The thyroid gland is a small, butterfly-shaped endocrine gland located in front of the neck. It uses iodine from the foods we eat to make two hormones: triiodothyronine (T3) and thyroxine (T4). These hormones travel from the thyroid gland through the blood to all parts of the body, where they help organs work well. They control how the body uses food for energy. Thus the functionality of the thyroid gland affects the body’s overall wellness.
It is important that the levels of these hormones are never too high or too low. Hypothalamus and pituitary glands in the brain work together to maintain these levels. The hypothalamus produces TSH Releasing Hormone (TRH) that signals the pituitary to tell the thyroid gland to produce more or less of T3 and T4 by either increasing or decreasing the release of a hormone called thyroid stimulating hormone (TSH).
- When T3 and T4 levels are low in the blood, the pituitary gland releases more TSH to tell the thyroid gland to produce more thyroid hormones.
- If T3 and T4 levels are high, the pituitary gland releases less TSH to the thyroid gland to slow production of these hormones.
It has been appreciated for a very long time that there is a complex relationship between thyroid disease, body weight and metabolism. 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). In fact, measurement of the BMR was one of the earliest tests used to assess a patient’s thyroid status.
There are two types of thyroid gland disorders:-
- Hypothyroidism (underactive thyroid) – it is when there is reduced thyroid hormone secretions. Some of the symptoms include:
- slowed down cell activity
- decreased heart rate
- heavy menstruation
- joint & muscle weakness
- feeling cold
- weight gain
- poor concentration
- Intestines work sluggishly, making one constipated
Since the BMR in the patient with hypothyroidism is decreased, an underactive thyroid is generally associated with weight gain. The cause of the weight gain in hypothyroid individuals is also complex, and not always related to excess fat accumulation. Most of the extra weight gained in hypothyroid individuals is due to excess accumulation of salt and water. Massive weight gain is rarely associated with hypothyroidism. In general, 2 to 5 kilograms of body weight may be attributable to the thyroid, depending on the severity of the hypothyroidism. Finally, if weight gain is the only symptom of hypothyroidism that is present, it is less likely that the weight gain is solely due to the thyroid.
- Hyperthyroidism (overactive thyroid) – it is when there is excess secretion of thyroid hormones. Some of the symptoms include:
- increased activity of body cells
- increased heart rate
- weight loss
- heat intolerance
- irritability or moodiness
- sore and gritty eyes
- hair loss
- missed menstrual periods
- increased activity of intestine causing frequent bowel motions or even diarrhea
Since the BMR in persons with hyperthyroidism is elevated, they experience weight loss. Furthermore, the likelihood of weight loss occurring is related to the severity of the overactive thyroid. Thus, if the thyroid is extremely overactive, the individual’s BMR increases which lead to increased caloric requirements to maintain that weight. If the person does not increase the calories consumed to match the excess calories burned, then weight loss will ensue. Since hyperthyroidism also increases appetite, some patients may not lose weight, and some may actually gain weight, depending on how much they increase their caloric intake.
The factors that control our appetite, metabolism, and activity are very complex and thyroid hormone is only one factor in this complex system. That is the reason why the thyroid gland is closely linked to body weight!