Can COVID-19 vaccines cause hyperthyroidism?

I’m starting today’s blog post with the BOTTOM LINE: COVID-19 has caused millions of disabilities and deaths worldwide. Over 2 BILLION people have had the COVID vaccine. The benefits of COVID-19 vaccination far outweigh any potential risks. Vaccination of the global population against COVID-19 remains strongly recommended.

As blogged previously, there is emerging data to suggest that infection with the COVID-19 virus may trigger the thyroid to become overactive (called hyperthyroidism).

Since I wrote that blog, I have had a couple of colleagues share with me that they have seen a few patients collectively who have developed thyroiditis, not in the context of a known coronavirus infection, but rather, after having had a COVID-19 vaccine.

Two recent papers have reported on 5 cases of hyperthyroidism after the COVID-19 vaccine.

Backgrounder: the three most common causes of hyperthyroidism are:

Graves’ disease (an autoimmune condition)
Thyroiditis, where the thyroid becomes temporarily overactive, then settles down on its own (can be autoimmune, caused by a virus (sometimes called subacute thyroiditis), can be seen after pregnancy; other causes exist as well)
Hyperfunctioning thyroid nodules

One paper, published in the Journal of Endocrinology & Metabolism, reported on three patients who developed subacute thyroidits after an inactivated COVID vaccine called CoronaVac (this is a conventional killed whole virus vaccine, prepared with the method used for vaccines for decades; it is not an mRNA vaccine). One developed thyroiditis 5 days after the first vaccination dose, and the other two were 4 and 7 days after the second dose. None had positive thyroid antibodies (a marker of autoimmune thyroid disease).

Another paper, published in the journal Thyroid, describes two patients who developed Graves’ disease, with onset 2-3 days after receiving the Pfizer mRNA COVID vaccine.

So how could a vaccine trigger the thyroid to become overactive? Adjuvants in vaccines are intentionally used as agents to enhance the immune response to a vaccine, and these can rarely trigger immune reactions in genetically predisposed individuals. This phenomenon has been reported with vaccines directed towards HPV (the virus that causes cervical cancer), hepatitis B, and influenza. Hyperthyroidism has been reported after vaccination for HPV and influenza.

It is important to note that:

Most cases of vaccine-associated hyperthyroidism either get better on their own, or are fairly straightforward to treat.
These are only a few case reports of hyperthyroidism after COVID-19 vaccination, and further data are needed to know whether these observations are coincidental, or actually cause and effect (causal).
As blogged last week, 10-20% of people admitted to hospital with a COVID-19 infection have elevated thyroid hormone levels.
As above, vaccine-induced hyperthyroidism has also been reported with vaccines that are long accepted as safe (HPV and influenza).

BOTTOM LINE: (I will say it again): COVID-19 has caused millions of disabilities and deaths worldwide. Over 2 BILLION people have had the COVID vaccine. The benefits of COVID-19 vaccination far outweigh any potential risks. Vaccination of the global population against COVID-19 remains strongly recommended.

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