Hashimoto’s disease causes your immune system to gradually destroy your thyroid gland, leading to symptoms like fatigue, weight gain, cold sensitivity, and dry skin. It is the most common cause of hypothyroidism (underactive thyroid), and it affects women far more often than men, with the highest rates occurring between ages 25 and 39. Because the damage happens slowly, symptoms can creep in over months or years before you realize something is wrong.
How Hashimoto’s Damages the Thyroid
Your thyroid is a small, butterfly-shaped gland at the front of your neck that produces hormones controlling your metabolism, heart rate, body temperature, and energy levels. In Hashimoto’s disease, your immune system mistakenly identifies thyroid tissue as a threat. White blood cells infiltrate the gland, causing inflammation and progressively destroying the cells that make thyroid hormones. Over time, scar tissue replaces healthy tissue, and the gland loses its ability to keep up with your body’s hormone needs.
This process also triggers the production of antibodies against your own thyroid. The most important one for diagnosis is called TPO antibody, which targets a key enzyme your thyroid needs to produce hormones. A second antibody, called thyroglobulin antibody, attacks the protein your thyroid uses as a building block. Both can be measured with a simple blood test.
The Most Common Symptoms
Because thyroid hormones influence nearly every organ system, the symptoms of Hashimoto’s are widespread and easy to mistake for other conditions. The hallmark signs, as the disease progresses into full hypothyroidism, include:
- Fatigue that doesn’t improve with rest
- Unexplained weight gain or difficulty losing weight
- Cold intolerance, feeling chilled when others are comfortable
- Joint and muscle pain
- Constipation
- Dry skin or dry, thinning hair
- Heavy or irregular menstrual periods
- Fertility problems
- Slowed heart rate
Many people also notice puffiness in the face, brain fog, and a general sense of sluggishness that makes even routine tasks feel harder than they should. The thyroid gland itself may enlarge, forming what’s called a goiter. This can make the front of your neck look swollen and create a sensation of fullness in your throat, though it typically isn’t painful.
Early Signs Before Full Hypothyroidism
Hashimoto’s doesn’t start with dramatic symptoms. There’s often a long subclinical phase where blood tests show a slightly elevated TSH level but your thyroid hormone levels remain in the normal range. During this stage, your thyroid is working harder to compensate for the ongoing immune attack, and you may feel mostly fine.
But even in this early window, subtle symptoms can appear. A study comparing patients with subclinical hypothyroidism to people with normal thyroid function found that dry skin, cold intolerance, and easy fatigability were significantly more common in the subclinical group. These symptoms improved once treatment began. If you’ve noticed these changes and can’t explain them, they may be worth mentioning to your doctor, especially if thyroid disease runs in your family.
The Temporary Hyperthyroid Phase
Here’s something that surprises many people: Hashimoto’s can briefly cause the opposite of hypothyroidism. Early in the disease, as immune cells destroy thyroid tissue, stored hormones spill into the bloodstream all at once. This creates a temporary surge that produces hyperthyroid symptoms: a racing heart, anxiety, tremors, weight loss, and feeling overheated.
This phase, sometimes called hashitoxicosis, typically lasts a few weeks to a few months and then resolves on its own. The thyroid gland during this time is usually firm and slightly enlarged but not tender. Because the symptoms look like other forms of hyperthyroidism, it can be misdiagnosed. The key difference is that hashitoxicosis is self-limiting. Once the stored hormones are depleted and the gland has sustained enough damage, the disease shifts into its more permanent hypothyroid pattern.
Effects on Your Heart, Cholesterol, and Metabolism
Thyroid hormones directly regulate how your body uses energy, how your heart contracts, and how your liver processes cholesterol. When those hormones drop, the ripple effects go well beyond feeling tired.
On the cardiovascular side, hypothyroidism reduces the heart’s pumping efficiency and increases resistance in blood vessels. This combination can raise blood pressure, particularly diastolic pressure (the bottom number). Even in the subclinical stage, studies have found higher blood pressure and cholesterol levels compared to people with normal thyroid function. The most common heart-related finding in subclinical hypothyroidism is a subtle stiffening of the heart muscle that reduces exercise tolerance, something you might notice as getting winded more easily than you used to.
Cholesterol is affected because thyroid hormones help your liver clear LDL (“bad” cholesterol) from the blood. When those hormones drop, LDL levels rise. This is one reason people with untreated Hashimoto’s often see their cholesterol climb even when their diet hasn’t changed. Treating the underlying thyroid problem typically brings these numbers back down.
Who Gets Hashimoto’s Disease
Women are overwhelmingly more likely to develop Hashimoto’s. Prevalence peaks between ages 25 and 39, with a recent large study of women finding the highest rates in the 25 to 29 and 35 to 39 age ranges. Having a first-degree relative with autoimmune thyroid disease significantly increases your risk, and people with other autoimmune conditions like type 1 diabetes, celiac disease, or rheumatoid arthritis are also at higher risk.
Genetics play a clear role. Specific variations in immune-regulating genes influence susceptibility. One particular genetic marker, a combination of two immune system gene variants called HLA-DRB1*03/*16, has been identified as a strong susceptibility factor. Another gene variant, HLA-DRB1*13, appears to be protective against the disease. You can’t test for these in a standard doctor’s visit, but the family history pattern is a useful clue. If your mother, sister, or aunt has thyroid problems, your own risk is meaningfully elevated.
How It’s Diagnosed
Diagnosis usually involves two types of blood tests. The first checks your thyroid function: TSH (thyroid-stimulating hormone) and free T4. In Hashimoto’s, TSH is typically elevated because your pituitary gland is sending louder and louder signals to a thyroid that can’t keep up. Free T4, the active thyroid hormone circulating in your blood, may be low or still in the normal range depending on how far the disease has progressed.
The second test looks for thyroid antibodies. TPO antibody levels above 5.6 IU/mL or thyroglobulin antibody levels above 4 IU/mL suggest an autoimmune process targeting the thyroid. A high TSH combined with elevated TPO antibodies is the classic diagnostic picture for Hashimoto’s. Some people test positive for antibodies years before their thyroid function actually changes, which is why repeat testing over time can be valuable if your initial results are borderline.
What Happens If It Goes Untreated
Left unmanaged, Hashimoto’s continues to destroy thyroid tissue until the gland can no longer produce adequate hormones. The symptoms described above worsen progressively. Cholesterol continues to climb, raising long-term cardiovascular risk. A goiter may grow large enough to cause visible neck swelling and difficulty swallowing. Fertility problems become more pronounced, and pregnancy complications, including miscarriage, become more likely.
In rare and extreme cases, severe untreated hypothyroidism can lead to a life-threatening condition called myxedema, marked by dangerously low body temperature, confusion, and organ slowdown. This is uncommon precisely because most cases are caught and treated long before they reach that point. The treatment itself is straightforward: a daily synthetic thyroid hormone pill that replaces what your gland can no longer make. Most people feel significantly better within weeks of starting it, though finding the right dose can take some adjustment.