In addition to thyroid hormone levels, vital signs, glucose, cholesterol levels and body composition determined by dual energy X-ray absorptiometry (DEXA), metabolic testing was done using an open-circuit indirect calorimeter testing to determine resting energy expenditure (REE) and other measures of metabolism (respiratory quotient (RQ) calculated as RQ = CO2/O2).
When compared to their values at baseline, TSH levels were uniformly lower – either just at the lower limit of normal or suppressed, FT4 levels were higher – although most within the normal range, T3 levels were lower, and, as expected T3/FT4 ratios were lower. Body mass index (BMI) and lean body mass, glucose and cholesterol were unchanged. However, heart rate and blood pressure were higher. REE was increased, but despite this, weight was stable. On the other hand the respiratory quotient (RQ) was lower and correlated over time with lower T3/FT4 ratios.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
Levothyroxine-induced TSH suppressive therapy in patients with thyroid cancer results in an increase in energy expenditure but also increases in heart rate and blood pressure, which may have adverse effects on the heart. This shows that levothyroxine can improve metabolic rate in hypothyroid patients. This also supports the updated recommendations from the American Thyroid Association limiting the use of levothyroxine-suppression to patients with high risk cancers or those with low to intermediate risk cancers who have not had an excellent response to therapy.
—Marjorie Safran, MD