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Over-active thyroid glands increase the risk of developing an irregular heart beat
06 May 2012
European Society of Endocrinology
Patients with slightly over-active thyroid glands who are not yet showing symptoms of the condition are at increased risk of atrial fibrillation – an irregular or abnormally fast heart beat – according to new research presented today (8 May 2012) at the International Congress of Endocrinology/European Congress of Endocrinology.
Researchers in Denmark studied the medical records of 525,100 people who consulted their general practitioner between 2000-2009 in Copenhagen and who underwent thyroid blood tests. They found that those with subclinical hyperthyroidism had between one and a half times and 1.8 times the risk of atrial fibrillation (AF) than people with a normally functioning thyroid gland.
The researchers also found that patients with under-active thyroid glands (hypothyroidism) had a lower than normal risk of AF: the risk was nearly halved in those with clinical signs of the condition, while the risk in patients with subclinical hypothyroidism (no obvious symptoms) was reduced by approximately a seventh (14%).
Dr Christian Selmer, a research fellow at Gentofte University Hospital in Copenhagen, Denmark, who presented the research, said: “This is the first study to investigate the relationship between thyroid dysfunction and the risk of atrial fibrillation in a very large group of primary care patients. We have found that there is a physiological relationship between all levels of thyroid dysfunction and the risk of atrial fibrillation, with a higher risk in patients with over-active thyroid glands, even when the condition is very mild or at the high end of ‘normal’, and a lower risk in patients with under-active thyroid glands.
“It’s important to emphasise that this is an observational study, and that we should be careful not to extrapolate our results to clinical practice before further prospective studies have been done. However, our results point out that doctors should be more aware of the risk associated with subclinical hyperthyroidism.”
Atrial fibrillation is the most common heart rhythm disturbance, and although it can affect anyone at any age it becomes more common the older you get, affecting approximately 10% of people over the age of 75. Symptoms include palpitations, fainting, chest pain and shortness of breath, and it also increases the risk of stroke. Treatment can include medication to control the heart rhythm and rate, and medication to prevent stroke.
Tests to investigate thyroid function include measuring the levels of thyroid stimulating hormone (TSH) in the blood. This is a messenger hormone produced by the pituitary gland that regulates the amount of hormonal output from the thyroid. When an over-active thyroid produces too much thyroid hormone, levels of TSH go down in order to lower production of the thyroid hormone. The range for ‘normal’ levels of TSH used by the Danish researchers was 0.2-4.0 microunits per litre of blood (mU/L).
The vast majority of the patients (96%) had normally functioning thyroid glands. However, 3,421 (0.7%) had clinical signs of an over-active thyroid gland and 5,414 (1%) had subclinical hyperthyroidism; 1,474 (0.3%) patients had clinical signs of under-active glands and 10,679 (2%) had subclinical hypothyroidism.
The researchers found that in patients with subclinical over-active thyroid glands, the risk of AF was increased the lower the levels of TSH. Those with TSH levels of less than 0.1 and between 0.1-0.2 mU/L had a 1.8 (80%) and 1.5 (50%) increased risk of AF, respectively, while those who were in the higher range of ‘normal’ (TSH levels of 0.2-0.4 mU/L) had an elevated risk of 1.3 (30%).
In contrast, patients with clinical and subclinical under-active thyroid glands had between nearly a half and seventh reduced risk of AF.
“These results show that subclinical hyperthyroidism and ‘high normal’ thyroid function are significant risk factors for atrial fibrillation, whereas hypothyroidism seems to be associated with a decreased risk,” concluded Dr Selmer.
Dr Selmer and his colleagues now intend to investigate the link between under- and over-active thyroid glands and blood clots, deaths from heart disease and deaths from any cause in the same group of Danish patients.