Slight Fluctuations in TSH: Can They Affect Your Weight?

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Research published in the May 2005 issue of the Journal of Clinical Endocrinology and Metabolism has finally put to rest a controversial disagreement in the endocrinology world regarding whether or not even slight hypothyroidism can cause or contribute to weight gain.

In the study, which looked more than 4600 research subjects, the researchers explored the connection between thyroid levels (measured by thyroid stimulating hormone -- TSH -- levels) and Body Mass Index (BMI) -- a calculation that looks at a person's height, compared to weight, and categorizes people in different weight segments, including underweight, normal weight, overweight, and obese.

What the researchers found was a positive association between BMI and TSH. This means that the BMI rose as TSH rose, showing a direct link between elevated TSH levels and body weight. There was also a negative association between BMI and Free T4, meaning that as Free T4 (a measure of the circulating storage thyroid hormone thyroxine in the bloodstream) rises, BMI drops.

Even among people who had a TSH levels that fell within the so-called "reference range" -- or "normal range" -- those whose levels fell on the higher end of the normal reference range for the TSH test  -- in this case a TSH of 4.5 -- weighed approximately 12 pounds more than those who had a TSH on the low end of the reference range, with a median TSH of 0.28.

Overall, researchers concluded that thyroid function -- even when TSH levels fell within the reference range - is a factor that helps determine body weight, and even slightly elevated TSH levels are associated with an increase in the occurrence of obesity.

Interestingly, thyroid function has the same impact on BMI as physical activity!

The researchers also found that even small variations in thyroid function -- within the reference range for Free T4 -- may contribute to the regulation of body weight in a population. It's thought that the circulating thyroid hormone may affect the body's "sleeping energy expenditure." According to the researchers,

"In a population where physical activity has been gradually diminished, even a relatively small contribution to energy expenditure mediated through thyroid hormones may be enough to accomplish increases in BMI."

The researchers concluded:

"...we suggest that differences in thyroid function within what is considered the normal range is associated with differences in BMI, caused by longstanding minor alterations in energy expenditure. This is more pronounced when mild hypo- or hyperthyroidism is present. The prevalence of such abnormalities in thyroid function are high and may be influenced by environmental factors. As small abnormalities in thyroid function are common, thyroid function may importantly influence the prevalence of obesity in a population."

The 2010 Study

Another study published in the Journal of Clinical Endocrinology and Metabolism confirmed these results

Bernadette Biondi's Thyroid and Obesity: An Intriguing Relationship, states:

"...subclinical and overt hypothyroidism correlated with a higher BMI and a higher prevalence of obesity in both smokers and nonsmokers. It has been noted that small variations in serum TSH caused by minimal changes in l-T4 dosage during replacement therapy are associated with significantly altered resting energy expenditure in hypothyroid patients. These studies support the clinical evidence that mild thyroid dysfunction is linked to significant changes in body weight and likely represents a risk factor for overweight and obesity."
...It is important to note that the increased prevalence of obesity worldwide may further confound the definition of the normal TSH range in population studies. More research is necessary to determine whether mild thyroid hormone deficiency and the consequent mild TSH increase, i.e. to the upper limit of the reference range, are involved in the development of obesity. Moreover, studies are required to establish the potential role of high leptin levels in increasing susceptibility to thyroid autoimmunity, which in turn entails a high risk of developing subclinical or overt hypothyroidism.

Obesity and thyroid dysfunction are common diseases, and consequently clinicians should be particularly alert to the possibility of thyroid dysfunction in obese patients."

MORE INFORMATION

Here are additional resources regarding weight gain, inability to lose weight, and the link to thyroid function:

Source: Knudsen N, et. al. "Small differences in thyroid function may be important for Body Mass Index and the occurrence of obesity in the population." J Clin Endocrinol Metab 2005 May 3

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