Why Thyroid Symptoms Get Worse After the Flu or a Cold
- valtality
- 5 days ago
- 7 min read
Winter is well and truly here. For many people, that means tissues in every handbag, hot cups of tea, and trying to avoid the latest virus making its way around the office or school.
But if you have hypothyroidism or Hashimoto's thyroiditis, you've probably noticed something else.
Every time you catch a cold or the flu, your thyroid symptoms seem to flare. Your energy disappears, brain fog returns, you feel colder than usual, the joints ache and motivation plummets.
Often your next thyroid blood test even looks different (usually shocking worse) from the last one.
If you've ever wondered whether viruses can actually affect your thyroid gland and your thyroid blood markers, the answer is yes.
Let's explore why.
The thyroid isn't an organ that works in isolation.
It's closely linked to your immune system, nervous system and metabolism.
Whenever your body encounters a virus—whether it's a common cold, influenza, COVID-19, Epstein-Barr virus (EBV), or another respiratory infection—it launches an inflammatory response designed to eliminate the infection. This response is essential for recovery.
However, inflammation also changes how your body produces, transports and converts thyroid hormones. For someone with an already healthy thyroid, these changes are often temporary and may go completely unnoticed. For someone living with hypothyroidism or Hashimoto's, however, they can be enough to trigger a noticeable worsening of symptoms.
How Colds, Influenza and Other Viruses Affect Your Thyroid
When a virus enters the body, immune cells release signalling molecules called cytokines, including:
Interleukin-6 (IL-6)
Tumour necrosis factor-alpha (TNF-α)
Interleukin-1β (IL-1β)
These cytokines help coordinate the immune response, but they also influence thyroid hormone production and metabolism.
Scientists often refer to this as Non-Thyroidal Illness Syndrome (NTIS) or Euthyroid Sick Syndrome.
This isn't a thyroid disease itself.
Instead, it's the body's temporary adaptation during illness.
Can a Cold or Flu Change Your TSH?
TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland to signal the thyroid to produce T4 and T3.
During an acute viral illness, TSH can become temporarily suppressed.
Several mechanisms contribute:
inflammatory cytokines suppress TSH release from the pituitary
changes occur in hypothalamic signalling
reduced calorie intake during illness influences hormone regulation
elevated cortisol during stress suppresses TSH secretion
In severe illness, TSH may become lower than expected. During recovery, it may temporarily rebound and become mildly elevated before eventually returning to baseline.
This is one reason thyroid function tests performed during an acute illness may not accurately reflect someone's usual thyroid function.
Can Viral Infections Affect Free T4?
Free T4 generally remains relatively stable during mild viral infections.
However, during more significant infections such as influenza, COVID-19 or pneumonia, changes can occur.
Possible findings include:
normal Free T4
mildly reduced Free T4
occasionally elevated Free T4 early in inflammatory thyroiditis
The degree of change often reflects the severity of the illness rather than permanent thyroid dysfunction.
Why T3 Often Drops During Viral Illness
One of the most consistent findings during viral illness is a reduction in T3.
This happens as most of your active thyroid hormone isn't produced directly by the thyroid.
Instead, T4 is converted into T3 by enzymes called deiodinases.
Inflammatory cytokines reduce the activity of these enzymes.
As a result:
less T4 becomes active T3
more T4 may instead convert into inactive Reverse T3 (rT3)
metabolism slows temporarily to conserve energy while the body fights infection
This explains why many people recovering from viral illnesses experience:
profound fatigue
poor concentration
feeling unusually cold
muscle weakness
reduced exercise tolerance
slower digestion
These symptoms can closely resemble hypothyroidism despite being part of the body's temporary response to illness. If you're confused by your TSH, Free T3, Free T4 or thyroid antibodies, my Thyroid Blood Test Masterclass explains what each marker means.
Can Viruses Increase TPO and Thyroglobulin (TG) Antibodies?
This is one of the most common questions I receive from patients living with Hashimoto's.
The answer is: they can—but it's more nuanced than many people realise.
Hashimoto's thyroiditis is an autoimmune condition.
Your immune system mistakenly attacks thyroid tissue, producing antibodies against thyroid proteins.
The two main antibodies are:
Thyroid Peroxidase Antibodies (TPOAb)
Thyroglobulin Antibodies (TgAb)
During viral infections, the immune system becomes highly activated.
This increased immune activity may temporarily increase autoimmune activity in susceptible individuals.
Several viruses have been investigated as possible triggers or contributors to autoimmune thyroid disease, including:
Epstein-Barr Virus (EBV)
Hepatitis C
SARS-CoV-2 (COVID-19)
Coxsackie virus
Cytomegalovirus (CMV)
Rubella virus
Parvovirus B19
Researchers believe this may occur through mechanisms such as:
molecular mimicry
bystander activation
chronic immune stimulation
activation of dormant autoreactive immune cells
While these mechanisms may contribute to autoimmune thyroid disease in genetically susceptible people, current evidence does not support the idea that every common cold or influenza infection will cause a lasting increase in thyroid antibodies.
Instead, repeated immune activation over many years may contribute to autoimmune progression in susceptible individuals.
Can Viral Infections Trigger Thyroiditis?
Yes. Some viruses can trigger subacute thyroiditis, also known as de Quervain thyroiditis.
This is an inflammatory condition where the thyroid becomes painful and temporarily releases stored thyroid hormone.
People may experience:
neck pain
pain swallowing
fever
rapid heartbeat
anxiety
sweating
temporary hyperthyroidism
After several weeks, thyroid hormone stores become depleted and many people temporarily become hypothyroid before eventually recovering. Most people regain normal thyroid function within several months, although a small percentage develop permanent hypothyroidism.
Why Hashimoto's Symptoms Often Get Worse During Winter
It's rarely just one thing.
Winter often combines several stressors at once:
increased exposure to viruses
reduced sunlight
lower vitamin D levels
poorer sleep
less physical activity
increased emotional stress
comfort foods that may not support optimal health
For someone already managing thyroid disease, these factors can combine to produce a noticeable flare in symptoms.
Should You Repeat Your Thyroid Blood Tests While You're Sick?
Generally, no.
Because viral illnesses can temporarily alter thyroid hormone levels, testing during an acute infection may produce misleading results.
Unless your practitioner specifically requests urgent testing or you develop symptoms suggestive of thyroiditis, it's usually better to wait until you've recovered before assessing thyroid function.
Many endocrinology guidelines recommend repeating thyroid function tests several weeks after recovery if abnormalities were found during an acute illness.
Supporting Your Thyroid During Cold and Flu Season
While we can't avoid every virus, we can reduce the burden they place on the body.
Some practical strategies include:
Prioritising quality sleep.
Eating adequate protein to support immune function.
Maintaining sufficient vitamin D levels where appropriate.
Managing stress as best you can.
Staying hydrated.
Washing hands regularly and minimising unnecessary exposure to illness.
Allowing yourself adequate recovery time instead of pushing through fatigue.
Most importantly, remember that worsening thyroid symptoms after a viral illness don't necessarily mean your thyroid condition has permanently deteriorated.
In many cases, your body is simply recovering from the additional workload placed upon it.
With time, rest and appropriate support, thyroid function often returns to its previous baseline.
Frequently Asked Questions
Can a cold affect thyroid blood tests?
Yes. During a viral infection, inflammatory chemicals produced by your immune system can temporarily alter thyroid hormone levels. TSH may decrease, T3 often falls first, and in more severe illnesses Free T4 may also change. These results usually return to normal after recovery.
Can influenza increase thyroid antibodies?
Influenza itself doesn't permanently raise thyroid antibodies in everyone. However, any significant viral infection activates the immune system, which may temporarily increase autoimmune activity in people already living with Hashimoto's thyroiditis.
Why do my thyroid symptoms get worse after the flu?
Many people notice increased fatigue, brain fog, constipation, muscle weakness and feeling cold after recovering from the flu. This often happens because inflammation reduces the body's ability to convert T4 into the active hormone T3 while your immune system is recovering.
Can viruses trigger Hashimoto's disease?
Researchers believe certain viruses, including Epstein-Barr virus (EBV), hepatitis C and SARS-CoV-2, may contribute to the development of Hashimoto's in genetically susceptible individuals. However, Hashimoto's usually develops through a combination of genetic, environmental and immune factors rather than one single viral infection.
Should I repeat my thyroid blood tests after a viral infection?
If your thyroid blood tests were performed while you were acutely unwell, many healthcare practitioners recommend repeating them several weeks after recovery, as temporary illness-related changes can occur.
Ready to Get Answers?
If you're struggling with thyroid symptoms, recurring winter illnesses, or confusing thyroid blood test results, I'd love to help.
Book your free 15-minute discovery call, and we'll discuss what's been happening, whether further investigation may be helpful, and what your next steps could look like.
Sometimes, one conversation is all it takes to start making sense of your symptoms.
References (APA 7th Edition)
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