ABSTRACT
Purpose We conducted this study to identify clinical predictors of normal thyroid-stimulating hormone (TSH) values over a one-year interval in patients treated for hypothyroidism.
Methods We retrospectively reviewed cases of patients treated for hypothyroidism by the Mayo Clinic Department of Family Medicine in 2006. For patients with a normal TSH value during the initial study period in 2006, we assessed the number who then had therapeutic and nontherapeutic TSH values 10 to 14 months later, and evaluated whether body mass index (BMI), age, sex, or dosage of levothyroxine replacement had predictive value of a normal TSH level.
Results The percentage of normal repeat TSH values significantly declined with increasing medication dosage (P=.01). Of those patients whose maintenance dosage was <75 mcg/d, 90.8% had normal repeat TSH values, compared with just 77.5% of those requiring ≥125 mcg/d, who had significantly lower odds of normal repeat TSH (odds ratio, 0.31; 95% confidence interval, 0.13–0.76; P=.01).
Conclusions Age, sex, and BMI were not predictive of stable TSH values in patients treated for hypothyroidism. The dosage of thyroid hormone replacement was predictive of normal TSH values, with dosages ≥125 mcg/d having significantly decreased odds of a normal repeat TSH on follow-up.
Once the level of thyroid-stimulating hormone (TSH) has been normalized in a patient treated for hypothyroidism, the American Association of Clinical Endocrinologists recommends yearly monitoring.1 Annual testing has become the default frequency of surveillance for many primary care providers, despite a relative paucity of data to support the recommendation as customary practice.
Factors that can warrant close monitoring of TSH levels. Elderly patients often require lower average doses of levothyroxine replacement of 1 mcg/kg of body weight.2 Factors that can influence the stability of the TSH level include age, lean body mass changes, pregnancy,3 and malabsorptive states. Concomitant medications, such as antacids, calcium,4,5 and selective serotonin reuptake inhibitors,6 may also affect TSH levels. Various formulations of levothyroxine from different manufacturers may have different bioequivalence; thus a change in brands of medication could affect TSH levels.7
Rationale for monitoring TSH levels. Subtherapeutic replacement may not alleviate potential secondary effects of hypothyroidism, including hyperlipidemia, or cardiovascular and neuropsychiatric effects,8 whereas supratherapeutic replacement is associated with an increased risk of atrial fibrillation9 and decreased bone mass in postmenopausal women.8,10 Due to the potential hazards of excess thyroxine replacement, as well as the desire to avoid inadequate replacement, it is necessary to monitor the patient’s response to replacement. But can the frequency of monitoring vary?
Less frequent monitoring may be acceptable for some. One retrospective study has suggested that an 18-month interval may be more appropriate for patients younger than 60 years taking a levothyroxine dose of 100 to 150 mcg/d.11 Total health care expenditures for chronic care patients could possibly be reduced by decreasing testing frequency. The objective of this retrospective study was to identify predictors of stable TSH values over one year in patients treated for hypothyroidism, which might allow for a longer monitoring interval.
Methods
Patient selection
We reviewed the electronic medical records of patients with hypothyroidism treated by the Mayo Clinic Department of Family Medicine from January 1, 2006 through January 1, 2007, and identified a random sample of 780 patients with a documented TSH value in the normal range (0.3-5.0 mIU/L) and no other exclusionary criteria (see below). We reviewed laboratory results to determine if repeat TSH assay(s) were performed during the subsequent 10 to 14 months. We chose this period in an attempt to approximate the one-year interval of monitoring used in standard practice to follow patients with hypothyroidism not requiring dosage changes.
Out of the 780 patients, we identified 452 who had repeat TSH measurements performed 10 to 14 months after documentation of a normal TSH value. We recorded and analyzed demographic data obtained at the time of the first TSH measurement, including age, sex, body mass index (BMI), and thyroxine dose, to determine if there were any identifiable characteristics predicting normal TSH values on repeat screening.
Exclusion criteria. We excluded 328 patients who had normal TSH levels recorded in 2006 but did not undergo a repeat TSH measurement in the subsequent 10 to 14 months or had a repeat TSH measurement done sooner than 10 months that necessitated a change in levothyroxine dosage. We also excluded individuals who were younger than 18 years at the time of the initial 2006 TSH value; who were pregnant during the study period; who had a history of thyroid cancer (due to different recommendations for TSH goals); or who were taking amiodarone or lithium during the study period.