Minimally symptomatic (subclinical) hypothyroidism

Subclinical hypothyroidism is defined by the presence of mild thyrotropin (TSH) elevation but normal blood free thyroxine and free triiodothyronine levels. The adjective ''subclinical'' seems awkward, if not inaccurate, given that it is arguable whether these patients are truly asymptomatic and in view of the support in the literature for a salutary effect of thyroid hormone therapy. In view of this and the evidence that the majority of such patients eventually evolve into overt thyroid failure, we propose that a more appropriate term is minimally symptomatic hypothyroidism (MSH). The most common causes of this syndrome are the same as those for overt hypothyroidism and include chronic autoimmune (Hashimoto's) thyroiditis, thyroid ablation with radioactive iodine, antithyroidal drugs, and thyroidectomy. Ideally, the diagnosis is best considered in an outpatient setting, because confounding factors in hospitalized patients, such as severe systemic illness and medications, can cause misleadingly elevated TSH levels. Although target organ dysfunction is not as evident as in overt hypothyroidism, well designed studies have reported subtle elevations in atherogenic lipoprotein fractions, suboptimal left ventricular function, and discrete neuropsychiatric abnormalities. In minimally symptomatic patients, it is important to reconcile optimal medical practice with the increasing demands of the current health care system for a judicious and cost effective approach to diagnosis and management. An initial clinical assessment for MSH could focus on identifying individuals vulnerable to thyroid disorders, such as patients with a family or past medical history of thyroid disease, patients with a goiter or history of recent pregnancy, patients taking medications that could interfere with thyroid function, or patients with hypercholesterolemia. TSH should be measured in all patients at risk, and measurement of thyroid antibody titers may be useful insofar as they confirm autoimmune thyroid disease and predict progression to frank hypothyroidism, especially in the geriatric population. We believe that there is reason to expect benefits from initiation of replacement therapy with levothyroxine, including relief of symptoms, improvement in lipid profiles and cardiovascular risk, and prevention of progression to overt hypothyroidism. Levothyroxine is given in the usual recommended doses, with an ultimate target dose of approximately 1.7 mu g/kg, titrated accordingly to maintain serum TSH within the normal (measurable) range. Although still controversial, one recent article suggests that screening women older than age 35 every 5 years for MSH may be as cost effective as screening for breast cancer or hypertension.