Underactive thyroid and associated risk for heart disease

Underactive thyroid and associated risk for heart disease

listen now download audio

8 November 2010

This transcript was typed from a recording of the program. The ABC cannot guarantee its complete accuracy because of the possibility of mishearing and occasional difficulty in identifying speakers.

Norman Swan: Good morning and welcome. Today on the Health Report, important information for pregnant women; the state of pregnancy care worldwide and how assuming childbirth is natural and should be left alone is killing untold hundreds of thousands of women worldwide each year, not to mention harming their babies; a simple way of helping women and babies at risk which is being trialled in Aboriginal communities here and a suspected risk factor for heart attacks and other manifestations of heart disease appears to have been confirmed. It's a silently underactive thyroid gland. The finding comes from a large international study which included people living in south west Western Australia.

Associate Professor John Walsh is an endocrinologist at Sir Charles Gairdner Hospital in Perth and the University of Western Australia.

John Walsh: The background is that we know that underactive thyroids are associated with an increased risk of heart disease but we don't know if mildly under-active thyroids which are called sub-clinical hypothyroidism are also associated with an increased risk of heart disease.

Norman Swan: So let's just go back to basics. Hypothyroidism, underactive thyroid, if you've got symptoms from it what do you feel?

John Walsh: The classic symptoms include tiredness, weight gain, dry skin and dry hair, constipation, puffiness, swelling of the ankles and around the eyes. But that's really only when the disease is very severe, with milder cases the symptoms can be non-specific or indeed have no symptoms at all.

Norman Swan: And it's under-diagnosed?

John Walsh: It is under-diagnosed.

Norman Swan: And why do you get it?

John Walsh: The commonest cause is auto-immune which is to say the immune system attacking the thyroid gland instead of fighting bugs like it's meant to.

Norman Swan: Why does full blown hypothyroidism, before we get to the sub-clinical, the one that you're not getting any symptoms from, why is that thought to increase the risk of heart disease?

John Walsh: The evidence that full blown hypothyroidism causes heart disease comes from autopsy studies done many, many years ago showing marked atherosclerosis in patients with that degree of thyroid failure. And the mechanisms include increases in circulating cholesterol concentrations and increased stiffness of the arteries and increased blood pressure.

Norman Swan: Because in some ways it's counterintuitive because the thyroid hormone increases your blood pressure, it increases your pulse, you'd think that hyperthyroidism is the one that causes the heart disease not the hypo activity of the gland.

John Walsh: Well that's right, in fact hyperthyroidism, overactive, and hypothyroidism, underactive, both cause heart disease but they do it in different ways and the blood pressure angle is interesting. Hyperthyroidism causes an increase in the systolic blood pressure, or the higher number that doctors measure when they take blood pressure. Hypothyroidism causes in increase in the diastolic or the lower blood pressure number so both can cause heart disease but different mechanisms.

Norman Swan: Do we know what proportion of the population have sub-clinical, in other words they've got no symptoms at all?

John Walsh: Well the term sub-clinical is unsatisfactory, it's actually applied to a biochemistry picture that you get back from blood tests so there's a big debate about whether it really is truly sub-clinical or whether it does cause symptoms or not. But 5% of the general population have the blood test pattern of sub-clinical hypothyroidism and if you look in some groups such as older women it will be much higher - 10% to 15%.

Norman Swan: And the blood test you're talking about is the hormone from the brain that's sent to the thyroid gland to tell it to produce more hormone and when it doesn't produce hormone this hormone goes a bit nuts whipping the thyroid into action.

John Walsh: That's correct, so the thyroid hormone levels themselves are in the normal range with sub-clinical hypothyroidism but the pituitary gland hormone TSH or thyroid stimulating hormone levels are elevated.

Norman Swan: This is the one from the brain?

John Walsh: Correct.

Norman Swan: What did you do in the study?

John Walsh: In the past there have been eleven large what's called cohort studies where you take a bunch of people at the beginning and follow them over the years and see who develops heart disease or dies and compare the outcomes in people with the thyroid disease with the people with normal thyroid disease at the beginning. And those studies have been inconclusive, two of the studies including ours from Busselton in Western Australia show that sub-clinical hypothyroidism was associated with cardiovascular disease whereas the other nine didn't. In this study the eleven groups got together, pooled all the data from those eleven studies into one large study and examined the studies in one enormous cohort study if you like.

Norman Swan: Before we get to the results, the Busselton study is a long running study here in Western Australia which has been following the health of a group of people in the community.

John Walsh: That's right, it started in the 1960s by a far sighted general practitioner Kevin Cullen and it's continued ever since with periodic surveys of the Busselton population.

Norman Swan: And when you brought together all this data what did you find?

John Walsh: We found very interesting results. Really the cardiovascular risk depended on the degree of sub-clinical hypothyroidism and there were two important findings. The first was that in people with the mildest form of this level whose TSH levels were up to 7 at the beginning of the study they did not have an increased risk of heart disease. And that's important because it means that if people have that profile on their blood tests and feel perfectly well with no symptoms they can safely be left alone and not treated. The flip side of the coin was that if people had higher levels of TSH above 10 they did have an increased risk of heart attack and other cardiovascular events during the follow up period. And if they had levels of between 7 and 10 which was intermediate, they had an increased risk of cardiovascular mortality, of dying from heart disease although the effect on cardiovascular events wasn't quite significant. So it was a bit mixed there and perhaps not totally convincing.

Norman Swan: There's some evidence that it was more portent in younger people this effect?

John Walsh: Yes, there's been a big debate about this, there's been suggestions that sub-clinical hypothyroidism might be a bad thing if you're a young person but not be bad for you if you're old. And the findings weren't quite clear cut on that and there was just a hint that in younger people there was an increased risk of cardiovascular events but it wasn't quite statistically significant.

Norman Swan: Could heart disease cause a high TSH?

John Walsh: Yes, and that's one of the confounders, you can get a pattern similar to sub-clinical hypothyroidism just in people who are sick. However that wouldn't explain this study because we took out everyone with heart disease at the beginning and when we restricted the analysis to those who didn't have heart disease at the beginning the effect was still there.

Norman Swan: What I'm getting at, is it cause and effect?

John Walsh: You've always got to be wary of concluding cause and effect from observational studies, so we can't conclude that firmly but it's very likely.

Norman Swan: Because the key here is that there is a treatment for a high TSH level, this hormone that comes from the brain and that's give thyroid hormone.

John Walsh: Yes, what is really needed to answer the question is an enormous trial of many thousands of people with half of them taking thyroxine and half of them taking placebo followed for many years.

Norman Swan: Thyroxine being the thyroid hormone?

John Walsh: Correct, but for various reasons that will probably never happen, it's probably not going to be funded, it would probably need to be too large to happen and take too long to be feasible. So we're probably never going to get the best level of evidence to answer that question.

Norman Swan: I mean the reality is we treat biochemical tests so people come in, you see people all the time, they come in with a high blood sugar you treat them for diabetes, they come in with a high cholesterol you treat them for cholesterol. Should doctors now treat a high TSH?

John Walsh: Well I think what we can say is that the best level of evidence would now say that if the TSH is above 10 and maybe above 7 that is associated with poor outcomes in the long term and those patients probably should be treated with thyroxine. The other side of the coin is that doctors can feel relaxed about not treating patients with lesser degrees of elevated TSH up to 7 if they have no symptoms, for example if it's been discovered on a routine health check and the patient's well.

Norman Swan: What's the potential for harm in giving the thyroxine given that there's no proof of benefit?

John Walsh: The potential for harm is probably only there if patients take too much in which case they end up with the other side of the coin a mildly over-active thyroid if you like resulting from the thyroid hormone tablets.

Norman Swan: It's not the thyroid that's over-active it's just the effects of the drug?

John Walsh: Correct, so too much thyroid hormone on board rather than the over-activity of the gland and that probably increases the risk of irregular heartbeats such atrial fibrillation which in turn can be a risk factor for stroke. And the concern is that when you study large numbers of people on thyroxine they quite often are not on the right dose, some are on too much, some are on too little.

Norman Swan: So how are consumers to know the best way of being on thyroxine?

John Walsh: The best way is simply to have a periodic measurement of your thyroid stimulating hormone level every 6 to 12 months and to have that in the reference range from the lab. So we've established that you don't have to treat patients who have a mildly elevated TSH if they don't have symptoms, the important exception to that rule would be women who are pregnant or who are contemplating pregnancy in whom we believe that even a mildly under-active thyroid might be bad for the baby's development. So that would be an important exception - women with even minor abnormalities of thyroid function tests might benefit from thyroxine if they're planning a pregnancy.

Norman Swan: Should the TSH level be a routine part of pregnancy screening?

John Walsh: That's a hugely controversial issue, my own belief is that it should, I think there is enough evidence for harm from thyroid abnormalities during pregnancy but consensus has not been reached on that yet.

Norman Swan: Associate Professor John Walsh is an endocrinologist at Sir Charles Gairdner Hospital in Perth and the University of Western Australia.

Reference:

Rodondi N et al. Subclinical Hypothyroidism and the Risk of Coronary Heart Disease and Mortality. JAMA 2010;304(12):1365-1374

http://www.abc.net.au/rn/healthreport/stories/2010/3055898.htm#transcript