Low thyroid function may be a factor in weight gain despite ‘normal’ tests

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When I was studying medicine I was even more cynical than I am now and so, I think, were quite a few of my friends. We had all established firmly in our minds the notion that eating few calories than the body ‘burned’ would result in weight loss, so generally had little compassion for individuals who claimed they did not overeat but still could not shift their excess weight. Also, around the time a study was published which claimed that the overweight tend to significantly underestimate the amount they eat. So, if someone claimed that they thought they had a ‘sluggish metabolism’ or had ‘a problem with their glands’ (low thyroid function) then our eyes would usually roll skywards, if not outwardly, then at least inwardly.

Age, experience and new belief that being an advocate to my patients is generally important has taught me better. I have had many, many experiences over the years which have convinced me that it is possible for individuals to gain weight while eating relatively little. Not uncommonly, the problem appears to indeed lie in a under-par metabolism, which itself is often the result of a genuine problem with the thyroid gland.

Conventional assessment of the thyroid usually involves blood tests to measure thyroid hormone levels. For a variety of reasons, though, these tests will not always pick up an underlying problem. To understand how this can be, we must first understand the physiology of the thyroid gland and the function of the chief thyroid hormones:

The thyroid produces a variety of hormones, the most plentiful of which is known as ‘thyroxine’ (also known as ‘T4’). Outside the thyroid, T4 is converted into another hormone called tri-iodothyronine (also known as ‘T3’). T3 is actually a more active form of thyroid hormone. T3 basically stimulates cells to burn fuel with oxygen to release energy, some of this being released as heat. Essentially, the more T4 and T3 there is around, the faster metabolism, the less tendency there is for weight gain and the warmer the body is.

The thyroid’s production of hormones is itself regulated by a tiny gland located at the base of the brain known as the ‘pituitary’. The pituitary is itself regulated by a part of brain known as the hypothalamus. In health, if the hypothalamus senses a drop in the levels of T4 and/or T3, it sends a signal to the pituitary, which in turn secretes a hormone known as thyroid stimulating hormone or ‘TSH’. As its name suggests, this hormone instructs the thyroid to produce more thyroid hormones. In theory, as the thyroid hormone levels rise, the hypothalamus instructs the pituitary to produce less TSH, which ensures thyroid hormone levels do rise too much. This mechanism is designed to ensure stable levels of thyroid hormones in the body.

However, like any other gland or organ in the body, the thyroid gland can weaken. In this case, despite high levels of TSH, it may still not be able to make the amounts of thyroid hormones necessary for optimal health. This low thyroid function state is known as ‘hypothyroidism’. One of the typical symptoms of this is weight gain, though fatigue, dry skin, dry hair, hair loss, low mood, sensitivity to cold and constipation are others.

The conventional way to test thyroid function is to measure blood levels of TSH. If this is raised, this suggests hypothyroidism. The diagnosis is usually confirmed by measuring T4 levels, which are characteristically low in cases of hypothyroidism.

While the TSH test is generally seen by doctors and endocrinologists as a sensitive and accurate guide to thyroid function, the reality is that this test has a number of deficiencies.

One major issue here relates to the ‘normal range’ that is set for TSH. Normal ranges are designed to encompass 95 per cent of people. What this means is that to have an elevated TSH, one needs to be in the top 2.5 per cent of the population for TSH levels. This means that however common hypothyroidism may be, only a relatively small proportion of the population can be diagnosed using this test.

Also, as was discussed earlier, low thyroid function can be related to low pituitary function. In traditional medicine, lower than normal levels of TSH are believed to signify this. However, before the pituitary is exhausted to this extent, it is possible for it to go through a phase where TSH levels are considered ‘normal’, though thyroid function is significantly compromised. While this notion is plausible, it is generally not accepted by endocrinologists (doctors specialising in hormone-related disease).

Another problem with conventional testing is that it relies on levels of TSH and, usually, T4. If there is enough T4 in the body, the brain can sense this and feel there is no need to increase TSH production. However, T4 is not very active in the rest of the body ” it seems T3 has a more important role to play in this respect. It is therefore possible for someone to have enough T4 but not enough T3, and be hypothyroid as a result. T3 levels are rarely checked in conventional medicine. As a result, someone with low T3 levels who is hypothyroid as a result, can get missed as a result of ‘normal’ TSH and T4 levels being found.

Yet another potential deficiency of conventional thyroid testing is that while it may show the level of hormones in the bloodstream, it does not tell us how active and effective those hormones are. It is now well recognised, for instance, that individuals can become resistant to the hormone insulin (known as ‘insulin resistance’), which may eventually lead to a problem with diabetes. In contrast, the concept of thyroid hormone weakness has yet to catch on in conventional medical circles.

So, for these and other potential reasons, just because someone has ‘normal’ thyroid hormone levels, that doesn’t guarantee by any means that their thyroid function is ‘normal’ or ‘optimal’ by any means.

The idea that a normal TSH means normal thyroid function was dealt a bit of a blow recently on the publication of a study in the Archives of Internal Medicine in a group of 2407 individuals, all with ‘normal’ TSH levels. Despite having acceptable TSH levels, the higher the TSH level was, generally speaking, the greater body weight was too [1]. This study is accompanied by an editorial [2] which examines the evidence from the study, along with three other studies [3-5] which all found evidence that as TSH rises, so does body weight.

They also cite evidence that as individuals lose weight, TSH levels come down.

At first sight, all this looks like weight gain may indeed be related to ‘worsening’ thyroid function. However, if this were the case, then conventional wisdom would dictate that levels of T3 and/or T4 would decrease as TSH levels rise. However, the authors of the editorial point to evidence which shows that in the very overweight, thyroid hormone levels tend to be raised, not lowered.

One might argue therefore that what is going on here is the body is recognising that it is carrying too much weight, and is attempting to boost its metabolism through the secretion of TSH. It’s almost as if the body is recognising it needs to shift some excess weight and is asking the thyroid to ramp up its production of thyroid hormones through increased secretion of TSH. They go on to postulate a few mechanisms through which fatty tissue in the body could ‘communicate’ with the body in an effort to tell it to rev-up the metabolism.

While the mechanisms here are not certain, what I think all this shows is that the relationship between thyroid function and weight is more complex than most of we doctors would have people believe. What is more, there may be more to an individual’s belief that their difficulty in losing weight is in some way related to thyroid function. The authors of the editorial conclude that we may agree with patients who suggest such a thing by saying: It may be the thyroid but we don’t exactly know how.�


1. Fox CS, et al. Relations of thyroid function to body weight: cross-sectional and longitudinal observations in a community-based sample. Arch Intern Med 2008;168(6):587-592

2. Weiss RE, et al. Doctor�could it be my thyroid? Arch Intern Med 2008;168(6):568-569

3. Knudsen N, et al. Small differences in thyroid function may be important for body mass index and the occurrence of obesity in the population. J Clin Endocrinol Metab. 2005;90(7):4019-4024

4. Nyrnes A, et al. Serum TSH is positively associated with BMI. Int J Obes (Lond). 2006;30(1):100-105.

5. Bastemir M, et al. Obesity is associated with increased serum TSH level, independent of thyroid function. Swiss Med Wkly. 2007;137(29-30):431-434.

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