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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.

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.

Filed under: Medical Conditions