Borderline TSH can strongly predict future hypothyroidism

Summary: Borderline levels of TSH (thyroid stimulating hormone) still within the reference ranges typically printed in laboratory reports can indicate low thyroid function (and predict hyperthyroid on the other end of the scale). A thorough assessment of the more than two dozen patterns of thyroid dysfunction is necessary for an accurate diagnosis.Clinicians and patients may often be misled by TSH levels that appear normal, but experienced practitioners know that they can mask the presence thyroid disorders. Because hypothyroidism affects function globally, a study just published in the Journal of Clinical Endocrinology & Metabolism that practitioners in all specialties should be vigilant. The authors state:

"Serum TSH in the upper part of the reference range may sometimes be a response to autoimmune thyroiditis in early stage and may therefore predict future hypothyroidism. Conversely, relatively low serum TSH could predict future hyperthyroidism...The objective of the study was to assess TSH within the reference range and subsequent risk of hypothyroidism and hyperthyroidism."

The authors examined 10,083 women and 5,023 men without previous thyroid disease who had a baseline TSH of 0.20–4.5 mU/liter for the predictive probabilities of developing hypothyroidism or hyperthyroidism according to categories of baseline TSH during follow-up 11 years later. Their data drew a strong result:

"During 11 yr of follow-up, 3.5% of women and 1.3% of men developed hypothyroidism, and 1.1% of women and 0.6% of men developed hyperthyroidism. In both sexes, the baseline TSH was positively associated with the risk of subsequent hypothyroidism. The risk increased gradually from TSH of 0.50–1.4 mU/liter [women, 1.1%; men, 0.3%] to a TSH of 4.0–4.5 mU/liter (women, 31.5%; men, 14.7%). The risk of hyperthyroidism was higher in women with a baseline TSH of 0.20–0.49 mU/liter (3.9%) than in women with a TSH of 0.50–0.99 mU/liter (1.4%) or higher (∼1.0%)."

Too many patients with thyroid dysfunction fall between the cracks of routine care. This evidence strongly supports the importance of a complete assessment of thyroid function when these disorders, especially autoimmune thyroid disease, are suspected. The authors conclude:

"TSH within the reference range is positively and strongly associated with the risk of future hypothyroidism. TSH at the lower limit of the reference range may be associated with an increased risk of hyperthyroidism."