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Side Effects of
Thyroid Hormones


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Side Effects of Thyroid Hormones


In a normally operating thyroid gland, thyroid hormones are produced in sufficient quantities. These hormones regulate the way the body uses energy. The thyroid gland takes iodine from injested food to produce thyroxine (T4) and triiodothyronine (T3). Metabolism regulation depends on every cell in the body, which in turn depends on thyroid hormones. The normal thyroid gland produces approximately 80% T4 and approximately 20% T3; however, T3 has about four times the hormone “strength” as T4.

When this system is out of balance, hormone replacement therapy, consisting of either levothyroxine (synthetic T4) treatment alone, or in conjunction with Liothyronine (synthetic T3), is indicated. Supplementation with thyroid hormones becomes necessary when the thyroid gland doesn’t function properly. There are many generic varieties of both synthetic T3 and T4. It is important that patients do not switch brands once the therapeutic dose is established. It is well known that different brands of synthetic thyroid hormones may have different effective concentrations of active drug. This is normally not true for generic pharmaceuticals labeled with the same dose.

Serious Adverse Reactions

Rare but potentially life-threatening side effects include serious conditions such as abnormal heart rhythm, angina, heart attack and others. Cardiac action potential is modulated by thyroid hormones, and thus an excess can cause overexcitation (Annals of Internal Medicine “The History and Future of Treatment of Hypothyroidism,” Jan 2016).

Common Side Effects

Side effects of thyroid hormones include:

  • Hot flashes
  • Sensitivity to heat
  • Nausea
  • Fever
  • Sweating
  • Headache
  • Nervousness
  • Irritability
  • (Annals of Internal Medicine “The History and Future of Treatment of Hypothyroidism,” Jan 2016).

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    Rxight® PGx testing is based on the analysis of 18 genes and their alleles to determine how well you metabolize over 200 common prescription and OTC medications, including levothyroxine (branded as Synthroid, Tirosint, Unithroid, Levothroid, Levoxyl, Novothyrox). The results of the Rxight® pharmacogenetic analysis helps determine the dosing and efficacy of these medications and thereby guides patients, pharmacists and providers in devising the safest and most efficacious treatment plan tailored to your unique genetics.

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

    Thyroid Hormones Tested Include:

    • Levothyroxine (Synthroid, Tirosint, Unithroid, Levothroid, Levoxyl, Novothyrox)

    Read more about Rxight® Genetic Testing For Medication Effectiveness