Thyroxine Therapy and TSH Levels

October 1, 2006

Recent laboratory guidelines from the National Academy of Clinical Biochemistry indicate that more than 95% of persons with normal thyroid function have a TSH level of less than 2.5 mIU/L.

The normal thyroid-stimulating hormone (TSH) reference range at our laboratory is 0.35 to 5.5 mIU/L. When I am treating a patient with levothyroxine, I do not adjust the dosage if the TSH level is within this range and the patient is otherwise asymptomatic. A colleague tells me that I should be adjusting the levothyroxine dosage to keep the TSH level below 2 mIU/L. He feels that lower TSH levels are associated with improved lipid profiles.

Some experts advocate a narrower TSH range, and others state that the wider range is appropriate. What do you suggest?

---- Bruce Schober, DO
     Easley, SC

Recent laboratory guidelines from the National Academy of Clinical Biochemistry indicate that more than 95% of persons with normal thyroid function have a TSH level of less than 2.5 mIU/L.1 The remainder, who have higher values, are outliers; most of them are likely to have underlying Hashimoto thyroiditis or another condition that results in an elevated TSH level. In large, truly "normal" populations, the mean TSH level for white Americans is 1.40 mIU/L, and in African Americans (who have a very low incidence of underlying early thyroid failure as a result of Hashimoto thyroiditis), it is 1.18 mIU/L.1 The upper limit of the reference range at the University of Southern California's reference laboratory has been dropped to 2.6 mIU/L. In my view, laboratories that still use reference ranges with an upper limit of 5.5 mIU/L are being irresponsible.

I would not start a patient on lifelong levothyroxine therapy on the basis of one marginally abnormal TSH level. Patients with a level in the range of 3 to 6 mIU/L should have their TSH level remeasured in 2 months along with testing for anti-TPO antibodies. However, if the marginal elevation is confirmed and the results of antibody testing are positive, I would start the patient on levothyroxine therapy.

In a patient who is already taking levothyroxine, has no history of thyroid cancer (which may require TSH suppression), and has a TSH level between 3 and 6 mIU/L, I would increase the dosage to titrate the TSH level down to somewhere between 0.5 mIU/L and, as a maximum, 2.0 mIU/L. The ideal level is one that is close to the average mean cited above of 1.40 mIU/L in white Americans or 1.18 mIU/L in African Americans.

---- Leonard Wartofsky, MD
     Chairman, Department of Medicine
     Washington Hospital Center
     Washington, DC

References:

REFERENCE:


1.

Wartofsky L, Dickey RA. The evidence for a narrower thyrotropin reference range is compelling.

J Clin Endocrinol Metab

. 2005;90:5483-5488.

FOR MORE INFORMATION:


Dickey RA, Wartofsky L, Feld S. Optimal thyrotropin level: normal ranges and reference intervals are not equivalent.

Thyroid

. 2005;15:1035-1039.