Signs Your Hashimoto’s Is Flaring (And What to Do About It)
If you have Hashimoto’s thyroiditis, you already know that living with this condition isn’t always predictable. Some weeks you feel almost normal. Other weeks, out of nowhere, your body feels like it’s working against you — and you can’t quite explain why.
That’s a flare. And if you’ve ever been caught off guard by one, you’re not alone.
Hashimoto’s is an autoimmune condition in which the immune system mistakenly attacks the thyroid gland, causing inflammation and — over time — reduced thyroid function. It’s the most common cause of hypothyroidism in the United States, and it affects far more women than men. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), Hashimoto’s disease affects about 5 in 100 people in the US.
Understanding what a flare looks like — and what triggers one — can be the difference between spiraling for weeks and catching it early. Here’s what you need to know.
What Is a Hashimoto’s Flare?
A Hashimoto’s flare-up is a period of intensified autoimmune activity where your symptoms worsen, sometimes suddenly and significantly. During a flare, your immune system ramps up its attack on the thyroid, which can destabilize your hormone levels and cause a cascade of symptoms throughout your body.
Flares aren’t just “feeling a bit off.” They can be physically and emotionally exhausting, and they can develop within hours and last anywhere from a few days to several weeks.
What makes Hashimoto’s particularly tricky is that flares can push your thyroid in either direction. Most of the time, a flare worsens hypothyroid (underactive thyroid) symptoms. But in some cases — especially in the earlier stages of the disease — the destruction of thyroid tissue can temporarily release a surge of stored thyroid hormone into the bloodstream, causing a transient hyperthyroid state known as hashitoxicosis. This is temporary, but it can feel alarming if you don’t know it’s possible.
Signs Your Hashimoto’s Is Flaring
Because Hashimoto’s affects your entire body through thyroid hormone levels, flare symptoms can show up in a lot of different ways. Here are the most common signs to watch for:
1. Crushing Fatigue That Sleep Doesn’t Fix
This isn’t regular tiredness. A Hashimoto’s flare can bring on a bone-deep exhaustion that doesn’t lift no matter how much you rest. You wake up tired. You feel drained by mid-morning. Simple tasks feel monumental. If your fatigue has suddenly intensified or returned after a period of feeling stable, your thyroid may be the culprit.
2. Brain Fog
Difficulty concentrating, forgetting words mid-sentence, feeling mentally “slow” or disconnected — this is brain fog, and it’s one of the most commonly reported Hashimoto’s symptoms. During a flare, it can feel like you’re thinking through wet concrete. According to the Mayo Clinic, slowed thinking and impaired memory are hallmark symptoms of an underactive thyroid.
3. Mood Changes — Especially Anxiety or Depression
Thyroid hormones play a significant role in regulating mood. When levels drop during a flare, many people experience increased feelings of depression, sadness, or emotional flatness. Conversely, if you’re in a hashitoxicosis phase, you may feel unusually anxious, jittery, or irritable. Sudden and unexplained mood shifts are worth paying attention to.
4. Increased Sensitivity to Cold
Feeling cold when others aren’t is a classic hypothyroid sign. During a flare, you might notice you’re reaching for an extra blanket, can’t seem to warm up, or that your hands and feet are constantly cold. This happens because low thyroid hormone slows your metabolism, which reduces heat production in the body.
5. Weight Changes
An unexplained uptick on the scale despite no changes to your diet or activity level can indicate your thyroid is struggling. Conversely, unexpected weight loss may signal a temporary hyperthyroid swing. Both warrant attention.
6. Muscle and Joint Aches
Hashimoto’s is an inflammatory condition, and during a flare, that inflammation doesn’t stay contained to the thyroid. Many people experience generalized muscle aches, joint pain, or stiffness — sometimes described as feeling like you have the flu without actually being sick.
7. Hair Loss or Changes in Hair Texture
Hair thinning is a common and distressing symptom of Hashimoto’s. During a flare, you might notice more hair in the shower drain, on your pillow, or in your brush. Hair may also become dry, coarse, or brittle. Thyroid hormones are essential to the hair growth cycle, so disruptions show up quickly.
8. Neck Swelling or Discomfort
Hashimoto’s can cause a goiter — an enlargement of the thyroid gland. During a flare, some people notice pressure, fullness, or tenderness in the throat or neck area. If you experience difficulty swallowing or breathing, contact your doctor promptly. The American Thyroid Association notes that while goiters from Hashimoto’s are usually painless, some people do experience discomfort.
9. Digestive Issues
Constipation is a classic hypothyroid symptom — a slowed metabolism means a slowed digestive system. During a flare, some people also experience bloating, nausea, or general GI discomfort. If your Hashimoto’s is in a temporary hyperthyroid phase, diarrhea may occur instead.
10. Heart Palpitations (Especially During a Hyperthyroid Swing)
If you’re experiencing a hashitoxicosis episode, you may notice a racing heart, palpitations, or an irregular heartbeat. According to research published in PMC, palpitations and arrhythmias are recognized symptoms of the hyperthyroid phase of Hashimoto’s. This symptom should always be evaluated by a doctor.
Common Hashimoto’s Flare Triggers
Understanding what sets off a flare can help you get ahead of them. According to Paloma Health, the most common triggers include:
Stress — Psychological and physical stress elevates cortisol, which can suppress thyroid function and trigger immune dysregulation. This is one of the most well-documented flare triggers.
Illness or infection — Viral infections in particular (including COVID-19 and Epstein-Barr virus) are known to provoke autoimmune activity. Getting sick can kick off a significant flare even weeks later.
Poor sleep — Chronic sleep deprivation elevates inflammatory markers and puts additional strain on an already-stressed immune system.
Dietary choices — For some people, gluten, dairy, soy, or high iodine intake can worsen autoimmune activity. This is highly individual — not everyone with Hashimoto’s reacts to the same foods — but these are worth noting if you notice patterns.
Overtraining — High-intensity exercise without adequate recovery can act as a physiological stressor. During a flare, your body needs gentler movement.
Hormonal shifts — Pregnancy, postpartum, perimenopause, and other hormonal transitions are high-risk windows for autoimmune flares. Many women are first diagnosed with Hashimoto’s around these life stages.
Missed or changed medication — If you’re on levothyroxine and miss doses or change brands, it can cause TSH fluctuations that worsen symptoms.
Environmental exposures — Pesticide exposure, BPA from plastics, and nutrient deficiencies (particularly selenium and vitamin D) have all been linked to worsened autoimmune activity.
What to Do When You’re Flaring
1. Contact Your Doctor and Check Your Labs
If your symptoms have noticeably worsened, it’s time to get bloodwork. TSH, Free T3, Free T4, and TPO antibody levels can help your doctor assess what’s happening. According to Cleveland Clinic, regular monitoring of thyroid levels is essential for managing Hashimoto’s effectively. Don’t wait and hope it passes — get the data.
2. Prioritize Rest — Without Guilt
This sounds simple but can be genuinely hard to do. Your body is under real physiological stress during a flare, and rest is not laziness — it’s medicine. Protect your sleep, reduce your schedule where possible, and give yourself permission to slow down.
3. Reduce Stressors Where You Can
You can’t eliminate stress entirely, but you can be intentional about reducing it during a flare. Gentle activities like slow walks, light yoga, breathwork, or spending time in nature can help lower cortisol levels and support your immune system.
4. Focus on Anti-Inflammatory Eating
A systematic review published in PMC found that nutritional intervention can positively impact Hashimoto’s by reducing inflammation and supporting thyroid function. Focus on whole, nutrient-dense foods — leafy greens, colorful vegetables, wild-caught fish, berries, olive oil, and nuts. Minimize ultra-processed foods, refined sugar, and alcohol during a flare.
Key nutrients to prioritize:
- Selenium — found in Brazil nuts, wild-caught fish, and eggs. Research from the American Thyroid Association suggests selenium may help reduce TPO antibodies, though supplementation should only be done under medical supervision.
- Vitamin D — deficiency is common in people with Hashimoto’s and is associated with increased autoimmune activity.
- Omega-3 fatty acids — found in fatty fish, walnuts, and flaxseed, these help reduce pro-inflammatory cytokines.
5. Move Gently — Not Intensely
Skip the intense workouts during a flare. High-intensity exercise is a physiological stressor, and your body doesn’t need more stress right now. Walking, restorative yoga, swimming, or gentle stretching are far better choices. Movement supports circulation, mood, and lymphatic drainage without demanding more from an already taxed system.
6. Track Your Symptoms
Keep a simple symptom journal — even just notes in your phone. Tracking what you ate, your stress levels, your sleep, and your symptoms can help you and your doctor identify patterns and triggers over time. This information is genuinely useful at appointments and can help you anticipate and prevent future flares.
When to Seek Immediate Medical Attention
Most Hashimoto’s flares, while miserable, are not medical emergencies. However, contact your doctor promptly if you experience:
- Heart palpitations, racing heart, or irregular heartbeat
- Difficulty swallowing or breathing due to neck swelling
- Severe depression or anxiety
- Sudden unexplained weight loss
- Extreme fatigue that prevents daily functioning
These symptoms warrant evaluation to rule out other causes or complications.
A Final Word
Living with Hashimoto’s means learning to listen to your body in a whole new way. Flares can feel like setbacks, but they’re also information — your body signaling that something needs attention.
The more you understand your own patterns and triggers, the more empowered you’ll be to manage them. You are not powerless here. With the right support, the right information, and a little self-compassion, you can navigate a flare and come out the other side.
And as always — you are not alone in this.
Xx Monti
Disclaimer: This post is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider about your specific symptoms and treatment plan.
