Stable TSH can be rechecked in 2 years

Clinical Question

How much do seemingly stable thyroid tests vary over time?

Bottom Line

Most patients receiving thyroid replacement therapy with less than 125 mcg levothyroxine per day can wait 2 years before monitoring with thyroid stimulating hormone (TSH) if their level is normal. Fewer than 1 in 10 patients who take less than 125 mcg levothyroxine per day with a normal TSH level will have an abnormal laboratory value 1 year later. The likelihood goes up to 26.7% if the dosage is higher than 125 mcg levothyroxine per day. Patients with TSH levels closer to the upper or lower limits of normal will also be slightly more likely to have an abnormal value in 1 year. (LOE = 1b)

Reference

Pecina J, Garrison GM, Bernard ME. Levothyroxine dosage is associated with stability of thyroid-stimulating hormone values. Am J Med 2014;127(3):240-245.

Study Design: Cohort (retrospective)

Funding: Self-funded or unfunded

Setting: Outpatient (primary care)

Allocation: Unknown

Synopsis

These authors identified 715 patients (84% female, average age = 54 years) in a single primary care practice who were treated for hypothyroidism and had a normal TSH value (0.3 mIU/L – 5.0 mIU/L). They recorded all subsequent TSH levels in these patients for the following 6 years. Age, sex, body mass index, and history of chronic autoimmune thyroiditis were not associated with the development of a subsequent abnormal TSH level, but the dosage of levothyroxine was. Approximately 1 in 4 patients taking more than 125 mcg levothyroxine per day (26.7%) had an abnormal TSH level 1 year later. Most of the patients taking lower dosages (91.1%) had normal TSH levels 1 year later.

Allen F. Shaughnessy, PharmD, MMedEd
Professor of Family Medicine
Tufts University
Boston, MA
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