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Fatigue, Weight Gain, and the Thyroid (or is the thyroid why I am so tired and can't lose weight)
Merril W. Edmonds MD, FRCP(C)Endocrinologist, London Health Sciences Centre
Mr. Jones, a 43-year-old, has always had trouble keeping his weight down. He was always dieting and exercising until a couple of years ago when he suddenly started to lose weight despite eating a lot more than usual. He was subsequently diagnosed with hyperthyroidism and treated with a drink of radioactive iodine. Within three months he was again putting on weight, developed muscle cramps, and was becoming cold and puffy. Hypothyroidism was diagnosed and he was started on thyroxine but he continued to gain weight. Repeat thyroid tests indicated his thyroid levels were back to normal but Mr. Jones found that he had to eat even less than before and exercise even longer in order to keep his weight down. He wonders if the tests are wrong and feels he should be taking more thyroid hormone.
Although Mrs. Smith and Mr. Jones are fictitious, they are typical of many patients that are referred to endocrinologists. Fatigue and weight gain are two of the most common symptoms that bring people to see their family doctor. Can the thyroid cause these symptoms? Can we be sure the tests are accurate? What would happen if I took a little more thyroid hormone than the doctor advised? Before we try to answer these questions it would be best to review the role of the thyroid and the tests we use to determine if the thyroid is functioning normally.
If thyroid levels increase, cells throughout the rest of the body increase in activity. As a result more energy is used up, fat and protein stores are mobilised and the weight decreases even though the appetite increases. More heat is produced as a by-product and the person becomes hot, and sweating increases in an effort to get rid of the heat. The brain works overtime resulting in irritability and shakiness, and sleep becomes difficult. The heart beats faster and harder. Bowel activity increases. Everything speeds up.
If thyroid hormone levels in the blood decrease even a little, the pituitary puts out more Thyroid Stimulating Hormone (TSH) which travels through the blood to the thyroid where it stimulates the thyroid gland to make and release more thyroid hormone. If thyroid hormone levels increase for some reason, the pituitary stops secreting TSH into the blood stream and the thyroid is no longer stimulated to make and secrete thyroid hormone into the circulation.
The main thyroid hormone produced by the thyroid gland is thyroxine (T4) but T4 is inactive and has to be converted to triiodothyronine (T3). This occurs throughout the body. It is T3 that the pituitary senses and if T3 levels are too high the pituitary stops putting out TSH, the thyroid stops working and the thyroid levels come back down to normal. If the T3 levels are too low the pituitary secretes more TSH and the thyroid starts releasing thyroid hormone again bringing the levels back up to normal.
Sometimes the thyroid is stimulated by antibodies (Graves' disease) produced by the immune cells within the thyroid, and the TSH levels as a result are low. Other times the thyroid may be blocked or destroyed by antibodies (Hashimoto's Thyroiditis) and the TSH levels become elevated. The TSH level in the blood is the most sensitive indicator of these common causes of change in thyroid function.
In addition new assays for TSH can measure very low levels and are very reliable. Consequently the measurement of TSH has become the main screening test for thyroid dysfunction. A slight increase in thyroid hormone levels (mainly T3) quickly decreases the TSH level. A slight decrease quickly causes the TSH level to become elevated. Very occasionally changes in thyroid function are due to changes in pituitary function.
An underactive pituitary results in decreased TSH, decreased stimulation of the thyroid and decreased thyroid levels. A pituitary tumour that secretes TSH results in increased TSH levels, increased stimulation of the thyroid and increased thyroid levels. Measuring TSH in pituitary problems could, therefore, lead to the wrong diagnosis.
This rarely causes a problem though, because a high TSH due to a pituitary problem will be associated with clinical features of an overactive thyroid, and a low TSH due to a pituitary problem will be associated with clinical features of an underactive thyroid. In addition when TSH levels are abnormal, free T4 and free T3 are measured and help make the right diagnosis.
An overactive thyroid initially may cause an increase in energy but the energy wasted in the process results in a decrease in total energy stores including the loss of protein and this eventually results in fatigue and loss of stamina and strength.
There are, however, many other causes of fatigue. Anemias of which there are many causes result in decreased oxygen delivery to cells throughout the body and thus fatigue.
Any chronic disease affecting the heart, kidneys, gut, and other organs can result in fatigue. Overwork, lack of sleep, depression, and boredom all sap a person's energy and result in fatigue.
These latter factors are probably the commonest causes of fatigue and certainly more common than changes in thyroid function.
Taking too much thyroid hormone may initially help the fatigue caused by these other disorders but in the long term the fatigue usually returns and too much thyroid hormone can result in a loss of bone mass (osteoporosis) and heart problems (irregular rhythms and heart failure).
The main cause for weight gain, however, is still not understood. In North America almost everyone eats more calories than needed each day for normal metabolism of the cells. Fortunately most of us can waste these extra calories probably by burning them off in the form of heat.
People who start to put on weight lose the mechanism for wasting extra calories. If any extra calories are consumed beyond what is needed, and this may be quite small, the extra calories can only be stored and the weight increases.
People who are overweight often eat significantly less than those who are lean and yet still gain because they can't waste any extra calories. Taking thyroid hormone will help someone who is hypothyroid lose weight.
Taking too much thyroid hormone will usually cause anyone to burn off or waste calories and lose weight but much of the weight lost will be protein and result in osteoporosis and problems with the heart. When the thyroid hormone is stopped or reduced to normal amounts the weight will increase again.
Some individuals, who have been hyperthyroid but have been treated and now have normal thyroid levels, experience more difficulty keeping their weight down as compared to before they ever had a thyroid problem.
Somehow too much thyroid hormone for a period of time may permanently impair an individual's ability to waste extra calories and the weight tends to increase even though the thyroid levels are normal. This applies to individuals who have been hyperthyroid because of a thyroid problem such as Graves' disease and those individuals who take too much thyroid hormone over a long time.
Because of this and the long-term side effects of taking too much, thyroid hormone is not a good way to lose weight.
The safest approach is lifestyle modification that results in decreased caloric intake by changing eating habits and increased expenditure of calories by exercising more. Even though weight control may be more difficult after being hyperthyroid, it is still quite possible.
Even though she did not have other symptoms and signs of an underactive thyroid it was still quite reasonable, with her family history, to measure the TSH level. The normal result though excludes a thyroid problem as a cause for her fatigue.
Mr. Jones lost weight when his thyroid was overactive. After he was treated and his thyroid became underactive he again put on weight.
Unfortunately, he experienced even more difficulty keeping his weight down than he had prior to having any thyroid problem even when his thyroid levels were brought back to normal.
Although this might in part be due to a permanent effect of being hyperthyroid on his ability to waste extra calories, it is not due to a current lack of thyroid hormone since his TSH is normal.
Taking more thyroid hormone might help him lose weight but much of the weight loss will be protein and he will increase his risks for osteoporosis and heart problems.
In addition taking too much thyroid hormone may make it even more difficult for him to lose weight if the dose of thyroxine is decreased to normal amounts again.
Changes in thyroid function can cause fatigue and changes in weight. Although thyroid problems may affect up to 8% of women and 1% of men, there are still many other more common causes for these symptoms.
It is relatively simple and inexpensive, however, to rule out thyroid problems as a cause for these symptoms by measuring TSH in the blood. A normal TSH excludes thyroid problems as causes for fatigue and weight gain and individuals can be reassured that their cruise control is functioning just fine.