NEW ORLEANS – Most clinical laboratories utilize an overly broad, out-of-date normal reference range for thyroid-stimulating hormone values in patients with hypothyroidism who are on thyroid hormone replacement therapy.
Today, most labs still define a normal TSH reference range as running from 0.5 at the bottom to a maximum of 4.5 or even 5.5 mIU/L on the basis of early cross-sectional studies that didn’t exclude patients with underlying thyroid disease.
Dr. Thomas L. O'Connell
Several recent studies, including the National Health and Nutrition Examination Survey III, suggest a much lower upper limit of normal once individuals with positive antithyroid antibodies or a family history of thyroid disease have been excluded. As recently as 2002, the American Association of Clinical Endocrinologists recommended a normal reference range of 0.3-3.0 mIU/L. The National Academy of Clinical Biochemistry has suggested that "normal" is 0.4-2.5 mIU/L. But most labs have yet to adopt either of these tighter standards, explained Dr. Thomas L. O’Connell, an endocrinologist at Duke University in Durham, N.C.
The implications for clinical practice have been laid out in a joint statement by the American Association of Clinical Endocrinologists, the Endocrine Society, and the American Thyroid Association (J. Clin. Endocrinol. Metab. 2005;90:581-5). The organizations recommended that if a patient being treated for hypothyroidism feels well and has a TSH level within the laboratory’s upper limit of normal, then fine, leave the dosing of thyroid hormone replacement therapy as is. But if the patient continues to have symptoms, then it’s appropriate to boost treatment to drop the TSH level to the lower half of the lab’s "normal" range.
"I do this all the time. It’s a perfectly reasonable thing to try. I’ll have patients come to me with a TSH of 5.0 and their physician says their thyroid function has been normalized because the lab has a normal range of 0.5-5.5 mIU/L or something like that, yet they still have fatigue and are gaining weight and have the other classic symptoms of hypothyroidism. So what I do is tweak their dose a bit and get their TSH below 2.5, and if need be close to 1.0. It’s often a matter of taking one extra half-pill 1 day per week or several days a week," the endocrinologist said at the annual meeting of the American College of Physicians.
He reported that he serves as a consultant to Sanofi Aventis and Amylin.