Age-Dependent TSH Reference Ranges in microIU/mL
Neonatal Period (Birth to 28 Days)
TSH levels are dramatically elevated immediately after birth and decline rapidly over the first weeks of life, requiring hour-specific and day-specific reference ranges for accurate interpretation. 1, 2
First 24 Hours of Life
- TSH reference range: 4.1-40.2 mIU/L (2.5th-97.5th percentile) 2
- Peak TSH occurs on day 1, with levels substantially higher than any other time in life 1
- Cord blood TSH is lower than day 1 values 1
25-48 Hours (Days 2)
49-72 Hours (Day 3)
73-96 Hours (Day 4)
- TSH reference range: 2.2-14.7 mIU/L 2
97-120 Hours (Day 5)
121-144 Hours (Day 6)
- TSH reference range: 1.4-12.7 mIU/L 2
145-168 Hours (Day 7)
- TSH reference range: 1.0-8.3 mIU/L 2
Days 10-14
- TSH levels stabilize below 10 mIU/L after 2 weeks postnatal age 1
- Mean thyroid volume by ultrasound: 0.72 ± 0.24 mL (range 0.36-1.62 mL) 1
Days 14-30
- TSH reference range: 1.90-10.34 mIU/L (2.5th-97.5th percentile) 3
- No significant difference in TSH values between day 14-21 and day 22-30 age groups 3
- Upper reference limit of approximately 10 mIU/L is established by this period 3
Plasma TSH Conversion from Newborn Screening
- Blood spot TSH upper reference: 3.04 mU/L (97.5th percentile) 4
- Plasma TSH upper reference intervals vary by assay method: 4
- Siemens Centaur: 7.6 mU/L
- Abbott Architect: 6.3 mU/L
- Roche Elecsys E170: 7.3 mU/L
- Siemens Immulite 2000: 8.3 mU/L
- Beckman Access HYPERsensitive: 6.5 mU/L
Infants Beyond Neonatal Period
Infants with mildly elevated TSH (10-20 mIU/L) at 14-30 days typically normalize on follow-up without treatment (mean decline from 11.41 to 4.42 mIU/L), suggesting transient elevation rather than true hypothyroidism. 3
Children, Adults, and Elderly
Standard Adult Reference Range (All Ages)
- TSH reference range: 0.45-4.5 mIU/L (2.5th-97.5th percentile in disease-free populations) 5, 6
- Geometric mean TSH in healthy populations: 1.4 mIU/L 5, 6
- This range applies across adult age groups from young adulthood through elderly 6
Age-Related Considerations in Older Adults
- Prevalence of subclinical hypothyroidism increases with age: 5
- 12% of persons aged ≥80 years with no thyroid disease have TSH >4.5 mIU/L, suggesting the standard population reference interval may be inappropriate for very elderly patients 5
- TSH secretion varies among subpopulations defined by age, with higher values considered potentially normal in the very elderly 5
Critical Interpretation Caveats
Assay-Specific Variations
- Different laboratory platforms produce varying results, making direct comparison problematic 7
- Most laboratories define abnormal TSH using 0.4-4.5 mIU/L, but this threshold is arbitrary and not based on adverse health outcomes 5
Physiological Variability
- TSH secretion is pulsatile with up to 50% variation in mean values day-to-day 5
- Serial TSH measurements at the same time of day show up to 40% variation 5
- A single abnormal TSH should never trigger immediate diagnosis or treatment 6
- 30-60% of mildly elevated TSH levels normalize spontaneously on repeat testing 8, 3
Non-Thyroidal Factors Affecting TSH
- Acute illness frequently suppresses TSH 5
- Medications (dopamine, glucocorticoids, octreotide, bexarotene, iodine) alter TSH levels 5
- Pregnancy (especially first trimester), anorexia nervosa, adrenal insufficiency, and pituitary adenomas interfere with normal TSH 5
- Recent iodine exposure (CT contrast) transiently affects thyroid function 8
Clinical Decision Thresholds
- TSH 0.45-4.5 mIU/L: Normal thyroid function 6
- TSH <0.45 mIU/L: Suggests subclinical or overt hyperthyroidism; measure free T4 and T3 6
- TSH 4.5-10 mIU/L: Subclinical hypothyroidism; measure free T4 and consider thyroid antibodies 6
- TSH >10 mIU/L: Generally warrants treatment consideration, particularly if symptomatic 6
- Confirm all abnormal values with repeat testing before making treatment decisions 6