The relation between an underactive thyroid and weight gain is quite popularly recognized. But what exactly is the underlying mechanism that is at work behind this close interconnection between the Thyroid and weight gain? To understand this mechanism, it is necessary to first recognize the relation between Basal Metabolic Rate (BMR) and body weight and then further attempt to know more about the effects of an underactive thyroid gland on the BMR.

Relation between Basal Metabolic Rate and Body Weight

• The Basal Metabolic Rate is the prime factor that determines the way in which the body utilizes the energy that is available from digestion and assimilation of foods.
• A lowering in the BMR leads to a proportional reduction in utilization of available calories for deriving energy.
• This leads to a negative energy balance in the body.
• This means that a low BMR is often associated with extra calories in the body, that are not being utilized and are hence being converted to fat and stored for further use
• A high BMR rate is associated with speedy and rapid utilization of the available calories and there is no negative energy balance as none of the calories obtained from assimilation of foods are left over to be stored.

Relation between Underactive Thyroid and Weight Gain

• An underactive thyroid gland leads to the clinical syndrome of Hypothyroidism, and it is associated with a slowing down of the Basal Metabolic Rate.
• As described in the previous section; lowering of BMR only leads to a negative energy balance and the body is incapable of utilizing the calorie resources that are available.
• This incapability of the body to utilize calories is translated in to weight gain due to the storing of the unused calories as fat cells.
• Well, this does not mean that the relation between underactive thyroid and weight gain is merely due to the low BMR rates, as there are many other factors like other hormones, proteins, enzymes, and chemicals etc which have their say in weight gain process.

Can The Underactive Thyroid And Weight Gain Associated With It Be Reversed?

• The treatment of an underactive thyroid gland or Hypothyroidism is done by prescribing thyroid hormone supplements which have to be consumed on a daily basis.
• The replenishing of the deficient thyroid hormones leads to improvement of the BMR and thus the negative energy balance is reversed.
• So after consuming Thyroid hormone supplements regularly, patients can expect to lose a major part of the weight gained due to Hypothyroidism.
• But there is associated salt and water retention in Hypothyroidism, which contributes towards weight gain.
• Using thyroid hormone supplements may help to improve this retention, and this can lead to approximately 10 % of the excess body weight being lost.
• But most of the times, the weight loss may not be very significant even after regular usage of Thyroid hormone supplements because other factors like consumption of unhealthy diet or lack of exercise etc may be simultaneously present.