The thyroid is a small butterfly shaped organ found at the base of the neck.
Hypothyroidism or an underactive thyroid is a condition where the thyroid gland is not producing enough thyroxine (T4) and triiodothyronine (T3).

The pituitary gland controls thyroid function. It is a small gland the size of a peanut at the base of the brain. When thyroxine and triiodothyronine hormones drop too low, the pituitary gland produces thyroid stimulating hormone (TSH).

The thyroid gland may become disrupted due to a number of reasons such as pregnancy, radiation exposure, thyroiditis, co-morbidity of other autoimmune disorders and inflammation of the thyroid gland.

Weight gain and chronic fatigue are the leading symptoms of an under-active thyroid. These symptoms can easily be misdiagnosed as other lifestyle changes, however, they could be the leading tell-tale symptoms that you are suffering from hypothyroidism.

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Unusual Weight Gain

According to the American Thyroid Association, only 5-10 pounds of weight gain can be attributed to hypothyroidism. Additionally, it is predicted that suffers will lose only (10%) of body weight following treatment.

Weight gain as a result of hypothyroidism can largely be attributed to salt and water retention as opposed to fat accumulation. The chances of gaining weight while suffering from hypothyroidism is less severe than the weight loss observed in hyperthyroidism.

If other symptoms of an underactive thyroid rectify themselves following treatment, it is more than likely that the weight gain isn’t attributable to thyroid malfunction alone but could include lifestyle factors such as diet and physical activity.

Body Weight, Metabolic Rate & The Thyroid Explained

The thyroid gland regulates your metabolic rate. Metabolic rate refers to the amount of energy used by an organism per unit of time. The metabolic rate can be measured by examining the amount of oxygen expended by the body over a period of time.
Traditionally, the associations between the metabolic rate, body weight and the function of the thyroid gland were examined using the BMR (Basal Metabolic Rate).

Basal Metabolic Rate (BMR) refers to the amount of oxygen expended when an organism is at rest. Earlier testing for thyroid function once used to measure the basal metabolic rate. Those with hypothyroidism are said to have a low basal metabolic rate meaning that the quantity of energy they can produce at rest is low.

The BMR test is no longer widely utilized for thyroid testing, just as the BMI test isn’t relied upon, due to the complex nature of body weight and the numerous factors that play a role in determining weight and muscle mass. However, the BMR and corresponding thyroid function acts as a valid explanation of why having an underactive thyroid may cause weight gain.

The BMR measures the relationship between energy consumption and expenditure within the body. Energy in food is measured in calories, calories refer to the amount of heat it takes to create energy for cell function. The BMR looks at the difference in the amount of calories the body obtains from food and the body's ability to use said calories as energy. A low BMR rate can be caused by low level thyroid function, making it increasingly difficult for the body to optimally use and expend the energy or calories obtained from food. A low basal metabolic rate equates to a slower metabolism which equates to a resistance in burning calories and subsequent weight gain.

Luckily, if hypothyroidism is identified and treatment is put in place, it is very possible to maintain a healthy weight.


Feeling cold when others are not or having a body temperature consistently below 37 °C (98.5 °F) is a symptom of hypothyroidism, and this symptom causes chronic fatigue in those who suffer from hypothyroidism.

Specifically, cold hands and feet and/or a sensitivity to the cold is associated with having an underactive thyroid. As the metabolic rate begins to slow down, your body attempts to conserve lost energy in the form of heat for vital organs. The body doesn’t have enough expendable energy to remain at a normal temperature or prevent increased sensitivity to the cold.

Low BMR compounded by a low basal body temperature. If the body cannot produce a sufficient amount of thyroxine, there is a higher chance of experiencing low body temperatures, and thus causing chronic fatigue.

Fatigue in sufferers of hypothyroidism can be easier to deal with in the late afternoon when body temperature is generally at its highest. Body temperatures are lowest in the late afternoon which could explain the “afternoon” slump, these symptoms are escalated with on-going thyroid problems. Often people who suffer from hypothyroidism function optimally for a few days and then experience a crash and burn. The only way to identify why you are suffering from fatigue is to test every aspect of your health,

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Buy an At-Home Thyroid Test

Understand your thyroid with our home thyroid testing options.

Worried about your fatigue? Watch Dr. Dominic Rowley explain why you might be tired all the time.

Do you think you might have an underactive thyroid? Find out more about the Signs and Symptoms of Thyroid Problems.

Written by Hannah Kingston | Approved by Medical Director Dominic Rowley