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Side Effects of Levothyroxine (Synthroid, Tirosint, Unithroid, Levothroid, Levoxyl, Novothyrox)


Side Effects of Levothyroxine (Synthroid)

Levothyroxine, referred as the brand Synthroid, is used for hypothyroidism (when the thyroid gland does not create enough thyroid hormone) treatment. In addition, Synthroid decreases the size of inflated thyroid glands and treats thyroid cancer.

About Levothyroxine and Endocrine Function

In a normally functioning endocrine system, thyroxine (a hormone) is produced by the thyroid gland. Some patients don’t produce enough of this hormone due to various conditions such as hypothyroidism, cancer, radiation treatment and other conditions such as Hashimoto’s thyroiditis, an autoimmune condition.


Levothyroxine supplementation (often called T4 supplementation) becomes necessary to properly regulate the endocrine system. The endocrine system is a collection of glands that secrete hormones directly into the circulatory system when are then carried toward distant target organs. They help integrate and control bodily metabolic activity and help the organs work properly. When thyroxine is lacking hypothyroidism, symptoms such as fatigue, weight gain, depression, memory problems, chronic constipation, feeling cold even when other people are warm, dry skin and hair loss occur.


Only a physician can diagnose hypothyroidism with appropriate blood tests. Because it is difficult to accurately measure a circulating hormone, several blood tests are prescribed to assess the effect of a levothyroxine prescription on thyroid status. Some of these tests include the free levothyroxine test (T4), the T3 test (triiodothyronine), a test for thyroid antibodies to diagnose an autoimmune condition, the TSH (thyroid stimulation hormone), and in some cases, an ultrasound of the thyroid gland.


A prescription for levothryoxine requires blood tests every three months (along with a subjective report of how the patient feels), followed by blood tests every six months for maintenance therapy. Some patients may have to take levothyroxine for the rest of their lives for normal endocrine function.

Common Side Effects

According to the FDA, report any of these common side effects of Synthroid to your provider:


  • Hair loss (temporary)
  • Signs of an allergic reaction: hives, swelling of your face, lips or tongue, or difficulty breathing
  • Headache
  • Sleep problems
  • Nervousness
  • Irritability
  • Hot flashes
  • Fever
  • Sweating
  • Fluttering in your chest
  • Changes in your appetite or weight loss

Drug Interactions

Remember to tell your physician about any other drugs you are taking as Synthroid interacts with a number of prescription medicines. Your provider will likely tell you to take Synthroid four hours before or four hours after any other medication for optimal absorption and benefits.

About Pharmacogenetic Testing

Pharmacogenetic research has shown there are gene variations responsible for drug metabolism. Enzymes encoded in genes are responsible for how a drug is metabolized. A variation in these genes will become a varied enzyme that will not function well as the gene without the variation. This can result into unwanted side effects.

Know Your Risk with the Rxight® Genetic Test

The Rxight® genetic test was developed by MD Labs to guide healthcare providers in finding the most safe and optimal dosages. Rxight® measures the ability to metabolize over 200 prescription and over-the-counter (OTC) medications. The test is a simple cheek swab that collects your DNA. After the test is completed, you will be given a detailed genetic analysis and the medications safe and ineffective for you.


To learn more about Rxight® pharmacogenetic testing, contact us at 1-888-888-1932 or through email at support@Rxight.com.


Contributors to this Article:
Michael Sapko, MD, PhD; and Deborah Kallick, PhD, Medicinal Chemistry

Read more about Rxight® Genetic Health Testing