 50MG, 75MCG, 88MCG, 100 MCG, 112 MCG,125MCG, 137 MCG,150 MCG, 175 MCG, 200 MCG, 300 MCG

  • Oral Capsule (Tirosint): 13 MCG, 25 MCG, 50 MCG, 75 MCG, 88 MCG, 100 MCG, 112 MCG, 125 MCG, 137 MCG, 150 MCG

  • Oral Solution (Tirosint-SOL): 13 MCG/ ML, 25 MCG/ML, 50 MCG/ML, 75 MCG/ML , 88 MCG/ML, 100 MCG/ML, 112 MCG/ML, 125 MCG/ML, 137 MCG/ML, 150 MCG/ML,175MCG/ML,200MG/ML

  • Powder for Injection: [100 MCG, 200 MCG, 500 MCG] /Vial

Levothyroxine (Synthroid) belongs to the class of medicines generally referred to as thyroid hormone replacement products. It is a synthetic thyroid hormone (T4), which is thought to work by exerting the same physiological effect in the body as do endogenous or biological T4.

Thyroid hormones exert a direct effect on the DNA. Once bound to their receptor proteins attached to the DNA, a series of physiological events occur in the body, which ultimately leads to their peripheral effects—restoring patients’ mental, metabolic, and physical state. While T3 is the physiologically active thyroid hormone in the body, it is mostly produced in the peripheral tissues through the conversion from T4; thus the reason for T4 being used as a synthetic replacement. It is also thought that T3 is not physiological stable outside of the thyroid gland.

In patients with hypothyroidism, physicians often prescribe levothyroxine-T4 alone or with a T3 combination as a replacement to help restore the deficiency. Also, levothyroxine is often prescribed as an adjunct to surgery and radioiodine treatment in the management of thyroid cancer.

When is a good time to take my medicine?

The absorption of levothyroxine following an oral administration is increased by fasting, and decreased in malabsorption syndromes and by certain foods such as soybeans. The prescribing information recommends taking your levothyroxine tablets on an empty stomach, 30 minutes to 1 hour before meal (breakfast). To accomplish this, most people wake up about 30 minutes early to take their medicine before getting their day started.

I take my stomach acid reduction medication (omeprazole) in the morning 30 minutes before breakfast. Can I take it together with my levothyroxine?

The absorption of levothyroxine following an oral administration can be affected when co-administered with other medications, including stomach acid suppressants. The prescribing information for levothyroxine states that its absorption depends greatly on the stomach acidity.

Medications that alter the acidity of the stomach including omeprazole may reduce the absorption of levothyroxine, and subsequently your free T4 level. Though the clinical significance of this interaction still remains unclear, it is still generally advised that you should take your levothyroxine first then wait at least 30 minutes to 1 hour before taking other medicines.

I was just started on levothyroxine a week ago; I don’t feel much of a difference yet. Should I stop it?

It generally takes about 4 to 8 weeks following the initiation of levothyroxine before a dose adjustment based on your clinical response is determined. It is possible that it may take several weeks before you notice any improvement of symptoms. This is also a direct relationship with the Thyroid Stimulating Hormone (TSH) level, which is often measured to help guide dose change. In addition, some of the clinical manifestations of hypothyroidism, including chronic skin changes, may take up to 3 months to resolve after serum TSH has returned to normal. Thus, always consult with your doctor or pharmacist before stopping or starting any medication.

What should I do if I take more than 1 dose of levothyroxine?

The signs and symptoms of overuse are those of hyperthyroidism and it is relatively easy to wind up with hyperthyroidism; this would most likely not occur immediately, but following consistent overuse of levothyroxine over several days. Overuse or overdose of levothyroxine is a major concern in older patients or those who have an underlying heart disease—due to a possible precipitation of irregular heart rhythm or chest pain. Click Chat now to be connected with one of our pharmacists. If you think you may have overdosed, call the national poison control center.

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  • Jeffrey Garber, Rhoda Cobin, Hossein Gharib, James Hennessey, Irwin Klein, Jeffrey Mechanick, Rachel Pessah-Pollack, Peter Singer, and Kenneth Woeber (2012) Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocrine Practice: November 2012, Vol. 18, No. 6, pp. 988-1028. Assessed 2020

  • Levothyroxine Sodium tablet, film coated, [prescribing information]. Princeton, NJ: Sandoz Pharma USA, Inc.2020

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