Thyroid weight gain myths and surprises

Patients often suspect a thyroid disorder is responsible for their weight gain, but the data doesn’t support a causal effect between hypothyroidism and obesity.

Patients often suspect a thyroid condition is responsible for their weight gain or their weight loss struggles. Much of the stimulus for this belief is based on the internet, according to Lisal J. Folsom, M.D., M.S., a pediatric and adult endocrinologist with Norton Community Medical Associates and Norton Children’s Endocrinology, affiliated with the UofL School of Medicine.

“A lot of misinformation is out there about the thyroid,” Dr. Folsom said. “There is no connection between obesity and hypothyroidism.”

In fact, weight gain is not typically a finding with new-onset hypothyroidism (underactive thyroid gland).

For example, a study of 1,400 children referred for obesity found only 0.5% had hypothyroidism.

Looking at children with autoimmune hypothyroidism, thyroid enlargement was common, but weight gain was rare, and their body mass index was equal to that of the general population.

Studies evaluating patients’ weight before and after treatment with thyroid medication have shown no significant amount of weight loss. Also, DXA (dual-energy X-ray absorptiometry) scans do not show any difference in fat or lean muscle mass before and after treatment.

“Neither the adult data nor the pediatric data show a clear relationship between treatment and body weight,” Dr. Folsom said.

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The TSH link isn’t what many think

Thyroid stimulating hormone, also called thyrotropin or TSH,  is inversely related to thyroid hormone. High TSH corresponds with a low thyroid hormone level. TSHis often slightly higher in obese individuals compared with lean individuals, but this is not driving obesity. It appears to be just the opposite.

“There is some evidence that the higher TSH may actually be a physiologically adaptive response to obesity,” Dr. Folsom said during her continuing medical education presentation, “Pediatric Thyroid Disorders.”

Higher TSH levels increase metabolism, suggesting this could be a compensatory response of the body to combat weight gain. Even with the higher TSH levels in obese patients, levels of thyroid hormone typically remain in the normal range.

The belief that weight gain and hypothyroidism are linked likely stems from a time before it was possible to test for TSH, according to Dr. Folsom.

The first test for TSH was introduced in the 1960s. Before that, patients tended to be diagnosed very late in their disease — when their hypothyroidism had developed into myxedema. At that point, they had gained weight because of edema and increased water weight, rather than increased adipose tissue.

Classic symptoms of hypothyroidism include fatigue, constipation, dry skin, hair loss and irregular periods, rather than weight gain.

Thyroid hormone metabolism effects

Thyroid hormone increases basal metabolic rate through the genetic expression of sodium-potassium-ATPase, also known as the “Na-K pump” or the sodium pump. It also stimulates both the metabolism of carbohydrates and the anabolism — or building up — of proteins.

Thyroid hormone increases our ability to absorb glucose, and to produce glucose through gluconeogenesis. It also helps the bodies store glucose through glycogen synthesis, and improves glucose oxidation — the ability to use glucose.

Hypothyroidism affects about 1 of every 500 school-age children. The most common cause among children is an autoimmune condition, Hashimoto thyroiditis or chronic lymphocytic thyroiditis.

In childhood, thyroid hormone stimulates bone and linear growth, and brain development. Hypothyroidism in children may result in poor linear growth. Physical exams may reveal a goiter or a slow heart rate because thyroid hormone stimulates the beta receptors on the heart muscle.

Thyroxine (T4) is the main hormone produced by the thyroid gland. Free thyroxine is the amount of thyroxine that is not bound to protein. Total T4 is free T4 plus T4 bound to plasma proteins, primarily thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin.

Screening a patient for thyroid dysfunction typically involves assessing blood tests for thyroid stimulating hormone (TSH) level and eitherfree thyroxine (T4) or total T4.

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