TSH is produced by the pituitary and controls the thyroid gland: it "orders" it to make the hormones T4 and T3.
The relationship here is inverse: the less thyroid hormone in the blood, the more the pituitary raises TSH, spurring the gland — and vice versa. That is why TSH is considered the most sensitive first marker: it goes out of range before T4 and T3 themselves do.
Reference ranges
The lab reference is usually 0.4–4.0 mIU/L, but many endocrinologists consider a narrower range optimal.
| Most labs' reference | 0.4 – 4.0 mIU/L |
| Optimal range | 0.5 – 2.5 mIU/L |
| Subclinical hypothyroidism | TSH above normal with normal free T4 |
| Overt hypothyroidism | TSH elevated, free T4 reduced |
| Hyperthyroidism | TSH reduced (often below 0.1), free T4/T3 elevated |
What an elevated level means
Elevated TSH means the pituitary is "spurring" the thyroid: it lacks the power to produce the needed amount of hormone.
Most often this is hypothyroidism: with normal free T4 it is called subclinical, with reduced free T4 — overt. The most common cause is autoimmune thyroiditis (Hashimoto's), so TPO antibodies are usually looked at along with TSH. Other causes: iodine deficiency, a state after surgery or radioiodine therapy, an insufficient levothyroxine dose.
Typical symptoms: fatigue, feeling cold, weight gain, dry skin, hair loss, constipation, "brain fog", low mood.
What a low level means
Low TSH indicates the opposite: an excess of thyroid hormone, and the pituitary "falls silent".
Causes: hyperthyroidism (Graves' disease, toxic nodular goiter), thyroiditis in the hormone-release phase, levothyroxine overdose.
Symptoms: palpitations, tremor, sweating, heat intolerance, anxiety, weight loss. A persistently low TSH raises the risk of atrial fibrillation and reduced bone density.
A separate, rarer variant is central hypothyroidism: TSH is low or "inappropriately normal" against a background of low free T4. Here the problem is in the pituitary itself, not the gland.
How the test is done
TSH is drawn from a vein. Strict fasting is not required, but it is better to take blood in the morning and always under consistent conditions: TSH has a pronounced daily rhythm — higher at night and early morning, lower toward evening. This matters so that results are comparable over time.
Key point: isolated TSH is not very informative. It is almost always interpreted together with free T4, and if needed — with free T3 and TPO antibodies.
What affects the value
- Biotin (vitamin B7) in high doses distorts the result — stop it 2–3 days before the test.
- Time of day — the TSH daily rhythm (higher in the morning, lower in the evening).
- Acute illness and severe stress — euthyroid sick syndrome.
- Pregnancy — norms differ, by trimester.
- Medications: levothyroxine, glucocorticoids, amiodarone, lithium, dopamine.
- Iodine and selenium deficiency — affect gland function and hormone conversion.
Calculate using this marker
Calculators where TSH (Thyroid-stimulating hormone) is used directly:
Frequently asked questions
Is TSH of 3.5 normal?
Formally yes: it falls within the 0.4–4.0 mIU/L reference. But it is in the upper half of the range. If there are hypothyroidism symptoms (fatigue, feeling cold, weight gain), it makes sense to check free T4 and TPO antibodies — that is how subclinical hypothyroidism is found.
Do I need to test TSH fasting?
Strictly fasting — not required. But it is better to test in the morning and under consistent conditions: TSH has a pronounced daily rhythm, and an evening result may differ noticeably from a morning one, which hinders comparing tests over time.
Why do symptoms persist with a normal TSH?
A normal TSH does not guarantee an adequate level of active T3 in the tissues. This happens with impaired peripheral conversion of T4 to T3 — then free T3 and the free-T3/free-T4 ratio are looked at, not just TSH.
Related terms
Sources
- Jonklaas J. et al. Guidelines for the Treatment of Hypothyroidism. Thyroid, 2014
- American Thyroid Association — Hypothyroidism
- MedlinePlus (NIH) — TSH (Thyroid-Stimulating Hormone) Test