The term 'subclinical
hypothyroidism' applies to patients who have mildly increased levels of serum
thyrotropin hormone (TSH) and normal levels of
thyroxine and
liothyronine (triiodiothyronine). This very common condition, also called 'mild thyroid failure', accounts for 75% of patients who have increased serum TSH. For patients with sustained increases above 10 mIU/L, there is uniform agreement that
thyroxine therapy is indicated.
Therapy for milder forms of
hypothyroidism is controversial. Some randomized clinical trials favor
therapy for mild thyroid failure, but they are inconclusive because they lack stratification for the subgroup of patients with TSH levels below 10 mIU/L. For this subgroup, we recommend individualized management. The presence of
goiter, positive thyroperoxidase (TPO)
antibodies, manic-
depressive disorder, fertility problems, or pregnancy or the anticipation of pregnancy favors the initiation of
therapy. Positive TPO
antibodies are a strong indication for
therapy because of the high likelihood in these patients of progression to overt
hypothyroidism; patients who are already receiving
thyroxine should have adjustments of their dosage. Children and adolescents with mild thyroid failure should also be treated because of possible adverse effects on growth and development. It has been suggested that subclinical
hypothyroidism is a cardiovascular risk factor, however further investigation is needed. The controversy surrounding
therapy will not be resolved until more randomized studies are available for the subgroup of patients with TSH <10 mIU/L, and until the question of cardiovascular risk factors is further clarified.