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 prior to 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
It is essential that people diagnosed with any form of diabetes (type 1, type 2, gestational diabetes) as well as hypothyroidism 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 highlighting 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), it is very important that you 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 style of eating 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.
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.
Joy Erdile is the Registered Dietitian at BetterByDesign Nutrition Ltd., and has been supporting clients’ nutritional needs since 2008. Joy has a post graduate degree in Human Nutirion, more than 17 years of experience in private practice, and is a published in mental health nutrition. She is licensed in BC, Alberta, and Ontario, and her areas of expertise range from routine to clinically complex cases. Joy is passionate about helping people feel better and restore their health, and believes that there is no one-size-fits-all approach, but that Nutrition is BetterByDesign©.