TSH
Also known as: Thyroid Stimulating Hormone, Thyrotropin
What Does TSH Measure?
TSH (Thyroid Stimulating Hormone), also called thyrotropin, is a hormone produced by the pituitary gland in the brain. A TSH blood test measures how much of this hormone is circulating in your bloodstream. TSH acts as a messenger, telling the thyroid gland — a butterfly-shaped gland in the front of your neck — how much thyroid hormone to produce. When thyroid hormone levels in the blood drop too low, the pituitary gland releases more TSH to stimulate the thyroid to produce more. When thyroid hormone levels are too high, TSH levels fall to signal the thyroid to slow down production.
Why Does TSH Matter?
TSH is considered the single most sensitive and reliable screening test for thyroid function. Because of this feedback relationship between TSH and thyroid hormones, even small changes in thyroid function cause significant shifts in TSH levels, making it an early and powerful indicator of thyroid disorders. Thyroid hormones regulate metabolism, energy, body temperature, heart rate, mood, and nearly every organ system in the body. Abnormal TSH levels can indicate hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), both of which, if untreated, can lead to serious health consequences including cardiovascular disease, infertility, osteoporosis, and neurological complications. TSH testing is also used to monitor patients on thyroid medication to ensure they are receiving the correct dose.
Normal Ranges
Males
0.4–4.0 mIU/L (some labs use 0.5–4.5 mIU/L)
Females
0.4–4.0 mIU/L; during pregnancy: 0.1–2.5 mIU/L (first trimester), 0.2–3.0 mIU/L (second trimester), 0.3–3.0 mIU/L (third trimester)
Children
Varies by age: newborns 1.0–39.0 mIU/L, infants 0.6–10.0 mIU/L, children 0.5–4.5 mIU/L
Causes of High Levels
- Hypothyroidism (underactive thyroid) — the most common cause, where the thyroid is not producing enough hormone so the pituitary overproduces TSH to compensate
- Hashimoto's thyroiditis — an autoimmune condition that gradually destroys thyroid tissue, reducing hormone output
- Thyroid surgery or radioactive iodine treatment — removal or damage to thyroid tissue reduces hormone production
- Iodine deficiency — inadequate iodine limits thyroid hormone synthesis, triggering elevated TSH
- Certain medications — including lithium, amiodarone, and some cancer immunotherapies that interfere with thyroid function
- Pituitary tumors (TSH-secreting adenomas) — rare tumors that produce excess TSH independently of normal feedback
Causes of Low Levels
- Hyperthyroidism (overactive thyroid) — excess thyroid hormone suppresses TSH production by the pituitary gland
- Graves' disease — an autoimmune condition where antibodies stimulate the thyroid to overproduce hormone, suppressing TSH
How to Improve Your TSH
Diet
- Ensure adequate iodine intake through iodized salt, seaweed, dairy products, and seafood — iodine is essential for thyroid hormone production
- Eat selenium-rich foods such as Brazil nuts (1–2 per day), tuna, sardines, and eggs, as selenium is critical for converting T4 to active T3 hormone
- Consume zinc-rich foods like oysters, beef, pumpkin seeds, and legumes to support thyroid hormone synthesis and pituitary function
- Limit raw cruciferous vegetables (broccoli, kale, cabbage) in very large amounts if you have hypothyroidism, as they contain goitrogens that can mildly interfere with iodine uptake — cooking deactivates these compounds
- Avoid highly processed foods, excess sugar, and refined carbohydrates, which promote inflammation and can worsen autoimmune thyroid conditions
Supplements
- Selenium: 100–200 mcg/day has been shown in studies to reduce thyroid antibodies in Hashimoto's thyroiditis and support T4-to-T3 conversion
- Iodine: Only supplement (150–220 mcg/day) if confirmed deficient, as both excess and deficiency of iodine can worsen thyroid disorders — consult a doctor before supplementing
- Vitamin D: 1,000–4,000 IU/day if deficient, as low vitamin D is strongly associated with autoimmune thyroid disease and supplementation may reduce antibody levels
Related Biomarkers
Frequently Asked Questions
What does it mean if my TSH is high but my T4 and T3 are normal?
This pattern is called subclinical hypothyroidism. It means your pituitary gland is working harder than normal (producing more TSH) to keep your thyroid hormone levels in the normal range. Your thyroid is underperforming but still compensating. Many people with subclinical hypothyroidism have no symptoms, while others experience fatigue, weight gain, or brain fog. Whether to treat it depends on how high the TSH is, your symptoms, whether you have thyroid antibodies, and whether you are pregnant. A TSH above 10 mIU/L typically warrants treatment, while levels between 4–10 mIU/L may be monitored or treated based on individual circumstances.
What is considered a dangerously high or dangerously low TSH level?
A TSH above 10 mIU/L is generally considered significantly elevated and associated with overt hypothyroidism requiring treatment. Very high levels (above 20–50 mIU/L) indicate severe thyroid failure. On the other end, a TSH below 0.1 mIU/L is considered suppressed and indicates hyperthyroidism or excessive thyroid hormone replacement. TSH near zero (below 0.01 mIU/L) can be associated with serious risks including atrial fibrillation, heart failure, and bone loss. A 'thyroid storm' is a rare, life-threatening emergency involving extremely low TSH with very high thyroid hormone levels. Always consult your doctor about abnormal results.
Can stress affect my TSH levels?
Yes, both physical and psychological stress can influence TSH levels. Chronic stress elevates cortisol, which suppresses TSH secretion from the pituitary and also impairs the conversion of T4 to active T3 in the tissues. Acute illness or major physical stress (such as surgery) can cause transient drops in TSH and thyroid hormones — a pattern known as 'non-thyroidal illness syndrome' or 'sick euthyroid syndrome.' For this reason, doctors generally avoid testing thyroid function during acute illness unless absolutely necessary, as results may be misleading.