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Thyroid Changes Within Norm Linked to Weight Gain 3-24-08
FRAMINGHAM, Mass., March 24 -- Changes in thyroid function within the normal
range may account for unexplained weight gain in some patients, according to an
analysis of Framingham study data.
Baseline serum thyrotropin (TSH) levels correlated with body weight in men and
women, and higher TSH values at follow-up correlated with weight gain in both
sexes (P=0.007, P<0.001 for trend), Caroline S. Fox, M.D., of the Framingham
Heart Study, and colleagues reported in the March 24 issue of Archives of
Internal Medicine.
"The identification of change in thyroid function as a risk factor for weight
gain may help guide research into the identification, prevention, and treatment
of individuals at risk for the development of excess adiposity," the authors
concluded.
"Our findings raise the possibility that modest increases in serum TSH
concentrations within the reference range may be associated with weight gain,"
they added.
Thyroid dysfunction has a well-established association with body weight. Weight
loss is a common manifestation of hyperthyroidism, and patients with
hypothyroidism often have weight gain. Weight changes associated with both
conditions usually can be corrected with treatment of the thyroid condition.
Whether variations in thyroid function within the normal range affected body
weight remained unclear, according to the authors. Population-based studies have
suggested a possible association, but most of the studies relied on
self-reported weight changes and lacked adequate control for baseline weight.
Data from the Framingham Offspring Study addressed some of the limitations of
prior evaluations of thyroid function and body weight. Dr. Fox and colleagues
analyzed data on 2,407 male and female participants who had mean baseline age of
48. All of the participants had weight and TSH measurements at baseline and
follow-up that averaged 3.5 years.
Stratification of baseline TSH values showed that weight increased progressively
with rising hormone levels. Across quartiles of TSH levels, baseline weight
increased from 64.5 kg to 70.2 kg in women and from 82.8 kg to 85.6 kg in men.
During follow-up, weight increased by an average of 1.5 kg in women and 1.0 kg
in men. Baseline TSH values did not correlate with weight gain. However, higher
TSH values at follow-up did predict weight gain that averaged 0.5 to 2.3 kg in
women and 0.4 to 1.3 kg in men across quartiles of hormone levels.
Physicians and patients alike have long speculated that changes in the
hypothalamic pituitary-thyroid axis contribute to weight gain and obesity, Roy
E. Weiss, M.D., Ph.D., and Rebecca L. Brown, M.D., of the University of Chicago,
noted in an accompanying editorial. However, obese patients have a low
prevalence of overt or subclinical hypothyroidism.
Moreover, studies linking weight gain to changes in TSH levels have not
demonstrated a reciprocal relationship between weight and thyroid hormone
levels.
"One would expect that, as concentrations of TSH rise, levels of
triiodothyronine (T3) and/or thyroxine (T4) would fall if the relationship is a
simple matter of decreased thyroid hormone levels," said Drs. Weiss and Brown.
Identifying mechanisms by which obesity might alter the HPT axis could reveal
other contributing metabolic abnormalities, they added. However, that evidence
does not yet exist.
"There is no evidence at this time to advocate lowering serum TSH concentrations
to treat obesity, but we can agree with our patients, 'It may be the thyroid but
we don't know exactly how,'" the editorialists concluded.