What is the normal reference range for serum thyroid‑stimulating hormone (TSH) in healthy adults, and how does it differ in children and during pregnancy?

Normal TSH Reference Ranges in Adults, Children, and Pregnancy

Standard Adult Reference Range

The normal TSH reference range for healthy adults is 0.45–4.5 mIU/L, though this range shifts upward with advancing age. 1

• The geometric mean TSH in disease-free adult populations is approximately 1.4 mIU/L 1
• TSH values within 0.45–4.5 mIU/L are considered physiologically normal and do not indicate thyroid dysfunction when free T4 is also normal 1

Age-Related Changes in Adult TSH Reference Ranges

TSH reference ranges increase progressively with age, and using standard adult ranges in elderly patients leads to substantial overdiagnosis of subclinical hypothyroidism.

• For adults under 40 years, the upper limit of normal TSH is approximately 3.6 mIU/L 1
• The upper limit rises to approximately 7.5 mIU/L in individuals over 80 years 1
• Approximately 12% of persons aged 80+ without thyroid disease have TSH levels >4.5 mIU/L, confirming that age-adjusted interpretation is essential 1
• Recent research demonstrates that implementing age-specific reference intervals reduces inappropriate subclinical hypothyroidism diagnoses: in women aged 50–60 from 13.1% to 8.6%, and in women aged 90–100 from 22.7% to 8.1% 2
• In men aged 60–70, age-adjusted ranges reduced diagnoses from 10.9% to 7.7%, and in men aged 90–100 from 27.4% to 9.6% 2

Age-Specific Upper Limits

The upper TSH reference limit increases from age 50–60 onward, while the lower limit remains stable until approximately age 80 2. This physiological shift means that a TSH of 5–6 mIU/L may represent normal aging rather than pathology in very elderly patients 1.

Pediatric TSH Reference Ranges

Children have distinctly different TSH reference ranges compared to adults, with higher values in younger age groups that gradually decline toward adult levels.

Pediatric reference ranges by age group (using electrochemiluminescence immunoassay) 3:

• Ages 4–6 years: TSH 0.62–4.90 μU/mL
• Ages 7–8 years: TSH 0.53–5.16 μU/mL
• Ages 9–10 years: TSH 0.67–4.52 μU/mL
• Ages 11–12 years: TSH 0.62–3.36 μU/mL
• Ages 13–14 years: TSH 0.54–2.78 μU/mL
• Ages 15 years: TSH 0.32–3.00 μU/mL

Using adult reference values in children can lead to misdiagnosis of hypothyroidism or failure to detect mild subclinical hypothyroidism, making age-appropriate ranges essential. 3

Pregnancy-Specific TSH Reference Ranges

TSH reference ranges are substantially lower during pregnancy, particularly in the first trimester, due to human chorionic gonadotropin (hCG) stimulation of the thyroid gland.

First Trimester Reference Ranges

• The first-trimester TSH reference range (2.5th–97.5th percentile) is approximately 0.20–4.60 mIU/L in iodine-sufficient populations 4
• Another study from an iodine-sufficient area reported first-trimester TSH of 0.1–4.4 mIU/L 5
• When more rigorous screening excludes thyroid antibodies and ultrasound abnormalities, the upper limit may be as low as 3.6 mIU/L 5

Physiological Basis

Normal pregnancy causes subnormal TSH concentrations with normal free T4 levels because hCG structurally resembles TSH and directly stimulates the thyroid gland. 6

• Using non-pregnant TSH reference ranges (0.45–4.5 mIU/L) in pregnant women is inappropriate and will miss significant thyroid dysfunction 6
• Both the American Thyroid Association and European thyroid associations recommend using local trimester-specific reference intervals 4

Monitoring in Pregnancy

For pregnant women with pre-existing hypothyroidism on levothyroxine:

• Check TSH every 4 weeks until stable, then every trimester 6
• Levothyroxine requirements typically increase by 25–50% during pregnancy 1
• Target TSH <2.5 mIU/L in the first trimester for women planning pregnancy or with known hypothyroidism 1

Critical Pitfalls to Avoid

Never apply standard adult TSH reference ranges to children or pregnant women—this leads to systematic misdiagnosis. 3, 6

• A TSH of 4.0 mIU/L may be normal in an 85-year-old but warrants evaluation in a 30-year-old 1
• A TSH of 3.5 mIU/L is normal in a non-pregnant woman but may indicate hypothyroidism requiring treatment in the first trimester of pregnancy 4, 5
• Children aged 4–8 years normally have higher TSH values (up to 5.16 μU/mL) than adults 3

TSH values exhibit substantial day-to-day variability (up to 50% fluctuation), so a single abnormal value should never trigger treatment decisions—always confirm with repeat testing after 3–6 weeks. 1