Analytical goals for thyroid function tests when monitoring patients with untreated subclinical hypothyroidism

Subclinical hypothyroidism is a common condition and it is often monitored biochemically without treatment. For thyroid function tests, analytical goals and critical difference between two tests have been calculated from data on individual variation in euthyroid individuals. Variation in thyroid function tests differs in subclinical hypothyroidism. We aimed to calculate the analytical goals in subclinical hypothyroidism, both in a scientific study setting and in a clinical setting from disease specific variation in thyroid function tests in patients with stable, untreated subclinical hypothyroidism.

The serum concentration of TSH, fT4 and fT3 were measured monthly for one year in 15 patients with untreated subclinical hypothyroidism and stable TSH.

The within-person coefficient of variation (CV%) was 16.6 for TSH, 4.1 for fT4 and 4.0 for fT3, and the calculated goals for analytical imprecision were with CV% of 8.3, 2.0 and 2.0 respectively. The within-batch analytical CV%s for TSH, fT4 and fT3 were 1.7, 3.2 and 2.7 respectively. The between-batch analytical CV% of TSH increased to 6.3.

In patients with stable untreated subclinical hypothyroidism the goal for analytical imprecision was met with a single analysis of serum TSH, whereas duplicate and triplicate serum measurements of fT3 and fT4 would be necessary. Adding between-batch analytical imprecision for TSH did not alter results, a setting that mirrors every-day clinic.