How much do seemingly stable thyroid tests vary over time?
Most patients receiving thyroid replacement therapy with less than 125 mcg of levothyroxine per day can wait two years before monitoring thyroid-stimulating hormone (TSH) levels if their results are normal. Fewer than one in 10 patients who take less than 125 mcg of levothyroxine per day with a normal TSH level will have an abnormal laboratory value one year later. The likelihood goes up to 26.7% if the dosage is higher than 125 mcg of 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 one year. (Level of Evidence = 1b)
These authors identified 715 patients (84% women; average age = 54 years) in a single primary care practice who were treated for hypothyroidism and had a normal TSH value (0.3 to 5.0 mIU per L). They recorded all subsequent TSH levels in these patients for the following six 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 associated. Approximately one in four patients taking more than 125 mcg of levothyroxine per day (26.7%) had an abnormal TSH level one year later. Most of the patients taking lower dosages (91.1%) had normal TSH levels one year later.
Study design: Cohort (retrospective)
Funding source: Self-funded or unfunded
Setting: Outpatient (primary care)
Reference: PecinaJGarrisonGMBernardMELevothyroxine dosage is associated with stability of thyroid-stimulating hormone values. Am J Med.2014; 127( 3): 240– 245.