The thyroid gland is a butterfly-shaped endocrine gland that is normally located in the lower front of the neck. Thyroid hormones help the body use energy, stay warm and keep the brain, heart, muscles, and other organs working as they should. 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. 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 influences other than the thyroid state.
Differences in BMRs are associated with changes in energy balance. Energy balance reflects the difference between the number of calories one eats and the number 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. During a week treatment of methimazole, body composition, resting respiratory expenditure REE , and handgrip strength were measured consecutively.
After methimazole treatment, body weight was initially increased 0—8 weeks , subsequently plateaued 8—24 weeks , and gradually decreased in the later period 24—52 weeks despite the decreased food intake. Body compositional analyses showed that the fat mass increased during an earlier period 4—12 weeks , while the lean mass increased significantly during the later period 26—52 weeks. Consistent with the lean mass changes, muscle strength also significantly increased during the later period.
However, long-term compositional changes moved in a beneficial direction increasing lean mass and reinforcing muscle strength, following decreasing fat percentages.
The thyroid hormone is an important determinant of the body composition by regulating the basal metabolic rate and thermogenesis [ 1 ]. Increased basal metabolic rate or resting energy expenditure has been observed in patients with hyperthyroidism [ 2 , 3 ]. Several studies have shown that hyperthyroidism-related weight loss reflects not only the depletion of body fat but also the loss of muscle mass in association with an accelerated whole body catabolism following increased thermogenesis and oxygen consumption [ 4 ].
Indeed, weight loss, despite the increased appetite and food intake, is one of the clinical manifestations of untreated hyperthyroidism. Increased food intake observed in patients with hyperthyroidism is not only a result of energy consumption but also a direct consequence of it affecting the appetite-regulating hypothalamic nuclei [ 2 , 5 ]. Meanwhile, treatment of hyperthyroidism consequently increases body weight and changes the body composition [ 4 , 6 , 7 ].
Whether it is a balanced weight gain composed of fat and lean mass is undetermined. Conflictingly, lean mass [ 6 ], fat mass [ 7 ], or both of them [ 3 , 4 ] have been reported as the major component of the treatment-related weight gain in hyperthyroidism. The earlier experience of weight loss following rapid weight gain during antithyroid treatment can give patients a reasonable concern whether these treatment-related changes are healthy or not.
The aim of this study was to investigate the change in body composition and metabolic rates consecutively during the treatment of hyperthyroidism.
Eight female patients were screened and 6 were enrolled. Two patients were excluded because of their intolerance to the resting metabolic rate RMR test. All 6 patients were treated with antithyroid drugs to achieve euthyroidism. Informed consent was obtained from all patients. To assess the dietary intake, a three-day dietary record was used.
Before each clinic visit, patients were asked to record the type and amount of each food consumed for 3 days two weekdays and one weekend day. Total energy and nutrient intake were calculated using the nutritional analysis program CAN-Pro version 5.
To evaluate the quality of life, SF was surveyed before and after 52 weeks of treatment. The score was converted into points.
Height, weight, waist circumference, hip circumference, body composition, blood pressure, and heart rate were measured at each clinic visit. Body composition including lean mass, fat mass, and fat content was measured by bioelectrical impedance with the body composition analyzer InBody , Biospace, Seoul, Korea. Respired gases oxygen and carbon dioxide were analyzed, and resting energy expenditure REE was calculated indirectly.
The predicted REE was calculated as follows: [ 9 ]. Muscle strength was evaluated with an electronic handgrip dynamometer Lavisen, Namyangju, Korea because it is feasible, convenient, and inexpensive. The handgrip strength was measured 3 times in each hand with at 1-minute intervals, and the average values were used for the analysis.
After the study was completed, free triiodothyronine fT3 and total thyroxine TT4 were measured with the stored blood at 0, 12, 26, and 52 weeks after the treatment. To compare parameters before and after the treatment, the paired Wilcoxon test was performed.
The change in food intake and muscle strength over time was analyzed using a linear mixed model. All statistical analyses were performed with SPSS version All patients preferred the antithyroid drug treatment and were treated with methimazole.
The average starting dose was Fifty-two weeks after the initial treatment, the dose was reduced to 6. The changes in the biochemical parameters are shown in Table 1.
Twenty-six weeks after the treatment, all patients had achieved a normal thyroid function: the thyroid hormone decreased gradually from week 4 to week 26, and TSH increased at a later time from week 12 to week 26 Table 1. After a week treatment of methimazole, the titer of anti-TSH receptor antibodies was decreased with marginal significance Systolic blood pressure decreased significantly while diastolic blood pressure remained unchanged Table 2. Heart rate also decreased significantly Table 2.
Although a beta-blocker was used in 2 patients until week 26 of the treatment, the heart rate remained decreased at week 52 of the treatment. Total cholesterol with both HDL and LDL cholesterols significantly increased after treatment, while triglyceride showed no change Table 1. At the time of diagnosis, the mean body weight and BMI were During the initial 8 weeks of treatment, the BMI significantly increased, reaching a peak at week 8, and then, it plateaued until week 26 Table 2.
At 52 weeks after the treatment, the BMI was slightly decreased but still higher than that at the baseline without any statistical significance. Although the hip circumference was slightly increased, the waist-to-hip ratio was not different after the treatment Table 2. Because a higher food intake with an increased appetite can be the major cause of weight gain, we first assessed the dietary energy intake using the three-day dietary record.
However, the total energy intake rapidly decreased from week 8 to week 12, and then, it was maintained up to week 52 ; Figure 1. There was no significant difference in the composition of macronutrients such as carbohydrates, proteins, and fats. Some studies have suggested a connection between hypothyroidism and vitamin D deficiency. For instance, research published in in the International Journal of Health Sciences found that people with hypothyroidism were deficient in vitamin D.
Yet a analysis of the U. Get enough shut-eye. Not getting enough sleep can lower your metabolic rate, according to the National Sleep Foundation, which recommends that most adults get about seven to nine hours of sleep a night.
Making these changes in your life can help you manage hypothyroidism and overcome the effects of slow metabolism that accompany it.
By subscribing you agree to the Terms of Use and Privacy Policy. Health Topics. Health Tools. Reviewed: August 19, A number of plasma membrane transporters have been identified, some of which require ATP hydrolysis; the relative importance of different carrier systems is not yet clear and may differ among tissues.
Once inside the nucleus, the hormone binds its receptor, and the hormone-receptor complex interacts with specific sequences of DNA in the promoters of responsive genes. The effect of the hormone-receptor complex binding to DNA is to modulate gene expression , either by stimulating or inhibiting transcription of specific genes.
For the purpose of illustration, consider one mechanism by which thyroid hormones increase the strength of contraction of the heart.
Cardiac contractility depends, in part, on the relative ratio of different types of myosin proteins in cardiac muscle. Transcription of some myosin genes is stimulated by thyroid hormones, while transcription of others in inhibited. The net effect is to alter the ratio toward increased contractility. For additional details on mechanism of action and how these receptors interact with other transcription factors, examine the section Thyroid Hormone Receptors.
It is likely that all cells in the body are targets for thyroid hormones. While not strictly necessary for life, thyroid hormones have profound effects on many "big time" physiologic processes, such as development, growth and metabolism, and deficiency in thyroid hormones is not compatible with normal health.
Additionally, many of the effects of thyroid hormone have been delineated by study of deficiency and excess states, as discussed briefly below.
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