Hypothyroidism (an underactive thyroid gland) is diagnosed through a blood test measuring three hormones, T3 (triiodothyronine), T4 (thyroxine), and TSH (thyroid-stimulating hormone or thyrotropin). If levels of T3 and T4 are low or normal and the level of TSH is abnormally high then hypothyroidism is deemed to be present. The reference range for TSH is 0.2 – 5.5 mU/L. This range was arrived at by measuring TSH levels in a large group of seemingly healthy people. Several researchers have recently questioned this approach. They point out that undiagnosed, subclinical hypothyroidism may be present in up to 40 per cent of women. This obviously could have skewed the results since these women would have been considered healthy even though they actually had low-grade hypothyroidism. A group of British researchers now suggest that the reference range for TSH may be too wide. They point out that when a group of individuals with no history of thyroid disease and no antibodies against thyroid peroxidase was tested their TSH values fell within a much narrower range of 0.48 – 3.60 mU/L. Other studies have shown that TSH values above 1.9 mU/L are associated with thyroid peroxidase antibody positivity indicating abnormal pathology in the thyroid. There is also evidence that individuals with TSH values greater than 2.0 mU/L have an increased risk of developing clinical hypothyroidism over the next 20 years. The researchers believe that a TSH range of 0.5 – 3.5 mU/L rather than 0.2 – 5.5 mU/L should be considered the norm, but point out that there is emerging epidemiological data suggesting that a TSH level above 2.0 mU/L may be associated with adverse effects.
Dayan, Colin M., et al. Whose normal thyroid is better – yours or mine? The Lancet, Vol. 360, August 3, 2002, pp. 353-54 (commentary)