What is TSH?
TSH stands for thyroid-stimulating hormone. It's made by your pituitary gland — a tiny gland at the base of your brain — and its job is to tell your thyroid how much hormone to produce. Think of TSH as a "request signal." When your thyroid is sluggish, your brain sends more TSH to push it harder. When your thyroid is overactive, TSH drops because the brain doesn't need to ask for more.
Your thyroid itself is a butterfly-shaped gland in your neck, and it controls some of the most fundamental things in your body: metabolism, energy levels, body temperature, weight, mood, and even how fast your heart beats.
Why it matters for longevity
An underactive thyroid (hypothyroidism) is one of the most common hormonal problems in the world — and women are 5 to 8 times more likely to be affected than men. It often creeps in slowly, showing up as fatigue, weight gain, brain fog, hair loss, dry skin, or feeling cold all the time. Many women chalk these symptoms up to stress or aging, when in reality their thyroid needs attention.
Even "subclinical" hypothyroidism — where TSH is mildly elevated but technically still in the "normal" range — has been linked to higher cholesterol, increased cardiovascular risk, and lower quality of life. Longevity-focused doctors take thyroid health seriously because an optimally functioning thyroid supports everything from your to your mood and cognitive sharpness over time.
What the numbers mean
The conventional reference range for TSH is:
- Normal: 0.4–4.0 mIU/L
- High (underactive thyroid): above 4.0 mIU/L
- Low (overactive thyroid): below 0.4 mIU/L
However, many integrative and longevity physicians consider a TSH of 0.5–2.5 mIU/L to be optimal. A TSH of 3.5 might be labeled "normal" on your lab report, but you could still feel tired, sluggish, or have trouble losing weight. If your TSH is above 2.5 and you have symptoms, it's worth a deeper conversation with your doctor — ideally including Free T3 and Free T4 tests for the full picture.
What affects your TSH
- Autoimmune conditions: Hashimoto's thyroiditis is the most common cause of hypothyroidism. Your immune system gradually attacks the thyroid, and it becomes less efficient over time. This is far more common in women.
- Stress: Chronic stress raises , which can interfere with thyroid hormone production and conversion.
- Nutrient deficiencies: Iodine, selenium, zinc, and iron are all essential for proper thyroid function. Low (iron stores) is especially common in women and directly affects thyroid health.
- Sleep deprivation: Poor sleep disrupts the hormonal cascade that keeps your thyroid running smoothly.
- Pregnancy and postpartum: Thyroid function often shifts during and after pregnancy. Postpartum thyroiditis affects about 5–10% of women.
- Age and menopause: Thyroid issues become more common as women age, particularly around and after menopause.
How to get tested
TSH is a simple blood test that does not require fasting. It's usually the first test doctors order when checking thyroid function. If your TSH comes back outside the optimal range, ask for a full thyroid panel that includes Free T3, Free T4, and thyroid antibodies (TPO and TgAb). This gives a much more complete picture. Testing once a year is a good baseline — more often if you're managing a known thyroid condition.
How to improve it
- Check your nutrients. Make sure you're getting enough iodine (from seafood or iodized salt), selenium (Brazil nuts are a great source), and zinc. If your ferritin is low, addressing iron stores can make a real difference.
- Manage stress. Since cortisol directly affects thyroid function, stress management isn't optional — it's essential. Daily walks, deep breathing, and adequate downtime all help.
- Prioritize sleep. Your thyroid hormones follow a circadian rhythm. Consistent, quality sleep supports healthy production and conversion of thyroid hormones.
- Move your body. Regular exercise supports thyroid function and boosts metabolism. Use our TDEE Calculator to understand how your metabolism and daily energy expenditure work together.
- Talk to your doctor about medication if needed. If your thyroid is truly underactive, lifestyle changes alone may not be enough. Thyroid hormone replacement is safe, well-studied, and can dramatically improve how you feel.