Highlights of the 74th Annual Meeting of the American Thyroid Association

Kenneth D. Burman, MD


February 05, 2003

In This Article

Subclinical Hypothyroidism

Subclinical hypothyroidism is characterized by normal total and free T4/T3 in conjunction with elevated TSH. Subclinical hypothyroidism should be thought of as a continuum between euthyroidism and overt hypothyroidism. When considered in this manner, it is realized that the term "subclinical hypothyroidism" is vague and misleading, although other terms such as mild hypothyroidism also are imprecise.

A patient with normal free and total T4/T3 may have signs and symptoms typical of hypothyroidism and, therefore, the disorder is not "subclinical." In addition, the signs and symptoms of subclinical and even overt hypothyroidism are relatively vague, and their presence or absence does not prove there is thyroid disease but is merely supportive of the diagnosis. Treatment with levothyroxine may or may not help ameliorate these symptoms.

Some patients have a normal free and total T4/T3 and have a TSH that is minimally increased, for example, in the 5-10 mU/mL range, whereas other patients will have a TSH that is 20-30 mU/mL. It is not clear that these 2 different types of patients should be approached similarly.

Recent studies have demonstrated that any given individual has very tightly controlled T4, T3, and TSH levels when assessed monthly throughout the year. The population-derived normal range is much broader than this individual variation. The result is that a patient can have T4/T3 within the population-derived normal range and still have a markedly abnormal TSH. If the normal free T4 range is 0.8-1.8 ng/dL but a given patient has his/her normal free T4 at 0.9 ng/dL, we indicate a patient has "overt hypothyroidism" when this free T4 decreases to 0.8 ng/dL, a decrease of less than 10%. On the other hand, if a given subject has his/her normal free T4 at 1.8 ng/dL and decreases to 0.9 ng/dL, this value is still within the normal range but represents about a 50% decrease, and we denote this as subclinical hypothyroidism.

Fatourechi and colleagues[8] studied the effect of levothyroxine therapy to treat patients with mild thyroid failure (subclinical hypothyroidism). They retrospectively reviewed the charts of patients with normalized TSH in the treatment group (n = 49) and compared their results with those of untreated patients (n = 26). LDL, HDL, and triglycerides were not significantly changed with treatment and normalization of TSH. These authors concluded that the possible decreases in LDL with levothyroxine therapy in patients with mild subclinical hypothyroidism are minimal, and prospective randomized trials are needed to better assess these changes.

Subclinical hypothyroidism is a controversial area, and further studies are needed to assess lipid, cardiac, and bone effects of untreated hypothyroid subjects compared with patients who have had their TSH normalized. The mood and memory effects also need study. From a clinical standpoint, subclinical hypothyroidism is common, and, after a thorough clinical evaluation, including routine laboratory studies, repeat free T4/T3 and TSH, and the measurement of thyroid antibodies, levothyroxine therapy is considered, especially for patients with a TSH greater than 10 mU/mL. Some clinicians will also consider ordering a thyroid sonogram and/or radioisotope scan.


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