With TSH testing, no lack of discord

William Check, PhD

June 2013—In 219 BCE, after he had unified the seven warring states to establish the nucleus of the Chinese empire, the First Emperor of China promulgated uniform administrative practices throughout the land. One section of his decree read:

All men under the sky toil with a single purpose. Tools and measures are made uniform. The written script is standardized. Wherever the sun and moon shine.

Perhaps the field of thyroid function testing could use a return visit from the First Emperor to make its tools and measures uniform. Disagreement swirls around even the most basic aspect of thyroid function testing—reference intervals for measurement of thyroid stimulating hormone.

“Reference intervals for TSH are all over the place,” Carole Spencer, MT, PhD, professor of medicine at the University of Southern California and technical director, USC Endocrine Laboratories, says, adding that there are several reasons. One is that the reference interval is very sensitive to individuals in the cohort who skew the upper limit, which makes the distribution non-Gaussian and requires log transformation of values. Older persons, in whom higher levels are seen, are an example. “The upper limit of the TSH interval may be 7.5 mIU/L for a group of healthy 80 year olds, whereas it might be down around 3 for healthy 20 year olds,” Dr. Spencer says.

Obesity is another example. “If you include obese individuals, that will increase TSH values independent of thyroid function,” Dr. Spencer continues. “You only have to have three or four really obese individuals in your cohort to skew the TSH upper limit. Many people don’t appreciate the effects of obesity on TSH. When a person loses weight, TSH comes way down.” And if those with thyroid autoimmunity are not screened out prospectively, she says, by testing for antibodies to TPO [thyroid peroxidase], that will also skew the upper limit.

“Everyone gets exercised about the accuracy of the TSH reference interval, but they fail to understand who this interval was calculated on. What it comes down to,” Dr. Spencer says, “is that clinical judgment is more important [for diagnosing hypothyroidism] than the TSH reference interval.”

Another source of uncertainty in thyroid function testing is the great difference between the population interval and individual variation, says Anthony Killeen, MD, PhD. “We don’t know each individual’s setpoint—their relation between TSH and free T4 [FT4],” Dr. Killeen, director of clinical laboratories and professor of laboratory medicine and pathology at the University of Minnesota Medical Center, Fairview, explains. “So we need to use the much wider population reference interval.” That requires clinical judgment. “At the upper end of normal, values around 4 mIU/L, we need to look at other aspects of the patient,” Dr. Killeen says. For instance, women older than age 55 will be at the upper end of the interval, where hypothyroidism is not uncommon. In Dr. Killeen’s view, the non-Gaussian skew at the upper end of the reference interval suggests that it also includes people with subclinical hypothyroidism. “Persons with higher values are more likely to have anti-TPO antibodies and to develop overt hypothyroidism,” he says.

A second controversy centers on the tools, the possibility of radically altering the method of measuring thyroid hormones. Immunoassays should be replaced by ultrafiltration (or equilibrium dialysis) followed by tandem mass spectrometry (UF/MS/MS), says Steven J. Soldin, PhD, senior scientist in the Department of Laboratory Medicine in the Clinical Center at the National Institutes of Health and adjunct professor of endocrinology and metabolism at Georgetown University Medical Center. Many years ago Dr. Soldin noticed that endocrinologists asked him about samples in which FT4 did not match TSH. He sent those to a reference laboratory for equilibrium dialysis followed by immunoassay.

“We saw that free T4 by [the reference laboratory method] correlated with log TSH, whereas the initial direct analogue free T4 by immunoassay did not,” Dr. Soldin says. “Everyone knows that direct analogue free T4 methods are suboptimal and correlate poorly with log TSH in patients with hypo- and hyperthyroidism. We have had two to three decades of frequently reporting the wrong results for free T3 and T4 by immunoassay, which clearly impacts the clinical diagnosis.”

Dr. Soldin has been working for more than 10 years on improving the UF/MS/MS method, which he has demonstrated to be analytically superior to immunoassay and which he has put into place at the NIH, Children’s National Medical Center, NMS Laboratories, and Georgetown University for clinical application.

Controversy No. 3 in thyroid function testing is whether to screen all pregnant women. The American Thyroid Association does not recommend universal screening, while other groups do, Dr. Killeen notes. Another important issue is that TSH levels in pregnancy are lower than those in nonpregnant women and that FT4 declines with gestation. Dr. Killeen and others recommend trimester-specific intervals for TSH.

Of the uncertainty regarding the reference interval for TSH, Dr. Spencer says that, in addition to the cohort used to define the population interval, “methodological factors also come into play.” Immunoassays employ monoclonal antibodies, which have limited and varying specificity to detect the epitopes of TSH. “Circulating TSH is heterogeneous, especially with respect to glycosylation,” Dr. Spencer says. “So which TSH molecules an assay detects will depend on the monoclonal antibody you select.” Antibody variability raises a whole different issue for clinical interpretation, she says. “Because of heterogeneity in glycosylation, not all molecular forms of TSH are bioactive.” In particular, although the upper limit of TSH is higher in older individuals, not all TSH in older persons may be bioactive.

Dr. Spencer’s summary: “There are a lot of unknowns here. We could spend the whole day arguing where the TSH upper limit should be set.”

To resolve this problem, Dr. Spencer invokes the point Dr. Killeen made. “In reality,” she says, “TSH population reference intervals are not a sensitive parameter for detecting thyroid dysfunction because all thyroid tests have a low index of individuality—the relationship between the between-person variation and the within-person variation.” One study that measured TSH in a group of subjects every month for a year found that, for a normal person, TSH levels varied by 0.5 to 0.75 mIU/L across the study. “So the reproducibility of TSH measurements within an individual is much narrower than the interval you see when you combine data among individuals to get a population reference interval,” Dr. Spencer explains. “If an individual starts to develop hypothyroidism, their TSH could rise to 2.7, which might be highly abnormal for that person but still well within the reference interval of the population.

“Do you want to treat that individual?” Dr. Spencer asks. “Whether you do would not be determined by the TSH reference interval, but by the person’s lipids, the presence or absence of anti-TPO antibodies, family history, and a whole number of other issues as to why that patient came in to see the doctor.” Say the patient was a pregnant young woman. Considering that the upper limit of normal TSH in the first trimester is 2.5, “If her TSH is 2.8 or 3, you might well treat her,” Dr. Spencer says. On the other hand, a 70-year-old woman with no antibodies, in an age group with an upper limit of normal of 5 or 6, probably wouldn’t be treated.

Dr. Spencer calls a population reference interval “a very insensitive way” to assess thyroid dysfunction in individuals. “It is necessary to look at each patient in a specific way. I totally think, unless you are dealing with a very elderly patient, a general reference interval of 0.3 to 3 is a good starting point. Then factor in patient-specific factors.”

Whether to treat subclinical hypothyroidism is also contentious. Subclinical hypothyroidism is usually due to autoimmune thyroid disease, such as Hashimoto’s thyroiditis. “If TSH is on the high side, 3 to 10 mIU/L, your next test should be anti-TPO antibodies. If you detect antibodies, likely there is some degree of clinical hypothyroidism.” These persons are at increased risk of progressing to overt hypothyroidism. Again, treatment would depend on the severity of the patient’s symptoms.

However, if the antibody test result is negative, Dr. Spencer says, “You must remember there are a number of reasons why TSH is high.” To the causes already mentioned, she adds polymorphism of TSH receptors in thyroid cells. “In these people it takes a higher level of TSH to do the job,” she explains. “So TSH may be high, TPO antibody negative, and nothing is wrong with them. They are euthyroid.”

Like Dr. Spencer, Dr. Killeen advocates weighing symptoms when interpreting TSH values. “Tiredness is a very common and vague complaint,” he notes. “Usually it does not indicate thyroid disease.” He suggests thyroid function testing only if tiredness is prolonged or debilitating. Dr. Killeen also points out that “textbook” symptoms of hypothyroidism—dry skin, diminished sweating, weight increase, periorbital puffiness—were described with more advanced hypothyroidism detected with older, less sensitive assays, not with the early forms detected with today’s more sensitive assays.

As for treating subclinical hypothyroidism, he notes that, while no association with mortality has been demonstrated, “Coronary heart disease events begin to rise above 7 mIU/L, becoming significant above 10 mIU/L.

“There has been a lot of disagreement on key questions” regarding TSH reference intervals, Dr. Killeen says. He favors the existing reference interval—0.3 to 4.0 mIU/L.