| 1 |
THYROID, BHR &
WEIGHT |
| ADDITIONAL
PATIENT RESOURCES
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| What
is the relationship between thyroid and weight?
It has been appreciated for a very long time that there is a complex
relationship between thyroid disease, body weight and metabolism.
Thyroid hormone regulates metabolism in both animals and humans.
Metabolism is determined by measuring the amount of oxygen used
by the body over a specific amount of time. If the measurement is
made at rest, it is known as the basal metabolic rate (BMR). Indeed,
measurement of the BMR was one of the earliest tests used to assess
a patient’s thyroid status. Patients whose thyroid glands
were not working were found to have low BMRs, and those with overactive
thyroid glands had high BMRs. Later studies linked these observations
with measurements of thyroid hormone levels and showed that low
thyroid hormone levels were associated with low BMRs and high thyroid
hormone levels were associated with BMRs. Most physicians no longer
use BMR due to the complexity in doing the test and because the
BMR is subject to many other influences other than the thyroid state.
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| What
is the relationship between BMR and weight?
Differences in BMRs are associated with changes in energy
balance. Energy balance reflects the difference between the amount
of calories one eats and the amount of calories the body uses. If
a high BMR is induced by the administration of drugs, such as amphetamines,
animals often have a negative energy balance which leads to weight
loss. Based on such studies many people have concluded that changes
in thyroid hormone levels, which lead to changes in BMR, should
also cause changes in energy balance and similar changes in body
weight. However, BMRs are not the whole story relating weight and
thyroid. For example, when metabolic rates are reduced in animals
by various means (for example by decreasing the body temperature),
these animals often do not show the expected excess weight gain.
Thus, the relationship between metabolic rates, energy balance,
and weight changes is very complex. There are many other hormones
(besides thyroid hormone), proteins, and other chemicals that are
very important for controlling energy expenditure, food intake,
and body weight. Because all these substances interact on both the
brain centers that regulate energy expenditure and tissues throughout
the body that control energy expenditure and energy intake, we cannot
predict the effect of altering only one of these factors (such as
thyroid hormone) on body weight as a whole. As a consequence, at
this time, we are unable to predict the effect of changing thyroid
state on any individual’s body weight. |
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| 2 |
HYPERTHYROIDISM |
| What
is the relationship between hyperthyroidism and weight?
Since the BMR in patients with hyperthyroidism (see Hyperthyroidism
brochure) is elevated, many patients with an overactive thyroid
do, indeed, experience some weight loss. Furthermore, the likelihood
of weight loss occurring is related to the severity of the overactive
thyroid. Thus, if the thyroid is extremely overactive, the individual’s
BMR increases which leads to increased caloric requirements to maintain
that weight. If the person does not increase the calories consumed
to match the excess calories burned, then weight loss will ensue.
As indicated earlier, the factors that control our appetite, metabolism,
and activity are very complex and thyroid hormone is only one factor
in this complex system. Nevertheless, on average the more severe
the hyperthyroidism, the greater the weight loss observed. Weight
loss is also observed in other conditions where thyroid hormones
are elevated, such as in the toxic phase of thyroiditis (see Thyroiditis
brochure) and if one is on too high a dose of thyroid hormone
pills. Since hyperthyroidism also increases appetite, some patients
may not lose weight, and some may actually gain weight, depending
on how much they increase their caloric intake.
Why do I gain weight when hyperthyroidism
is treated?
Because the hyperthyroidism is an abnormal state, we can predict
that any weight loss caused by the abnormal state would not be maintained
when the abnormal state is reversed. This is indeed what we find.
On the average, any weight lost during the hyperthyroid state is
regained when the hyperthyroidism is treated. One consequence of
this observation is that the use of thyroid hormone to treat obesity
is not very useful. Once thyroid hormone treatment is stopped, any
weight that is lost while on treatment will be regained after treatment
is discontinued. |
| 3 |
HYPOTHYROIDISM AND THYROID HORMONE |
| What
is the relationship between hypothyroidism and weight gain?
Since the BMR in the patient with hypothyroidism (see Hypothyroidism
brochure) is decreased, an underactive thyroid is generally
associated with some weight gain. The weight gain is often greater
in those individuals with more severe hypothyroidism. However, the
decrease in BMR due to hypothyroidism is usually much less dramatic
than the marked increase seen in hyperthyroidism, leading to more
modest alterations in weight due to the underactive thyroid. The
cause of the weight gain in hypothyroid individuals is also complex,
and not always related to excess fat accumulation. Most of the extra
weight gained in hypothyroid individuals is due to excess accumulation
of salt and water. Massive weight gain is rarely associated with
hypothyroidism. In general, 5-10 pounds of body weight may be attributable
to the thyroid, depending on the severity of the hypothyroidism.
Finally, if weight gain is the only symptom of hypothyroidism that
is present, it is less likely that the weight gain is solely due
to the thyroid.
How much weight can I expect to
lose once the hypothyroidism is treated?
Since much of the weight gain in hypothyroidism is accumulation
in salt and water, when the hypothyroidism is treated one can expect
a small (usually less than 10% of body weight) weight loss. As in
the treatment with hyperthyroidism, treatment of the abnormal state
of hypothyroidism with thyroid hormone should result in a return
of body weight to what it was before the hypothyroidism developed.
However, since hypothyroidism usually develops over a long period
of time, it fairly common to find that there is no significant weight
loss after successful treatment of hypothyroidism. Again, if all
of the other symptoms of hypothyroidism, with the exception of weight
gain, are resolved with treatment with thyroid hormone, it is less
likely that the weight gain is solely due to the thyroid. Once hypothyroidism
has been treated and thyroid hormone levels have returned to the
normal range on thyroid hormone, the ability to gain or lose weight
is the same as in individuals who do not have thyroid problems.
Can thyroid hormone be used to help
me lose weight?
Thyroid hormones have been used as a weight loss tool in the past.
Many studies have shown that excess thyroid hormone treatment can
help produce more weight loss than can be achieved by dieting alone.
However, once the excess thyroid hormone is stopped, the excess
weight loss is usually regained. Furthermore, there may be significant
negative consequences from the use of thyroid hormone to help with
weight loss, such as the loss of muscle protein in addition to any
loss of body fat. Pushing the thyroid hormone dose to cause thyroid
hormone levels to be elevated is unlikely to significantly change
weight and may result in other metabolic problems.
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| Thyroid
and Weight Brochure for Saving and Printing (PDF
File, 48KB) |
© 2005 American Thyroid Association. All rights reserved.
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