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 can point to hypothyroidism, a suppressed TSH to hyperthyroidism or overtreatment; it is interpreted together with fT4 and fT3.
| Parameter | Value |
|---|---|
| Unit | mU/l (mIU/l) |
| Reference Range | 0.4–4.0 mU/l (per clinical guidelines) |
Agree on your personal target range with your doctor.
↓ What a low value means
Low TSH (< 0.4 mU/l) can point to an overactive thyroid or over-dosed levothyroxine therapy. Whether hyperthyroidism is present, and which form, only becomes clear together with fT4, fT3 and the clinical context: with normal fT4 and fT3 it is called subclinical, with elevated levels overt hyperthyroidism. Pregnancy in the first trimester or, rarely, a disorder of the pituitary gland (then with low fT4) can also lower TSH. Possible symptoms of an overactive thyroid: palpitations, weight loss, sleep disturbance, sweating, tremor, anxiety, osteoporosis risk with sustained suppression. With TSH near zero (< 0.1 mU/l), especially with symptoms, prompt medical evaluation is indicated. Subclinical hyperthyroidism (TSH 0.1–0.4, fT4 normal) goes along with an elevated risk of atrial fibrillation and fractures, especially in older adults.
↑ What a high value means
Elevated TSH (> 4.0 mU/l) can point to an underactive thyroid or, rarely, increased TSH production by the pituitary. Whether hypothyroidism is present, and which form, only becomes clear together with fT4 and the clinical context: with elevated TSH and normal fT4 it is called subclinical, with low fT4 overt hypothyroidism. The TSH level alone does not decide this; the NICE guideline NG145 also classifies a TSH of 10 mU/l or more with normal fT4 as subclinical hypothyroidism. Causes: Hashimoto's thyroiditis (most common), iodine deficiency, previous thyroid surgery, radiation therapy. Whether and how treatment is given is decided by your doctor.
✓ What can influence this value
The TSH value is influenced by thyroid function and conditions such as Hashimoto's thyroiditis (the most common cause of elevated values), iodine supply, thyroid surgery, and radiation therapy. Levothyroxine therapy changes the value; the absorption of levothyroxine can be impaired by calcium, iron, and coffee. Further influences are the time of day of the blood draw (highest values in the morning) and pregnancy, in which HCG lowers the TSH value in the 1st trimester. The assessment is made by the treating doctor.
When to test
Fasting is not needed. TSH follows a daily rhythm and is highest in the morning, so the time of the blood draw affects the value. TSH is measured with suspected thyroid dysfunction, with symptoms or a family history, and during ongoing therapy. When levothyroxine is taken, from what age and how often the value is measured is decided by your doctor.
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.
How is a TSH value in the upper normal range assessed? +
The laboratory's reference range applies (0.4–4.0 mU/l per the AWMF guideline); the doctor makes the assessment. The laboratory norm 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 reference values apply, commonly cited as < 2.5 mU/l in the 1st trimester and < 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.
TSH
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.