Hypothyroidism and the TSH Reference Range: Medical Journals and Associations

Purpose of this compilation

To show the flaws in using only the TSH (thyroid stimulating hormone) level as a diagnostic and assessment tool. This test can miss numerous cases of hypothyroidism and result in undertreatment because the range is too wide.

See also the links in the right column as well as in our T3 References and Desiccated Thyroid sections.

A. TSH levels above 3
B. TSH levels above 2

A. TSH levels above 3

1. "2003 Campaign Encourages Awareness of Mild Thyroid Failure, Importance of Routine Testing" (US, 2003)

Until November 2002, doctors had relied on a normal TSH level ranging from 0.5 to 5.0 to diagnose and treat patients with a thyroid disorder who tested outside the boundaries of that range. Now AACE encourages doctors to consider treatment for patients who test outside the boundaries of a narrower margin based on a target TSH level of 0.3 to 3.0⁴. AACE believes the new range will result in proper diagnosis for millions of Americans who suffer from a mild thyroid disorder, but have gone untreated until now.

2. "Thyroid Awareness Month: 2001" (US, 2001)

Even though a TSH level between 3.0 and 5.0 uU/ml is in the normal range, it should be considered suspect since it may signal a case of evolving thyroid underactivity.

B. TSH levels above 2

1. "NACB: Laboratory Support for the Diagnosis and Monitoring of Thyroid Disease: Published Guidelines" (US, 2002)

...given the high prevalence of mild (subclinical) hypothyroidism in the general population, it is likely that the current upper limit of the population reference range is skewed by the inclusion of persons with occult thyroid dysfunction....

...In the future, it is likely that the upper limit of the serum TSH euthyroid reference range will be reduced to 2.5 mIU/L because >95% of rigorously screened normal euthyroid volunteers have serum TSH values between 0.4 and 2.5 mIU/L....

A serum TSH result between 0.5 and 2.0 mIU/L is generally considered the therapeutic target for a standard L-T4 replacement dose for primary hypothyroidism.

2. "Hypothyroidism: screening and subclinical disease" (England, 1997)

...a high thyroid stimulating hormone concentration (>2 mU/l) was associated with an increased risk of future hypothyroidism....The simplest explanation is that thyroid disease is so common that many people predisposed to thyroid failure are included in a laboratory's reference population, which raises the question whether thyroxine replacement is adequate in patients with thyroid stimulating hormone levels above 2 mU/l.

3. "The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey" (1995, UK)

A logit model indicated that increasing values of serum TSH above 2mU/l at first survey increased the probability of developing hypothyroidism which was further increased in the presence of anti-thyroid antibodies.