Subclinical (Mild) Thyrotoxicosis

Abstract

Subclinical thyrotoxicosis is defined as a condition with lowered (suppressed) TSH, but T4 and T3 are within their reference ranges. The designation subclinical is somewhat misleading because it has been demonstrated that many patients do in fact have thyrotoxic symptoms but in a milder form than what is in generally referred to as thyrotoxicosis. The designation mild thyrotoxicosis is therefore more frequently used. In Sweden, the prevalence is 2–4% and increases with age.

Biochemical subclinical thyrotoxicosis is often found in connection with routine determination of thyroid hormones. If the patient does not have a previously known thyroid disease or is receiving substitution treatment, the diagnosis should be confirmed by repeated measurement of TSH and free T4 after at least 4–8 weeks. The reason is that TSH suppression can occur during serious general illness and persist for a time after (Fig. 2.14). Studies in outpatients has demonstrated that suppressed TSH is normalized in 40% at follow-up after a few months in such cases. Table 22.1 summarizes the causes of subclinical thyrotoxicosis, which are also discussed below.