DISCUSSION
In the present study, we have not been able to demonstrate that the use
of established risk factors for thyroid cancer is useful to discriminate
obese patients with a higher risk of harbouring thyroid nodules and,
therefore candidates to rule out thyroid cancer.
No professional medical society recommends population-based screening
for thyroid cancer [20], and in the cases where this strategy has
been implemented it has rendered no benefit in clinical terms [21].
However, there is a continued increase in thyroid cancer prevalence in
most countries that is generally considered as overdiagnosis and
overtreatment [22] and probably indicates, among other reasons, a
continued non-judicious use of thyroid ultrasound in some clinical
settings [23, 24]. Our aim in this study was to analyse the
effectiveness of a screening strategy advocated as cost-effective and
therefore recommendable from a theoretical framework [15], adding
knowledge to the sensible use of thyroid ultrasound. Our data do not
support the advocated strategy and therefore supports the current
recommendations in asymptomatic obese adults [13].
A significant difference between the theoretical framework used in the
seminal study [15] and our actual results is the prevalence of risk
factors. The main discrepancy lies in our higher prevalence of
hypothyroidism. Although the prevalence of high TSH may be different in
different populations [25, 26], the reported prevalences, also for
treated hypothyroidism [27], are generally lower than in our study.
A possible explanation for our higher prevalence could be the presence
of obesity itself, since it has been reported that high TSH levels are
more prevalent in obese patients, at least in children and adolescents
[28] and morbid obese patients [29]. The prevalence of TPOAb was
similar to some other studies [26] but again higher than usually
expected.
Different studies have shown an association between thyroid cancer and
the presence of autoimmune thyroiditis [30, 31] or higher TSH levels
[32]. Although these associations are not a constant finding [33,
34], some authors have suggested a role for periodic US to detect
thyroid malignancy in these situations [30]. In our population the
presence of these potential risk factors was not associated to an
increased risk of thyroid nodularity or cancer. We can hypothesize that
this finding may be due to a different relevance of these factors in the
obese population, specially the higher incidence of high TSH. In this
sense it has been argued that higher TSH levels in obese patients are
not an evidence of thyroid dysfunction but, rather, an adaptation
process [35]. Interestingly, in a recent report, thyroid nodules
were less common in children and adolescents with autoimmune thyroiditis
or high TSH than in their counterparts [36].
The percentage of thyroid nodules with indication for FNAB was higher
than expected [15]. This result is in line with other authors’
results [37] and suggests that the prevalence of thyroid nodularity
in the obese population is probably higher than in the non-obese
population.
Several meta-analyses have found an association of obesity with an
increased thyroid cancer risk [3]. Although the present study was
not designed nor powered to evaluate the incidence of thyroid cancer or
the utility of generalized US screening for its detection in obese
patients, we found an incidence above the reported incidence rates for
the general population [8] although in line with other populations
where a systematic screening has been performed [21]. Most detected
thyroid cancers in our study were of low risk and good prognosis and
since any screening strategy should counterbalance potential health
benefits with the risk of generating an excess in diagnosis, treatment
and costs [8, 21], it does not seem that any screening strategy of
thyroid cancer in obese individuals will be cost-effective and, in any
case, the use of the assessed risk factors is not useful.
Furthermore, this screening strategy results in the detection of a
considerable number of nodules without indication for FNAB that could be
subjected to follow-up increasing resource use and costs and generating
anxiety for patients without any noticeable benefit.
Our study has some limitations. We designed the study to detect
differences in the incidence of thyroid nodules and not thyroid cancer,
which is the final purpose of the screening strategy. However, detection
of thyroid nodules is the first step in the diagnosis of most thyroid
cancers [16] and therefore the absence of an increased incidence of
thyroid nodules in the obese population with additional risk factors
makes the screening strategy ineffective.
Some variability may rise from the fact that ultrasound was not
performed centrally. However, the use of a specific classification
system might have reduced the effects of this limitation [38].
Additionally, we used as a surrogate for the presence of thyroiditis the
positivity of TPOAb. Although other serological markers can be used,
TPOAbs are considered the best serological marker to establish a
diagnosis of Hashimoto’s thyroiditis [39]. However, an association
between thyroid cancer and anti-thyroglobulin antibodies (TgAb) has been
reported [31], and therefore we cannot rule out that the use of
TgAbs would have changed the results.
Also, the population studied is predominantly morbid obese patients,
which may not be representative of other obese populations. It seems
reasonable, however, to consider that our study focuses on the obese
population at highest risk of cancer and in which the effect of
additional risk factors should be more evident. Moreover, BMI was not
different between patients without or with thyroid nodules.
Furthermore, there is some risk of selection bias. For unknown reasons,
not all obese patients attended were offered participation and, this
fact may have selected the population finally evaluated. Additionally,
we decided to exclude patients with other autoimmune diseases. The
reason for doing so, was to not overestimate de prevalence of high
TPOAbs as it is known that the prevalence of thyroid autoimmunity is
increased in the presence of other autoimmune disorders [40].
Despite these limitations, the number of patients evaluated give us
confidence that the results are reliable and therefore we can conclude
that the use of the studied risk factors (specifically TSH and TPO) is
not useful to discriminate obese patients with a higher risk of
harbouring thyroid nodules.