Last clinically reviewed by Joy Y. Erdile, MSc, RD on April 23, 2026.
Introduction
Did you know that thyroid medication can worsen blood sugar? The “highlights of prescribing information” sheets available to pharmacists and doctors for medications such as Synthroid® (generic: levothyroxine) and Cytomel® (generic: liothyronine) warn that both these types of thyroid medication can worsen blood sugar control in diabetics and increase the need for diabetes medications, including insulin. I didn’t know this.
Even though I had been diagnosed with type 2 diabetes more than twelve years ago and with hypothyroidism this past August, when I was prescribed thyroid medication, neither the pharmacist nor my doctor mentioned this potential side effect. He was well aware that for three years before my diagnosis of hypothyroidism, I had been in remission of diabetes, controlling my blood sugar through diet alone.
When I began taking thyroid medication, I started to periodically feel unwell, similar to how I felt with high blood sugar. I began to test my blood glucose more often and discovered it was routinely spiking as high as the mid-to-high 10 mmol/L (~190 mg/dl) for seemingly no reason.
A blood glucose reading showing an unexpected spike after starting thyroid medication
The Connection Between Thyroid and Glucose
People diagnosed with any form of diabetes (type 1, type 2, gestational diabetes), as well as those with hypothyroidism, must know that their thyroid medication can impact their blood sugar control. While those with type 1 diabetes monitor their glucose closely, many people with type 2 diabetes rarely check regularly and may miss these significant spikes.
The “Highlights of Prescribing Information” for both Synthroid® and Cytomel® warn that therapeutic use of these medications in patients with diabetes mellitus may worsen glycemic control and result in increased insulin requirements. They recommend carefully monitoring blood sugar after starting, changing, or discontinuing thyroid hormone therapy.
Page 1 of the Synthroid prescribing information highlights warnings for diabetic patients
Understanding the Biological Mechanism
A paper published recently explains how thyroid hormones contribute to a rise in blood glucose. In the liver, thyroid hormones increase the expression of glucose transporter 2 (GLUT2), leading to increases in both gluconeogenesis and glycogenolysis. Additionally, thyroid hormones increase lipolysis in adipose tissue, which further stimulates hepatic gluconeogenesis [7].
Natural Desiccated Thyroid (NDT) Warnings
Even products like Armour Thyroid® or ERFA desiccated thyroid® contain the same warnings in their product monographs. They state that thyroid hormone therapy in patients with concomitant diabetes mellitus aggravates the intensity of symptoms, requiring appropriate adjustments of therapeutic measures.
Precautions listed in the Erfa Thyroid product monograph regarding diabetes
Final Thoughts…
If you have any type of diabetes and have also been diagnosed with hypothyroidism (including Hashimoto’s disease), you must monitor your blood glucose regularly. Contact your doctor if you notice a worsening in your blood sugar control to evaluate your need for dosage adjustments or additional medication.
Consider adopting a diet that is lower in carbohydrates. According to a 2019 consensus report from the American Diabetes Association, reducing overall carb intake has “the most evidence for improving glycemia” [3]. However, if you take certain medications, please seek medical oversight before adopting a very low-carbohydrate diet.
More Info
Learn about me and the dietary support that I can provide for managing your blood sugar with concurrent hypothyroidism by viewing the Comprehensive Dietary Package. If you were previously diagnosed with type 2 diabetes and recently diagnosed with hypothyroidism, view the Hypothyroid Management Package.
To your good health!
Joy
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References
- U.S. Food and Drug Administration. (2020). Highlights of Prescribing Information for Synthroid (levothyroxine sodium). [https://www.accessdata.fda.gov/drugsatfda_docs/label/2020/021402s034lbl.pdf]
- U.S. Food and Drug Administration. (2018). Highlights of Prescribing Information for Cytomel (liothyronine sodium). [https://www.accessdata.fda.gov/drugsatfda_docs/label/2018/010379s054lbl.pdf]
- Evert, A. B., et al. (2019). Nutrition Therapy for Adults With Diabetes or Prediabetes: A Consensus Report. Diabetes Care, 42(5), 731-754. [https://doi.org/10.2337/dci19-0014]
- AbbVie Inc. (2022). Armour Thyroid (thyroid tablets, USP) Prescribing Information. [https://dailymed.nlm.nih.gov/dailymed/getFile.cfm?setid=56b41079-60db-4256-9695-202b3a65d13d&type=pdf]
- ERFA Canada 2012 Inc. (2017). Thyroid (Thyroid Tablets, USP) Product Monograph. [https://pdf.hres.ca/dpd_pm/00034857.PDF]
- Talwalkar, P., Deshmukh, V., & Bhole, M. (2019). Prevalence of hypothyroidism in patients with type 2 diabetes mellitus and hypertension in India. Diabetes, Metabolic Syndrome and Obesity, 12, 369-376. [https://doi.org/10.2147/DMSO.S181470]
- Eom, Y. S., Wilson, J. R., & Bernet, V. J. (2022). Links between Thyroid Disorders and Glucose Homeostasis. Diabetes & Metabolism Journal, 46(2), 239-256. [https://doi.org/10.4093/dmj.2022.0013]
© 2025 BetterByDesign Nutrition Ltd.
I am a Registered Dietitian Nutritionist and the owner of BetterByDesign Nutrition Ltd. With a postgraduate degree in Human Nutrition and a background as a published mental health nutrition researcher, I have been dedicated to supporting my clients’ clinical needs since 2008.
I hold active professional licenses in BC (CHPBC), Alberta (CDA), and Ontario (CDO), allowing me to provide regulated Medical Nutrition Therapy across these provinces. My expertise spans chronic disease management, complex digestive health, and therapeutic diets. I am deeply passionate about helping people reclaim their health, rooted in my firm belief that Nutrition is BetterByDesign©.