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Hyperthyroidism and weight gain: a limited impact

Through its hormones, the thyroid helps to regulate numerous functions, most notably cerebral function, cardiac rhythm, intestinal transit and metabolism. Overall, hypothyroidism tends to slow these activities down and puts the body into sleep mode. Hypothyroidism manifests itself through symptoms characteristic of the condition, one of which is often weight gain. What are the mechanisms at work in the body, how can you tell if you are affected by this condition, and if you are, how can you limit its effects?

The thyroid plays a major role in your metabolism

Hyperthyroidism and weight gain
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 A small gland situated a the base of the neck, in front of the trachea, the thyroid is essential for your body’s healthy functioning. Malfunctions, such as an underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism), can have repercussions for brain function, heart beat, intestinal transit, renal function, body temperature and also weight, one of the main complaints among patients affected by these disorders.


The thyroid secretes two main hormones, T3 (triiodothyronine), produced in small quantities and T4 (thyroxin), its inactive precursor which is transformed according to the body’s needs. This secretion depends on a third hormone, TSH (thyreostimulin), which is released through hypophysis. When the levels of T3 and T4 are reduced (hypothyroidism), hypophysis secretes more TSH to stimulate their  release. In contrast, when the levels of T3 and T4 are higher, TSH secretion lowers.

How does hypothyroidism work?

           Thyroid function is assessed through testing the blood for these three hormones1. Hypothyroidism is apparent, or confirmed, when TSH is superior to 4mUI/l with a reduced level of T4. The body is in sleep mode like this, with a noticeably slower heart beat and intestinal transit, considerable physical and mental fatigue, and lack of appetite but a tendency to put on more and more weight. Full hypothyroidism affects between 3-4% and 10% of the population.2


Hypothyroidism is subclinical3 when TSH levels are lower than 4mUI/l with T4 at a normal level. The consequences are “inconsistent, non-specific, non discriminatory and generally negligible”, in particular when TSH remains under 10mUI/l. Subclinical hypothyroidism does not necessarily need treatment. It develops in a third of cases into full hypothyroidism2,4.


Among people affected by hypothyroidism, many complain of weight gain or difficulty losing weight, including when hypothyroidism is subclinical or treatment is given to ensure TSH levels remain normal. But what exactly are the effects of these disorders on weight?

Hypothyroidism, limited impact on weight

There is an inverse relationship between the level of TSH and body mass index (BMI). On average, BMI rises by 0.41kg per m2 per unit of TSH in women, and 0.48kg per m2 for men, which corresponds to 1kg for a woman who is 1m65 and weighs 60kg. This weight gain is therefore limited to just a few kilos, according to starting weight and the level of TSH.5,6

In fact, according to Dr. Jocelyne M’Bemba*, except for when TSH levels are extreme, weight gain never exceeds a few kilos: “It is partly linked to the appearance of oedema. However, when treatment is balanced and hormonal levels are brought back down to normal amounts, with TSH especially under 2.3mUI/l, there is no reason why this weight gain should remain or continue. If this does occur then it is unlikely to be connected to hypothyroidism.”

How can more significant changes be explained?

Almost a third of the body’s resting energy expenditure (REE) could be controlled by the thyroidal hormones7. In patients treated for hypothyroidism, with TSH levels between 0.1 and 10mUI/l, REE was reduced by 15% 8, which could lead to a slight weight gain occasionally observed in patients.

Hypothyroidism can also encourage weight gain in an indirect way. People who are sensitive to tiredness, another symptom of the illness, might have a tendency to reduce their physical activity. Others, knowing they have the illness, could be more anxious and compensate for this by comfort eating.

There are other reasons why thyroid disorders are generally associated with weight. Apart from anxiety caused when discovering the illness, people are more preoccupied by their weight and can, without intending to, start eating an unbalanced diet thanks to a preventative diet regime (the yo-yo effect).


Finally, hypothyroidism is also more common in older patients, due to retirement and often connected to reduced activity. It also sometimes occurs after the menopause which causes other hormonal imbalances also blamed for weight gain.

But not all the actions of thyroidal hormones are known. Some authors have even suggested the possibility of an inverse relationship: in modifying the adipose tissue, weight gain contributes to a rise in TSH levels.6

Limiting weight gain

“There can be many explanations for fluctuations in weight” Dr. M’Bemba comments. Once hypothyroidism is confirmed, the first step is treatment where synthetic T4 is prescribed. After four to six weeks, sometimes a little more, TSH will return back to normal.” After this, the levels will not change except in certain situations (changes in weight, pregnancy or an illness...).


Certain patients sometimes ask if their dose can be increased in order that the resulting hyperthyroidism will help them lose weight more quickly. “This is absolutely not recommended due to the risk it poses for cardiac health” Dr. M’Bemba warns. For the same reason, hyperthyroidism must be treated even if the patient is worried about putting on weight.”


Hypothyroidism does not require a special diet. A balanced and healthy food intake combined with regular physical activity (three to four sessions a week) is already enough to make sure you dodge those extra pounds.




Sources :


*Interview with Dr Jocelyne M'Bemba, endocrinologist at the Hôtel Dieu (AP-HP, Paris) Hospital and the private Val d'Yerres Hospital (Yerres).


1- T3 and T4 are transported in the blood using proteins. Only a fractional amount (less than 0.1%) circulates ‘free’ (F) - biologically active. It is in this form (FT3/FT4) that they are adjusted for controlled use in the laboratory. The presence of certain antibodies can interfere with the result of adjustments.


2- Subclinical hypothyroidism in adults: diagnosis and management - professional recommendations. French Society of Endocrinology (SFE)/ Superior Health Authority (HAS).


3- Subclinical hypothyroidism is also referred to as asymptomatic or concealed. Its prevalence relies on the threshold levels of TSH used to define the disorder. According to the SUVIMAX study, these levels are 4%  and 11.1% respectively for men and women, for a TSH of between 4 and 9.9mUI/l and 0.2 and 0.7% for a TSH superior to 10mUI/l (when TSH increases significantly, T4 is most often reduced, causing patients to experience full hypothyroidism).


4- Subclinical hypothyroidism in adults: diagnosis and management - professional recommendations. French Society of Endocrinology (SFE)/ Superior Health Authority (HAS).

Posted 06.04.2012


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