Is there a need to redefine the upper normal limit of TSH?
Level 5 - mechanism / opinion, no new human data
Narrative review and expert opinion summarizing population studies without a systematic review methodology
PubMed 16645008 · doi:10.1530/eje.1.02136
What was done
This narrative review evaluated evidence from large population studies regarding whether to lower the conventional upper normal reference limit of serum thyroid-stimulating hormone (TSH) from 4.0–5.0 mU/l to 2.0–2.5 mU/l, and assessed the clinical implications for diagnosing and treating subclinical hypothyroidism.
What was found
The abstract reports no quantitative pooled data or effect estimates. It states that applying strict inclusion criteria in general population studies (no personal or family history of thyroid disease, no thyroid antibodies, and normal thyroid ultrasound) did not unequivocally yield an upper limit of 2.0–2.5 mU/l. The authors conclude that evidence is insufficient to justify lowering the upper normal limit, recommend maintaining the 0.4–4.0 mU/l reference interval, and state that classifying or treating individuals with TSH between 2 and 4 mU/l likely does more harm than good.
Why it matters
Lowering the TSH threshold would reclassify millions of healthy individuals as having subclinical hypothyroidism, risking widespread overtreatment with levothyroxine without proven benefit.
Limits
This is a narrative review without systematic search or meta-analytic methodology. The abstract provides no primary quantitative data, sample sizes, or formal risk estimates.
Cited by
- supports Conventional laboratory reference ranges define normal TSH levels as anywhere between 0.5 and 5.0 mIU/L.