How thyroid malfunction affects your heart

Abnormal function of the thyroid gland can lead to multiple heart problems

Key Points


  • Thyroid hormones can directly affect the function of the heart
  • People suffering from hypothyroidism or hyperthyroidism have a higher risk of developing heart disease
  • People who have suffered some forms of heart disease have low levels of thyroid hormones and have poorer health outcomes
  • Treatment with thyroid hormone therapy may be safe and beneficial for patients with chronic heart failure
  • Optimal gut microbiota composition and function is linked to a healthy thyroid function
  • A healthy diet and optimal lifestyle changes can benefit thyroid function
  • Modern Functional Medicine offers a comprehensive approach to test for thyroid malfunction, elucidate the key underlying problems and identify optimal treatments


The thyroid gland is an important component of the endocrine system – the system responsible for the production of all the hormones in our body. Many hormonal dysfunctions that affect our health can be traced back to the malfunctions of the thyroid gland.

The thyroid gland


      • Location

        Located below the larynx and at the front of the throat, the thyroid is a gland composed of two sections or lobes, which lie on either side of the trachea (Figure 1).


Thyroid gland

Figure 1. A normal thyroid gland.


      • Function

        The thyroid gland secretes the hormones triiodothyronine (T3) and thyroxine (T4), which are involved in the regulation of metabolism, as well with the growth and development of children. Some key facts about these thyroid enzymes include:


            • T3 is considered the biologically active hormone.
            • T4 must first be converted to T3 to interact with thyroid hormone receptors.
            • Most of the T3 enzyme present in the body is produced by conversion of T4, through the action of deiodinating enzymes.
            • In the heart, cardiomyocites (heart cells) produce two key deiodinating enzymes, called type II iodothyronine deiodinase (DIO2) and thyroxine 5-deiodinase (DIO3)


Alterations to the optimal function of this gland, and on the levels of the hormones it produces, can result in metabolic malfunction, including heart problems, neurological disorders, constipation, weight gain or weight loss, depression, or hair loss, among others.


Thyroid gland and health

Alterations to the optimal functioning of the thyroid gland result in abnormal levels of thyroid hormones, which can have wide-ranging and serious consequences to your health.

The most common forms of thyroid malfunctions include:


      • Hypothyroidism

        When this gland is underperforming and producing less than optimal hormone levels, there can be a wide range of consequences for our health, including fatigue, cold sensitivity, constipation, dry skin and unexplained weight gain. This condition has been estimated to have a prevalence of nearly 3% in Australia and 5% worldwide1. There are two main types:


              • Overt hypothyroidism, which occurs when there are elevated serum levels of TSH (Above 10mU/l) and low serum levels of the hormone T4. This condition affects between 0.2% and 2.0% of the global population.


              • Subclinical hypothyroidism, which occurs when there are elevated levels of the TSH hormone and normal levels of the thyroid hormones. Subclinical hypothyroidism can be mild (TSH between 4.0–4.5mU/l, but less than 10.0 mU/l) or severe (TSH more than 10.0mU/l).


      • Hyperthyroidism

        This occurs when the thyroid gland produces an excess of hormones, leading to symptoms like unexpected weight loss, rapid or irregular heartbeats, sweating and irritability. There two main types:

              • Overt hyperthyroidism, which occurs when laboratory tests indicate low levels of the thyroid-stimulating hormone (TSH) and high levels of thyroid hormones, above the laboratory reference range.


              • Subclinical hyperthyroidism, defined as having low levels of TSH (<0.3 or 0.4mU/l), and T3 and T4 serum levels are within the reference range.


Focus on Iodine, diet and health

Iodine is an important mineral, used by the thyroid gland for hormone synthesis. Hence, it is important to consume minimum amounts of this mineral. Current recommendations vary according to age (Table 1).


Table 1. Recommended dietary intakes*

Infants aged 0–6 months*  90 µg
 Infants aged 7–12 months*  110 µg
 Girls and boys aged 1–3 years  90 µg
 Girls and boys aged 4–8 years  90 µg
 Girls and boys aged 9–13 years  120 µg
 Girls and boys aged 14–18 years  150 µg
 Women and men aged 19 years and over  150 µg
 Pregnant women  220 µg
 Lactating women  270 µg

*According to Australian government recommendations.


Iodine is found in a wide variety of foods, including seafood, dairy products, eggs, and some vegetables like Brussel sprouts or broccoli (Table 2). Iodine is also commonly found in cooking salt, which is added during or after cooking.

While iodine is an important micronutrient, excessive iodine intake can result in a malfunction of the thyroid gland. The thyroid gland is a highly flexible organ, being able to adapt and tolerate high levels of dietary iodine. For example, for most adults, excess iodine intake of up to 2 grams/day (about 13,000 times the recommended dietary intake) can be tolerated without any health effects2.

However, chronic excessive iodine intake can result in conditions like goitre (an enlargement of the thyroid gland), hypothyroidism and hyperthyroidism, thyroid autoimmunity, Grave’s disease, and thyroid cancer3. Hence, it pays to know how much iodine you are ingesting on a daily basis. Below are some common sources of iodine on everyday foods (Table 2). Among foods, seaweed is particularly rich in iodine, which, depending on the species, can contain between 10 to 2,000% of the recommended daily intake of iodine4.


Table 2. Iodine content of various foods, according to Food Standards Australia & NZ

Food micrograms of iodine per 100 g micrograms of iodine per serve serve size
Oysters 160 144 6 oysters – 90g
Sushi (containing seaweed) 92 92 1 sushi roll – 100g
Canned salmon 60 63 1 small tin – 105g
Bread (except organic bread) 46 28 2 slices bread -60g
Steamed snapper 40 50 1 fillet – 125g
Cheddar cheese 23 4 2.5 cm cube – 16g
Eggs 22 19 2 eggs – 88g
Ice cream 21 10 2 scoops – 48g
Chocolate milk 20 60 1 large glass – 300ml
Flavoured yoghurt 16 32 1 tub – 200g
Regular milk 23 57 1 large glass – 250ml
Canned tuna 10 10 1 small tin – 95g
Bread, organic 3 2 2 slices – 60g
Beef, pork, lamb <1.5 <1.5 2 loin lamb chops
Apple, oranges, grapes, bananas <0.5 <0.6 1 apple


Thyroid function and heart disease

Tissues found in and around your heart respond to alterations in the levels of hormones produced by the thyroid gland. Studies have shown that even small changes in the concentrations of thyroid hormones can affect the function of the cardiovascular system. Some key findings about the link between the thyroid gland and heart disease include:

      • Altered levels of thyroid hormones can influence the health of the hearth, with studies showing that patients with hypothyroidism or hyperthyroidism have an increased risk of developing heart disease


      • Heart cells (cardiomyocytes) have specific receptors that bind to thyroid hormones5.


      • Thyroid hormone activity regulates myocardial contractility and systolic function, as well as regulating the expression of certain genes important for heart function6-8.


      • Subclinical hypothyroidism and subclinical hyperthyroidism are associated with a 20 -80% increase in vascular morbidity and mortality9-10


There are several risk factors associated with heart disease and thyroid function, including:


      • Hyperlipidaemia

        This term refers to high levels of fats in the blood. Thyroid hormones are involved with the metabolism of fats and the risk of atherosclerosis11. People suffering from overt hypothyroidism and subclinical hypothyroidism have a high chance of also developing hyperlipidaemia, which carries an increased risk of developing atherosclerosis12-15.


      • Blood pressure and vascular function

        Studies have shown that patients suffering from overt hyperthyroidism tend to develop systolic hypertension16-17. However, the link between subclinical hyperthyroidism and hypertension is less supported, with some studies supporting a link18-19 and others not20-21. Both overt and subclinical hypothyroidism has been associated with diastolic hypertension and impaired vascular function12.


      • Thrombosis

        This condition occurs when blot clots form within a blood vessel, vein or artery, potentially blocking it or reducing the normal flow of blood. Both overt and subclinical hyperthyroidism has been associated with dysfunction in the normal process of blood coagulation22-23. The link between thrombosis and subclinical hypothyroidism, in contrast, remains uncertain, with studies showing conflicting results5.


      • Heart Failure

        this is a complex condition, influenced by various factors, which results in impaired function of the heart as a blood pump. The most common form of heart failure is coronary artery disease, a condition associated with myocardial infarction (heart attack) in most patients24. The myocardium (heart muscle) is very sensitive to the levels of circulating thyroid hormone. Abnormal levels of thyroid hormones can lead to heart failure5. Severe hypothyroidism has also been shown to affect the function of the pericardium (a double-walled sac containing the heart), causing pericardial effusion. This condition occurs when there is an abnormal accumulation of fluid inside the pericardium25.



Factors affecting thyroid function


Optimal thyroid function depends on multiple factors, from the diet you follow to your genetic makeup and the composition of your gut microbiome. Also, certain diseases can affect the function of the thyroid gland, like viral or bacterial infections, autoimmune diseases, cancer, genetic disorders, and certain cancer treatments.

Among these factors, the role of the gut microbiota has received special attention in recent years. Gut dysbiosis is often found in patients suffering from different thyroid dysfunctions.

Two key mechanisms linking the gut microbiota, thyroid health and heart disease are:


      • Abnormal intestinal permeability

        Our intestines represent an important line of defence against pathogens. Intestinal permeability refers to the ability of the intestinal mucosal lining to act as a barrier, which prevents the passage of pathogens, toxins and other harmful molecules into our body. Alterations to the normal barrier function of our intestines have been linked to multiple chronic diseases, including coeliac disease, obesity, diabetes, Irritable Bowel Syndrome, Chron’s disease, food allergies, and others. Multiple factors are associated with intestinal permeability, including diet, food additives, gut dysbiosis and exposure to certain environmental factors26-27. One consequence of increased intestinal permeability is an increased immune response, due to the passage of toxins and other antigens that trigger the immune system. Chronic inflammation can affect multiple aspects of your health, as it can target multiple tissues in the body. Alterations to thyroid function have been linked to chronic inflammatory responses28-29.


      • Nutrient absorption

        Another important role of the gut microbiome is to facilitate the absorption of micronutrients into our body. In fact, some vitamins, like vitamin K and B, are synthesized by gut bacteria30. However, gut bacteria also have important roles with our body’s uptake of micronutrients, such as iodine, selenium, zinc, and iron, which are critical for optimal thyroid function. These micronutrients are essential for the synthesis and function of thyroid hormones. Studies have shown that gut microbiota composition can affect the absorption of these micronutrients. For example, reduced abundance of the gut bacteria Firmicutes and Bacteroidetes has been linked to inflammatory bowel disease, a condition characterised by poor iodine absorption. Iron is more efficiently absorbed in an acidic environment in the intestine, and a healthy gut microbiota can favour this process by helping maintain acidic conditions in the intestine28.


Modern Functional Medicine and Thyroid Health


At the Australian Centre for Functional Medicine, we take into consideration multiple factors that are associated with heart disease. Thyroid function is just one of multiple factors that can be at play in the development of heart disease.

We use comprehensive testing to understand the health of your endocrine system, which includes your thyroid gland, and various other factors that affect the health of your heart. We perform stool, blood, breath, and urine testing, targeting multiple biomarkers that can inform us about your health. Based on the results of these tests, we can evaluate levels of relevant hormones and identify mechanisms that may lead to heart disease. With this information at hand, we can design an optimal, research-based treatment plan.


BECOME A PATIENT TODAY and learn about the status of your thyroid gland, your endocrine system and about multiple other factors that can affect your heart.





  1. Taylor PN, Albrecht D, Scholz A, et al. Global epidemiology of hyperthyroidism and hypothyroidism. Nat Rev Endocrinol. 2018;14(5):301-316. doi:10.1038/nrendo.2018.18 Read it!
  2. Bürgi H. Iodine excess. Best practice & research Clinical endocrinology & metabolism. 2010 Feb 1;24(1):107-15. Read it!
  3. Farebrother J, Zimmermann MB, Andersson M. Excess iodine intake: sources, assessment, and effects on thyroid function. Annals of the New York Academy of Sciences. 2019 Jun;1446(1):44-65. Read it!
  4. Yeh TS, Hung NH, Lin TC. Analysis of iodine content in seaweed by GC-ECD and estimation of iodine intake. journal of food and drug analysis. 2014 Jun 1;22(2):189-96. Read it!
  5. Jabbar A, Pingitore A, Pearce SH, Zaman A, Iervasi G, Razvi S. Thyroid hormones and cardiovascular disease. Nature Reviews Cardiology. 2017 Jan;14(1):39-55. Read it!
  6. He H, Giordano FJ, Hilal-Dandan R, Choi DJ, Rockman HA, McDonough PM, Bluhm WF, Meyer M, Sayen MR, Swanson E, Dillmann WH. Overexpression of the rat sarcoplasmic reticulum Ca2+ ATPase gene in the heart of transgenic mice accelerates calcium transients and cardiac relaxation. The Journal of clinical investigation. 1997 Jul 15;100(2):380-9. Read it!
  7. Holt E, Sjaastad I, Lunde PK, Christensen G, Sejersted OM. Thyroid hormone control of contraction and the Ca2+-ATPase/phospholamban complex in adult rat ventricular myocytes. Journal of molecular and cellular cardiology. 1999 Mar 1;31(3):645-56. Read it!
  8. Kaasik A, Paju K, Vetter R, Seppet EK. Thyroid hormones increase the contractility but suppress the effects of β-adrenergic agonist by decreasing phospholamban expression in rat atria. Cardiovascular research. 1997 Jul 1;35(1):106-12. Read it!
  9. Hak AE, Pols HA, Visser TJ, Drexhage HA, Hofman A, Witteman JC. Subclinical hypothyroidism is an independent risk factor for atherosclerosis and myocardial infarction in elderly women: the Rotterdam Study. Annals of internal medicine. 2000 Feb 15;132(4):270-8. Read it!
  10. Walsh JP, Bremner AP, Bulsara MK, O’Leary P, Leedman PJ, Feddema P, Michelangeli V. Subclinical thyroid dysfunction as a risk factor for cardiovascular disease. Archives of internal medicine. 2005 Nov 28;165(21):2467-72. Read it!
  11. Cappola AR, Ladenson PW. Hypothyroidism and atherosclerosis. The Journal of Clinical Endocrinology & Metabolism. 2003 Jun 1;88(6):2438-44. Read it!
  12. Klein I, Danzi S. Thyroid disease and the heart. Circulation. 2007 Oct 9;116(15):1725-35. Read it!
  13. Duntas LH. Thyroid disease and lipids. Thyroid. 2002 Apr 1;12(4):287-93. Read it!
  14. Ineck BA, Ng TM. Effects of subclinical hypothyroidism and its treatment on serum lipids. Annals of Pharmacotherapy. 2003 May;37(5):725-30. Read it!
  15. Zhang T, Xia Y, Han TL, Zhang H, Baker PN. five serum fatty acids are associated with subclinical hypothyroidism in a chinese pregnant population. Scientific reports. 2020 Apr 21;10(1):1-9. Read it!
  16. Ichiki T. Thyroid hormone and vascular remodeling. Journal of Atherosclerosis and Thrombosis. 2016 Mar 1;23(3):266-75. Read it!
  17. Ching GW, Franklyn JA, Stallard TJ, Daykin J, Sheppard MC, Gammage MD. Cardiac hypertrophy as a result of long-term thyroxine therapy and thyrotoxicosis. Heart. 1996 Apr 1;75(4):363-8. Read it!
  18. Völzke H, Ittermann T, Schmidt CO, Dörr M, John U, Wallaschofski H, Stricker BH, Felix SB, Rettig R. Subclinical hyperthyroidism and blood pressure in a population-based prospective cohort study. European journal of endocrinology. 2009 Oct 1;161(4):615-21. Read it!
  19. Cai Y, Ren Y, Shi J. Blood pressure levels in patients with subclinical thyroid dysfunction: a meta-analysis of cross-sectional data. Hypertension Research. 2011 Oct;34(10):1098-105. Read it!
  20. Walsh JP, Bremner AP, Bulsara MK, O’Leary P, Leedman PJ, Feddema P, Michelangeli V. Subclinical thyroid dysfunction and blood pressure: a community‐based study. Clinical endocrinology. 2006 Oct;65(4):486-91. Read it!
  21. Osman F, Franklyn JA, Holder RL, Sheppard MC, Gammage MD. Cardiovascular manifestations of hyperthyroidism before and after antithyroid therapy: a matched case-control study. Journal of the American College of Cardiology. 2007 Jan 2;49(1):71-81. Read it!
  22. Dörr M, Robinson DM, Wallaschofski H, Schwahn C, John U, Felix SB, Völzke H. Low serum thyrotropin is associated with high plasma fibrinogen. The Journal of Clinical Endocrinology & Metabolism. 2006 Feb 1;91(2):530-4. Read it!
  23. Erem C. Blood coagulation, fibrinolytic activity and lipid profile in subclinical thyroid disease: subclinical hyperthyroidism increases plasma factor X activity. Clinical endocrinology. 2006 Mar;64(3):323-9. Read it!
  24. Mozaffarian D, Benjamin EJ, Go AS, Arnett DK, Blaha MJ, Cushman M, De Ferranti S, Després JP, Fullerton HJ, Howard VJ, Huffman MD. Executive summary: heart disease and stroke statistics—2015 update: a report from the American Heart Association. Circulation. 2015 Jan 27;131(4):434-41. Read it!
  25. Lim AS, Paz-Pacheco E, Reyes M, Punzalan F. Pericardial decompression syndrome in a patient with hypothyroidism presenting as massive pericardial effusion: a case report and review of related literature. Case Reports. 2011 Oct 7;2011:bcr0420114117. Read it!
  26. Llewellyn SR, Britton GJ, Contijoch EJ, Vennaro OH, Mortha A, Colombel JF, Grinspan A, Clemente JC, Merad M, Faith JJ. Interactions between diet and the intestinal microbiota alter intestinal permeability and colitis severity in mice. Gastroenterology. 2018 Mar 1;154(4):1037-46. Read it!
  27. Thevaranjan N, Puchta A, Schulz C, Naidoo A, Szamosi JC, Verschoor CP, Loukov D, Schenck LP, Jury J, Foley KP, Schertzer JD. Age-associated microbial dysbiosis promotes intestinal permeability, systemic inflammation, and macrophage dysfunction. Cell host & microbe. 2017 Apr 12;21(4):455-66. Read it!
  28. Knezevic J, Starchl C, Tmava Berisha A, Amrein K. Thyroid-Gut-Axis: How Does the Microbiota Influence Thyroid Function?. Nutrients. 2020 Jun;12(6):1769. Read it!
  29. Mancini A, Di Segni C, Raimondo S, Olivieri G, Silvestrini A, Meucci E, Currò D. Thyroid hormones, oxidative stress, and inflammation. Mediators of inflammation. 2016 Oct;2016. Read it!
  30. Ramakrishna BS. Role of the gut microbiota in human nutrition and metabolism. Journal of gastroenterology and hepatology. 2013 Dec;28:9-17. Read it!