The blood spot thyrotropin method is not adequate to screen for hypothyroidism in the elderly living in abundant-iodine intake areas: comparison to sensitive thyrotropin measurements

Abstract

We investigated whether the blood spot thyrotropin (TSH) method was adequate for screening elderly subjects with abundant iodine intake (median excretion 330 microg/g creatinine) for hypothyroidism. In 97 healthy adults (group A), 210 nursing home residents (group B) and 265 elderly subjects living at home (group C) serum (sensitivity < 0.02 mU/L, cost 1.2 U.S. dollars [USD]) and blood spot TSH (sensitivity < 1.0 mU/L, cost 0.4 USD) were measured, and the sensitivity and specificity of different blood spot TSH cutoff points to detect cases with elevated serum TSH were calculated. Elevated (> 3.5 mU/L) serum TSH levels (group A, 6.2%; group B, 16.2%; group C, 22.3%; B > A, p = 0.025; C > A, p < 0.001) were detected with the required sensitivity of greater than 0.9 only if the cutoff point of the blood spot TSH was set as low as 2.5 mU/L, but this led to a considerable loss of specificity. At cutoff point 2.5 mU/L, the rate of positivity was 39.3% and the cost of blood spot screening/person increased to 0.88 USD, considering that positive cases have to be rechecked by serum TSH to exclude false positivity. Cases with significantly elevated (> 10.0 mU/L) serum TSH (group A, 1.03%; group B, 2.85%; group C, 2.20%) were detected at blood spot cutoff points 10.0-4.0 mU/L with a sensitivity of 1.0 and without considerable loss of specificity. We conclude that while screening for hypothyroidism in the elderly population with abundant iodine intake is justified by the high prevalence of elevated ultrasensitive serum TSH values, the sensitivity of the blood spot method is insufficient to detect the subclinical hypothyroidism accurately and would, therefore, fail to detect most affected subjects.