Context/Objective: Most patients with subclinical hypothyroidism (SH) are regularly monitored when treatment is not started. We have studied how interval between follow-up visits and how different T4 estimates influence diagnostic outcome in a cohort of patients with untreated SH and studied if assessment of clinical symptoms and signs aids evaluation of individual SH patient.

Design/Patients: During a year monthly measurements of TSH and three different T4 estimates and recording of hypothyroid symptoms and signs were performed in 21 patients with SH confirmed on two occasions three months apart.

Results: One patient was euthyroid at all visits and one started treatment for profound overt hypothyrodism. The remaining patients were SH 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 characterised as SH after one year (p=0.016). Diagnosis of overt hypothyroidism differed between T4 estimates (p=0.005) and was highly dependent on T4 reference limits. Hypothyroid clinical score did not correlate to biochemical diagnoses (p=0.29).

Conclusion: The monitoring procedure itself may influence the outcome of control of SH. Specifically, the interval between visits, the type of T4 estimate used and the lower T4 reference limit influenced the outcome when untreated SH patients were followed for one year. Hypothyroid clinical score did not aid the evaluation in individual SH patients.