Interval between tests and thyroxine estimation method influence outcome of monitoring of subclinical hypothyroidism

Most patients with subclinical hypothyroidism are regularly monitored when treatment is not started. We have studied how interval between follow-up visits and how different T(4) estimates influence diagnostic outcome in a cohort of patients with untreated subclinical hypothyroidism, and studied whether assessment of clinical symptoms and signs aids evaluation of an individual subclinical hypothyroidism patient.

During 1 yr, monthly measurements of TSH and three different T(4) estimates, and recording of hypothyroid symptoms and signs were performed in 21 patients with subclinical hypothyroidism confirmed on two occasions 3 months apart.

One patient was euthyroid at all visits, and one started treatment for profound overt hypothyroidism. The remaining patients were subclinical hypothyroidism at 74%, overtly hypothyroid at 22%, and had normal thyroid function tests in 4% of the visits. Increasing frequency of visits associated significantly with decreasing number of patients characterized as subclinical hypothyroidism after 1 yr (P = 0.016). Diagnosis of overt hypothyroidism differed between T(4) estimates (P = 0.005) and was highly dependent on T(4) reference limits. The hypothyroid clinical score did not differ between biochemical diagnoses (P = 0.29).

The monitoring procedure itself may influence the outcome of control of subclinical hypothyroidism. Specifically, the interval between visits, type of T(4) estimate used, and lower T(4) reference limit influenced the outcome when untreated subclinical hypothyroidism patients were followed for 1 yr. The hypothyroid clinical score did not aid the evaluation in individual subclinical hypothyroidism patients.