Thyroid TSH
Reference Range Checker
See if your TSH level falls in the normal range for adults or by pregnancy trimester, with subclinical vs overt classification.
| Group | Normal Range | Notes |
|---|---|---|
| Non-pregnant adults | 0.4 – 4.0 | Standard reference; varies slightly by lab |
| Pregnancy — 1st trimester | 0.1 – 2.5 – 4.0 | hCG lowers TSH; ATA 2017 fallback upper limit ~4.0 |
| Pregnancy — 2nd trimester | 0.2 – 3.0 | Use lab-specific range where available |
| Pregnancy — 3rd trimester | 0.3 – 3.0 | Use lab-specific range where available |
| Pregnancy — treatment threshold | > 10.0 | Overt hypothyroidism; treatment generally recommended |
Where available, laboratory-specific and trimester-specific reference ranges are preferred over these general fallback figures.
What Is a Normal TSH Level?
TSH is typically the first screening test ordered when thyroid dysfunction is suspected, precisely because it's a highly sensitive indicator that often becomes abnormal before other thyroid hormone levels shift measurably — but a full diagnosis usually requires additional testing (particularly free T4) alongside clinical evaluation of symptoms.
Why Pregnancy Changes the Reference Range
TSH reference ranges are meaningfully lower during pregnancy, particularly in the first trimester, due to a specific hormonal mechanism: human chorionic gonadotropin (hCG), which rises sharply in early pregnancy, has a weak but real thyroid-stimulating effect of its own. This hCG-driven stimulation naturally suppresses TSH during the first trimester, meaning a TSH level that would be considered borderline-low in a non-pregnant adult can be entirely normal and expected in early pregnancy. The American Thyroid Association's 2017 guidelines specifically recommend using laboratory-specific, trimester-specific reference ranges where available, since these vary by population and testing method — when no local range exists, a fallback upper limit of approximately 4.0 mIU/L in early pregnancy is commonly used, a meaningful revision from older, stricter fixed targets like 2.5 mIU/L that research has since suggested were overly conservative and led to overdiagnosis of subclinical hypothyroidism in some populations.
Subclinical vs. Overt: Why Free T4 Matters
📊 Subclinical Hypothyroidism
Elevated TSH with a normal free T4 level. Often mild or asymptomatic, and management decisions (especially in pregnancy) are more individualized.
⚠️ Overt Hypothyroidism
Elevated TSH combined with a low free T4 level. Generally more symptomatic and more consistently warrants treatment.
This distinction genuinely matters for both symptom expectations and treatment decisions — a TSH level of 6 mIU/L with normal free T4 (subclinical) is managed quite differently than the same TSH level with low free T4 (overt), which is exactly why this calculator includes an optional free T4 input to add this important layer of context when available.
Why TSH Above 10 Is a Clear Treatment Threshold in Pregnancy
Research has consistently found that a TSH level above 10 mIU/L during pregnancy indicates overt hypothyroidism and is associated with meaningful risks to both maternal and fetal health if left untreated, including increased risk of miscarriage, preterm delivery, and impaired fetal neurodevelopment — which is why treatment is generally recommended without delay at this threshold. The picture is genuinely more nuanced for TSH levels between 2.5 and 10 mIU/L in pregnancy: research, including a notable NEJM trial (Casey et al., 2017), found that starting levothyroxine treatment for milder subclinical hypothyroidism or isolated hypothyroxinemia after the first trimester did not improve child cognitive outcomes — one reason clinicians still hold varying views on treatment thresholds in this intermediate range, and why individualized clinical judgment matters more here than at the clearer >10 threshold.
How UK NHS Guidance Compares
UK laboratories and NHS guidance use broadly similar TSH reference ranges to those cited here, though — as is standard practice internationally — exact upper and lower limits vary somewhat between individual UK labs based on their specific testing equipment and local population data, rather than there being one single nationally mandated number. For pregnancy specifically, UK obstetric and endocrinology guidance similarly emphasizes using trimester-specific and, where possible, laboratory-specific reference ranges rather than a single fixed figure, aligning closely with the American Thyroid Association's approach. One area of ongoing international discussion is the optimal treatment threshold for mildly elevated TSH in pregnancy (the 2.5-10 mIU/L range) — this remains a genuinely debated area in both US and UK clinical practice, with individual case factors like thyroid antibody status and symptom presence often weighing more heavily than the specific TSH number alone.
Common Symptoms Worth Discussing With a Doctor
- Hypothyroidism (high TSH) symptoms: fatigue, weight gain, cold intolerance, dry skin, constipation, depression
- Hyperthyroidism (low TSH) symptoms: weight loss, heat intolerance, rapid heartbeat, anxiety, tremor, difficulty sleeping
- Symptoms alone aren't diagnostic — they overlap substantially with many other conditions, which is exactly why TSH testing (not symptom pattern-matching) is the standard first step
Related Health Context
Thyroid function connects to broader metabolic health in several ways. If your evaluation includes weight or metabolic concerns, our HOMA-IR Calculator and Biological Age Calculator cover related metabolic markers. If you're managing thyroid concerns during pregnancy specifically, our Pregnancy Calculator can help track your current trimester precisely for reference-range purposes.
Frequently Asked Questions
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