Thyroid Medication can Worsen Blood Sugar Control in People with Diabetes

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, the pharmacist didn’t mention it when I first filled my prescription, nor was I provided with any printed information when my thyroid medication was delivered. My doctor didn’t mention it either but understandably, he knew I had been in remission of diabetes for three years prior to be diagnosed with hypothyroidism and probably didn’t think of me as being diabetic.  He was well-aware that for three years prior to be diagnosed with hypothyroidism, I did not meet the diagnostic criteria for type 2 diabetes either on the basis of fasting blood glucose or HbA1C because I controlled my blood sugar through diet.

When I began taking thyroid medication after my diagnosis of hypothyroidism, I began to periodically feel unwell as I did when I had high blood sugar.  I began to test my blood glucose more often and discovered that it was routinely spiking as high as the mid- to high 10 mmol/L (~190 mg/dl) for seemingly no reason. 

Effect of thyroid medication on blood sugar

I was totally puzzled as to why.  I didn’t eat simple carbs or starch-based food. I wasn’t sick, or under any new stress. I was sleeping well, was  properly hydrated, and there was no reason that I could think of that my blood sugar would keep spiking. In my search for answers, I stumbled across information that indicated that it should be well known that thyroid medication can worsen blood sugar control in people diagnosed with diabetes. For some reason, this information was not communicated to me, and when I asked others with both disorders, they were also unaware.

While I have already been eating low carb for the past three years, I began eating very low carb in order to get a better handle on the blood glucose spikes, and it is helping. I am also, in conjunction with my doctor, adjusting the timing of my thyroid medication around the timing of my meals to minimize the impact of the thyroid medication on my blood sugar and will continue to monitor my blood sugar several times per day. If need be, I will have my doctor either prescribe a medication such as Metformin to support normal blood sugar and/or trial different doses of thyroid medications. The important factor is I now know and can monitor this and make changes, as necessary. 

How many people have no idea?

Diabetes and Thyroid Medication 

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, as well as their need for diabetes medications, if they take any. Since those with type 1 diabetes and gestational diabetes have to monitor their blood glucose very closely, they would notice any changes, but many people with type 2 diabetes rarely regularly check. 

Thyroid Medication — highlights of prescribing information (product monographs) 

The “Highlights of Prescribing Information” sheets for both Synthroid® and Cytomel® that are available to doctors and pharmacists warn that therapeutic used of “these medications in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent [i.e., diabetes medications] or an “increase in insulin requirements.” In addition, there is a warning on prescribing both medications to “carefully monitor glycemic control (i.e., blood sugar) after starting, changing, or discontinuing thyroid hormone therapy.” This means that people with diabetes need to be continuing to monitor their own blood sugar and contact their doctors if there is a need to address worsening blood sugar control related to taking thyroid replacement medication.

The “Highlights of Prescribing Information” for Synthroid® [1] is a 19-page product monograph about this medication which states that it does “not include all the information needed to use this medication safely and effectively.” For that, there is the need to read the full prescribing information. In other words, these 19 pages are only a summary of all that is needed to use the medication safely and effectively! 

Below is page 1 of the 19 of the “Highlights of Prescribing Information” for Synthroid®.

Highlights of Prescribing Information for Synthroid – page 1 of 19 highlighted

If the relevant section wasn’t highlighted in yellow by me, how likely would it be that a pharmacist or physician would have noticed this warning amongst the 19 pages of fine type?

The “Highlights of Prescribing Information” for Cytomel® [2] is an 11-page product monograph about that medication. It also states that it does “not include all the information needed to use this medication safely and effectively.” Those sheets are just a summary, and there is the need to read the full prescribing information.

Below is page 1 of the “Highlights of Prescribing Information” for Cytomel®.

Highlights of Prescribing Information for Cytomel – page 1 of 11 highlighted

Again, if the relevant section wasn’t highlighted in yellow by me, how likely would it be that a pharmacist or physician would have noticed this warning amongst the 11 pages of fine type?


[Post publication note (May 1, 2023)] 

Since natural desiccated thyroid (NDT) also called natural desiccated extract (NDE) are not approved by the FDA or Health Canada as medications, there are no Prescribing Information sheets for products such as the US product Armour Thyroid®, or equivalent ERFA desiccated thyroid®,  in Canada, but both products contain the same warning.

Page 3 of 24 of the Product Monograph from Armour®  contains a warning under Contraindications;

“Thyroid hormone therapy in patients with concomitant diabetes mellitus or diabetes insipidus or adrenal cortical insufficiency aggravates the intensity of their symptoms. Appropriate adjustments of the various therapeutic measures directed at these concomitant endocrine diseases are required.”

Page 3 of 24 of the Product Monograph from Armour thyroid®

Page 1 of 9 of the Product Monograph from ERFA desiccated thyroid®  contains the same warning under Precautions;

page 4 of 9 Product Monograph Erfa Thyroid®

Recently published studies report that 11%–23% of people with type 2 diabetes also have hypothyroidism [6] making it essential that people with both diagnoses know about the possible effect of thyroid replacement on blood sugar control.

For those interested in the mechanism, a paper published last month explains how thyroid hormones contribute to a rise in blood glucose.  In the liver, thyroid hormones increases expression of glucose transporter 2 (GLUT2), which increases in both gluconeogenesis and glycogenolysis.  In liver, the thyroid hormone T3 increases gluconeogenesis by increasing activity of phosphoenolpyruvate carboxykinase (PEPCK), and in adipose tissue, thyroid hormones increase lipolysis, resulting in an increase in free fatty acid that stimulates hepatic gluconeogenesis [7].

Additional References

4.  Product monograph for Armour Thyroid:

5. Product monograph for Erfa:

6. Talwalkar P, Deshmukh V, Bhole M. Prevalence of hypothyroidism in patients with type 2 diabetes mellitus and hypertension in India: a cross-sectional observational study. Diabetes Metab Syndr Obes. 2019 Mar 20;12:369-376. doi: 10.2147/DMSO.S181470. PMID: 30936734; PMCID: PMC6431000.

7. Eom YS, Wilson JR, Bernet VJ. Links between Thyroid Disorders and Glucose Homeostasis. Diabetes Metab J. 2022 Mar;46(2):239-256. doi: 10.4093/dmj.2022.0013. Epub 2022 Mar 24. PMID: 35385635; PMCID: PMC8987680.

Final Thoughts…

If you have any type of diabetes and have also been diagnosed with hypothyroidism (including Hashimoto’s disease, the autoimmune form), it is very important that you monitor your blood glucose regularly. 

Contact your doctor if you notice a worsening in your blood sugar control, and to have your doctor evaluate your need for an increased dosage of existing diabetes medications, or the introduction of these medications if you don’t currently take any.

Consider adopting a style of eating that is lower in carbohydrate than you currently eat. According to a 2019 consensus report from the American Diabetes Association, reducing overall carb intake has “the most evidence for improving glycemia [blood sugar]” [3] . But please keep in mind that while a low carb diet is safe and effective for those with diabetes, if you take certain types of medications it is necessary to have medical oversight before adopting a very low carbohydrate (“keto”) diet.

More Info

If you have diabetes or pre-diabetes* and would like information on how I can support you in better managing your blood sugar, please send me a note through the Contact Me form at the top of this page.

To your good health!


*Please note that I do not design Meal Plans for people currently taking insulin or insulin-analogue medication for diabetes as I do not have CDE certification.

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  1. Food and Drug Administration, Highlights of Prescribing Information for Synthroid,
  2. Food and Drug Administration, Highlights of Prescribing Information for Cytomel,
  3. Evert, AB, Dennison M, Gardner CD, et al, Nutrition Therapy for Adults With
    Diabetes or Prediabetes: A Consensus Report, Diabetes Care, Ahead of Print, published online April 18, 2019,


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