TSH
Thyroid-stimulating hormone produced by the pituitary gland
Also known as
Definition
TSH (thyroid-stimulating hormone, also called thyrotropin) is produced in the pituitary gland and regulates thyroid hormone production via a feedback loop. It is the most sensitive and widely used screening marker for thyroid dysfunction. An elevated TSH indicates hypothyroidism; a suppressed TSH suggests hyperthyroidism or overtreatment.
| Parameter | Value |
|---|---|
| Unit | mU/l (mIU/l) |
| Reference Range | 0.4–4.0 mU/l (per clinical guidelines) |
| Optimal Range (lab2go) | 0.5–2.5 mU/l (lab2go Optimum Zone for well-being) |
↓ What a low value means
Low TSH (< 0.4 mU/l) indicates over-stimulation of the thyroid or over-dosed levothyroxine therapy. Clinical features: hyperthyroidism symptoms (palpitations, weight loss, sleep disturbance, sweating, tremor, anxiety, osteoporosis risk with sustained suppression). TSH near zero (< 0.1 mU/l): overt hyperthyroidism — urgent evaluation needed. Subclinical hyperthyroidism (TSH 0.1–0.4, fT4 normal): elevated risk of atrial fibrillation and fractures, especially in older adults.
↑ What a high value means
Elevated TSH (> 4.0 mU/l) points to hypothyroidism or pituitary over-stimulation. TSH 4–10 mU/l: subclinical hypothyroidism (fT4 still normal) — treatment decision individualized by symptoms, age, and antibody status. TSH > 10 mU/l: overt hypothyroidism; levothyroxine therapy usually indicated. Causes: Hashimoto's thyroiditis (most common), iodine deficiency, post-thyroid surgery, radiation therapy.
✓ How to optimize this marker
Hashimoto's (most common cause of elevated TSH): selenium (100–200 µg/day) can lower antibody titers and improve quality of life. Adequate iodine (iodized salt, not excessive). Reduce gluten with coexisting celiac disease. Levothyroxine: standardized morning administration fasting, 30–60 min before breakfast (calcium, iron, and coffee impair absorption). Do not self-medicate with T3/T4.
When to test
No fasting required. Morning (8–10 AM) due to circadian rhythm (TSH highest in the morning). Take levothyroxine only after the blood draw. First screening recommended at age 45, or earlier with symptoms or family history. With Hashimoto's and ongoing therapy: check every 6–12 months or 6–8 weeks after dose adjustment.
Frequently asked questions
Why does TSH fluctuate with the same fT4? +
TSH is highly sensitive and responds logarithmically to minimal fT4 changes. Small deviations in fT4 produce pronounced TSH swings. TSH also varies throughout the day (morning peak). Multiple measurements or comparison with one's personal baseline improve interpretive value.
Should TSH be below 2.5 mU/l? +
Many functional medicine practitioners prefer a TSH target of 0.5–2.5 mU/l for optimal well-being. The laboratory norm up to 4.0 mU/l is a statistical cutoff from the general population. With symptoms despite a TSH within range, fT3, fT4, and antibody testing can provide additional clues.
What does TSH mean during pregnancy? +
Pregnancy-specific TSH targets apply: < 2.5 mU/l in the 1st trimester, < 3.0 mU/l in the 2nd/3rd trimester. HCG stimulates the thyroid and physiologically lowers TSH in the 1st trimester. Untreated hypothyroidism increases risks of miscarriage and developmental disorders in the child.
Sources
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.