JAMA 2004;291: 228-38

Definition of Subclinical Hypothyroidism
Patients with subclinical thyroid disease have few or no symptoms or signs of thyroid dysfunction and thus by its very nature subclinical thyroid disease is a laboratory diagnosis. Subclinical hypothyroidism is defined as a serum thyroid stimulating hormone (TSH) above the defined upper limit of the reference range, with a serum free thyroxine (T4) within the reference range. Other causes of a raised TSH, a past history of thyroid disease and patients on T4 hormone treatment need to be excluded. It is therefore critically important that the reference limits for TSH be standardised. The TSH method used should have a high functional sensitivity (at least 0.02 mU/L), although this is of most importance for the diagnosis of subclinical hyperthyroidism.