11 Mar 2025 The Link Between Thyroid Health and Weight Gain
Is Your Metabolism Slowing You Down?
Weight gain and difficulty losing weight can be incredibly frustrating, especially when it feels like you are doing everything right – following a healthy diet, exercising regularly, and maintaining a healthy lifestyle. However, despite your best efforts, the scales refuse to budge. One often-overlooked cause of weight gain is thyroid dysfunction, which can dramatically affect your metabolism.
Thyroid hormones are essential for regulating metabolism, and when they are out of balance, they can cause significant changes in how your body processes energy. Weight gain and difficulty losing weight are among the most common signs of thyroid problems, particularly when the thyroid becomes underactive.
How Does the Thyroid Influence Metabolism
The thyroid, a small butterfly-shaped gland at the base of the neck, is vital for overall health as it produces hormones that regulate metabolism. The primary thyroid hormones—triiodothyronine (T3) and thyroxine (T4)—affect nearly every cell in the body. They influence how your body utilises energy, burns calories, and stores fat.
When the thyroid functions optimally, metabolism runs smoothly, maintaining energy levels and stabilising body weight. However, if thyroid function is disrupted, metabolism may slow down or accelerate, leading to weight fluctuations.
Hypothyroidism, or an underactive thyroid, is a primary cause of weight gain. This condition occurs when the thyroid produces insufficient hormones, slowing metabolism. A study in The Journal of Clinical Endocrinology & Metabolism found that individuals with hypothyroidism experience slower and more difficult weight loss compared to those with normal thyroid function.
So, how do you know if you may have an underactive thyroid? Are there any symptoms? Sometimes, this condition may go unnoticed for years. Common symptoms of hypothyroidism, aside from weight gain, may also include:
- Fatigue
- Dry skin and hair
- Sensitivity to cold
- Constipation
- Depression
- Muscle weakness
While weight gain is a key symptom, not everyone with hypothyroidism experiences significant changes in weight. However, for those affected, even a minor decrease in metabolic rate can make weight loss particularly challenging.
Research indicates that individuals with hypothyroidism expend fewer calories than those with normal thyroid function. Even with exercise, the body burns fewer calories due to a lower basal metabolic rate (BMR). Additionally, hypothyroidism can lead to muscle mass reduction, further lowering calorie expenditure. This makes weight loss more complex and requires a strategic approach involving nutrition, exercise, and lifestyle modifications.
Conversely, hyperthyroidism occurs when the thyroid overproduces hormones, accelerating metabolism and often leading to unintended weight loss. Symptoms include a rapid heartbeat, anxiety, and insomnia. While hyperthyroidism can result in weight loss, it can also cause severe health complications if untreated, such as heart problems and bone loss.
What is the Role of Thyroid Hormones in Metabolism?
Thyroid hormones, particularly T4 and T3, are essential for metabolism regulation. T4, the primary hormone produced by the thyroid, is converted into T3—the active form responsible for metabolic activity.
T4 is made in large amounts but remains inactive until converted into T3 in the liver, kidneys, and the gut. Gut health issues, such as dysbiosis or inflammatory conditions, can impair this conversion, reducing active T3 levels and slowing metabolism.
T3 also plays a direct role in metabolic processes by stimulating enzyme production involved in energy generation and fat oxidation. Low T3 levels result in inefficient energy production, leading to fat accumulation, particularly around the abdomen. Additionally, low T3 is linked to reduced muscle mass, further impairing the body’s ability to burn calories efficiently.
Thyroid hormones also impact other body systems, including the heart and digestive tract. A sluggish metabolism due to hypothyroidism can lead to a slower heart rate, low blood pressure, and digestive issues such as constipation—all contributing to weight management challenges.
How Nutrition helps support Thyroid health and Metabolism
Key Macronutrients for Optimal Thyroid Function and Metabolic Health
Proper nutrition plays a fundamental role in supporting thyroid health and maintaining a healthy metabolism. Many nutrients are essential for thyroid function and incorporating them into your diet can help regulate metabolism, support weight management, and prevent dysfunction.
Balancing Proteins and Healthy Fats:
Macronutrients such as proteins and dietary fats are important for optimal thyroid function and metabolism. Proteins facilitate hormone transport by supporting the synthesis of carrier proteins like thyroxine-binding globulin (TBG), which are necessary for transporting thyroid hormones throughout the bloodstream. Insufficient protein intake can impact hormone distribution and metabolism, potentially leading to symptoms of hypothyroidism. Opting for high-quality protein sources which include grass-fed meat, wild-caught fish, organic poultry, organic eggs, and legumes can help you optimise protein levels in the body.
Healthy dietary fats are other key macronutrients necessary for hormone production, cholesterol serves as a precursor for steroid hormone production, including thyroid hormones. Furthermore, fats help in the absorption of fat-soluble vitamins such as vitamin D, which is a powerful immune modulator and is essential in supporting thyroid health. Healthy fats include monounsaturated fats (olive oil, avocados), polyunsaturated fats (Omega-3s from fish and seeds), and saturated fats from quality sources like grass-fed butter and coconut oil. Adequate intake of these nutrients supports metabolic health and hormone balance.
Let’s take a closer look at omega-3 fatty acids, which play a fundamental role in fighting chronic inflammation and supporting cell integrity. Chronic inflammation is a major contributor to thyroid dysfunction, particularly in autoimmune conditions such as Hashimoto’s thyroiditis and Graves’ disease. Omega-3 fatty acids help modulate inflammation and support cell membrane integrity, which is crucial for effective thyroid hormone signalling. of omega-3s include wild-caught fatty fish (salmon, mackerel, sardines), flaxseeds, chia seeds, and walnuts.
Maintaining optimal thyroid function also depends on a well-balanced intake of essential micronutrients that support hormone production, conversion, and overall regulation. Here are some of the main ones you should watch out for when it comes to thyroid health.
Iodine: The Building Block of Thyroid Hormones
Iodine is an important element for the synthesis of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Low iodine levels can lead to goitre, hypothyroidism, and metabolic imbalances. Levels can be optimised by consuming rich dietary sources of iodine such as seaweed, wild-caught fish, organic dairy, and iodised salt. However, excessive iodine consumption may trigger or worsen autoimmune thyroid disorders such as Hashimoto’s thyroiditis; this is why iodine should be consumed with caution and a qualified nutritionist should be seen before taking any nutritional supplements.
Selenium: Supporting Hormone Conversion and Antioxidant Defence
The conversion of the inactive T4 into the metabolically active T3 hormone depends on adequate selenium levels. Selenium also contributes to thyroid antioxidant defence, protecting the gland from oxidative stress and inflammation. Good dietary sources of selenium include Brazil nuts, sunflower seeds, seafood, and organic eggs. Research suggests that adequate selenium intake may help reduce thyroid autoimmunity and inflammation.
Iron: Thyroid Hormone Production and Metabolic Regulation
Iron is essential for thyroid health and metabolism due to its role in thyroid hormone production and function. It is an important component of thyroid peroxidase, the enzyme responsible for adding iodine to tyrosine amino acids in thyroglobulin, a protein necessary for thyroid hormone production. If iron levels in the body are low this process can be impaired, leading to reduced thyroid hormone levels and potentially contributing to hypothyroidism. Iron is also necessary for the conversion of thyroxine (T4) to the active hormone triiodothyronine (T3), and low iron levels can hinder this conversion, further affecting metabolic processes. Moreover, iron deficiency anaemia is commonly seen in people with thyroid dysfunction, highlighting the strong connection between iron status and thyroid health. Dietary iron comes in two forms: heme iron, predominantly found in animal-based foods such as grass-fed meat,oysters and sardines, and non-heme iron, primarily present in plant-based sources—beans, leafy green vegetables, and nuts.
Zinc: Essential for Thyroid Hormone Synthesis and Function
Zinc participates in over 300 enzymatic reactions, metabolic functions, regulation of gene expression, and immune modulation. This key mineral is important for thyroid hormone production, conversion, and receptor activation. Low zinc levels may lead to symptoms such as fatigue, weight gain, and impaired immune function, which are commonly associated with hypothyroidism. Rich food sources of zinc include oysters, grass-fed red meat, organic poultry, and pumpkin seeds. Some studies indicate that zinc supplementation can improve thyroid function, particularly in individuals with subclinical hypothyroidism. While low zinc levels are linked to significant health risks, an excess of this trace element can also have harmful effects. This is why it is essential to consult with a qualified nutritionist before taking any zinc supplements.
How is our environment affecting thyroid and metabolism
Due to modern agricultural practices, soil depletion, and nutrient-deficient animal feed, dietary intake may sometimes be insufficient. Choosing organic produce where possible can help reduce exposure to harmful pesticides and ensure higher nutrient density, as organic farming tends to preserve soil quality and biodiversity. Growing your own food is another excellent way to enhance nutrient intake while avoiding synthetic chemicals that may disrupt the body’s hormonal balance. If you live in a flat without space to cultivate your own food, consider reaching out to your local council. They often offer allotments—parcels of land rented to people for growing fruits, vegetables, and flowers. Availability and regulations can vary by location, so contacting your local council is a good starting point to explore this option.
Unfortunately, modern world and big cities are full of environmental toxicants—such as pesticides, heavy metals, and industrial pollutants—which can further interfere with thyroid hormone production, metabolism, and action. These endocrine-disrupting chemicals are found in non-organic food, plastics, and contaminated water sources, making dietary choices critical for supporting healthy thyroid function. Choosing a reliable water filtration system is a first step to ensuring contaminant-free water. Reverse osmosis (RO) is a widely used purification method known for its effectiveness in removing a broad spectrum of contaminants, including harmful bacteria, viruses, heavy metals, and chlorine.
Opting for organic, whole foods and limiting exposure to processed, chemically treated products can have a significant impact on protecting overall health. While nutritional supplements can also be beneficial in cases of dietary insufficiency, consulting a psa-accredited registered nutritional therapy practitioner is essential to ensure they align with individual health needs and do not contribute to imbalances or toxicity.
For more on Endocrine disruptors click here.
Conclusion
Thyroid function and metabolism are strongly connected, and disruptions in thyroid health can significantly impact weight management. Whether dealing with hypothyroidism, which slows metabolism and promotes weight gain, or hyperthyroidism, which speeds it up, thyroid dysfunction makes maintaining a healthy weight more challenging.
Supporting thyroid health through proper nutrition—including essential nutrients like iodine, selenium, zinc, iron, and Omega-3s—is vital for sustaining metabolic efficiency and preventing weight fluctuations.
By understanding the complex relationship between thyroid health and metabolism, you can make informed dietary and lifestyle choices to promote optimal thyroid function and overall well-being.
Author: Simona Yakarova, BANT Registered Nutritionist®
For more on Thyroid Health, see the latest edition of the NED Journal with all the latest science and clinical considerations.
REFERENCES:
- Anna-Mariia Shulhai, Rotondo, R., Maddalena Petraroli, Patianna, V., Predieri, B., Iughetti, L., Esposito, S. and Maria Elisabeth Street (2024). The Role of Nutrition on Thyroid Function. Nutrients, [online] 16(15), pp.2496–2496. doi:https://doi.org/10.3390/nu16152496.
- Biondi, B. and Cooper, D.S. (2007). The Clinical Significance of Subclinical Thyroid Dysfunction. Endocrine Reviews, 29(1), pp.76–131. doi:https://doi.org/10.1210/er.2006-0043.
- Calder, P.C. (2010). Omega-3 Fatty Acids and Inflammatory Processes. Nutrients, 2(3), pp.355–374.Available at : https://pmc.ncbi.nlm.nih.gov/articles/PMC3257651/
- Czarnywojtek, A. et al. (2023) ‘The role of Vitamin D in autoimmune thyroid diseases: A Narrative review,’ Journal of Clinical Medicine, 12(4), p. 1452. https://doi.org/10.3390/jcm12041452.
- Garofalo, V., Condorelli, R.A., Cannarella, R., Aversa, A., Calogero, A.E. and La Vignera, S. (2023). Relationship between Iron Deficiency and Thyroid Function: A Systematic Review and Meta-Analysis. Nutrients, [online] 15(22), p.4790. doi:https://doi.org/10.3390/nu15224790.
- Hu, Y., Feng, W., Chen, H., Shi, H., Jiang, L., Zheng, X., Liu, X., Zhang, W., Ge, Y., Liu, Y. and Cui, D. (2021). Effect of selenium on thyroid autoimmunity and regulatory T cells in patients with Hashimoto’s thyroiditis: A prospective randomized‐controlled trial. Clinical and Translational Science, 14(4), pp.1390–1402. doi:https://doi.org/10.1111/cts.12993.
- Kitada, M., Ogura, Y., Monno, I. and Koya, D. (2019). The impact of dietary protein intake on longevity and metabolic health. EBioMedicine, 43, pp.632–640. doi:https://doi.org/10.1016/j.ebiom.2019.04.005
- Macvanin, M.T., Gluvic, Z., Zafirovic, S., Gao, X., Essack, M. and Isenovic, E.R. (2022). The protective role of nutritional antioxidants against oxidative stress in thyroid disorders. Frontiers in Endocrinology, [online] 13, p.1092837. doi:https://doi.org/10.3389/fendo.2022.1092837
- Ofori, S., Dwomoh, J., Owusu, P., Kwakye, D., Osei Kyeremeh, Frimpong, D. and Aggrey, M. (2024). Dietary Fat Intake on Metabolic Health: An in-Depth Analysis of Epidemiological, Clinical, and Animal Studies. American Journal of Biomedical and Life Sciences, 12(4), pp.68–77. doi:https://doi.org/10.11648/j.ajbls.20241204.12.
- Severo, J.S., Morais, J.B.S., de Freitas, T.E.C., Andrade, A.L.P., Feitosa, M.M., Fontenelle, L.C., de Oliveira, A.R.S., Cruz, K.J.C. and do Nascimento Marreiro, D. (2019). The Role of Zinc in Thyroid Hormones Metabolism. International Journal for Vitamin and Nutrition Research. Internationale Zeitschrift Fur Vitamin- Und Ernahrungsforschung. Journal International De Vitaminologie Et De Nutrition, [online] 89(1-2), pp.80–88. doi:https://doi.org/10.1024/0300-9831/a000262.
- Xu, L., Liu, Z., Xu, X., Wan, Q., Zhen, J., Jiang, L., Man, B. and Li, C. (2024). Thyroid autoimmunity is associated with dietary fat consumption. The Journal of Clinical Endocrinology & Metabolism. [online] doi:https://doi.org/10.1210/clinem/dgae629.
- Yang, Z., Zhou, Y., Feng, Z., Rui, X., Zhang, T. and Zhang, Z. (2019). A Review on Reverse Osmosis and Nanofiltration Membranes for Water Purification. Polymers, 11(8), p.1252 doi:https://doi.org/10.3390/polym11081252.
- Yavuz, S., Salgado Nunez del Prado, S. and Celi, F.S. (2019). Thyroid Hormone Action and Energy Expenditure. Journal of the Endocrine Society, [online] 3(7), pp.1345–1356. doi:https://doi.org/10.1210/js.2018-00423.
- Ylli, D. and Wartofsky, L. (2018). Can We Link Thyroid Status, Energy Expenditure, and Body Composition to Management of Subclinical Thyroid Dysfunction? The Journal of Clinical Endocrinology & Metabolism, 104(1), pp.209–212. doi:https://doi.org/10.1210/jc.2018-01997.
- Zimmermann, M.B. and Boelaert, K. (2015). Iodine deficiency and thyroid disorders. The Lancet. Diabetes & Endocrinology, [online] 3(4), pp.286–295. doi:https://doi.org/10.1016/S2213-8587(14)70225-6.
NOTES TO EDITORS:
BANT is the leading professional body for Registered Nutritional Therapy Practitioners in one-to-one clinical practice and a self-regulator for BANT Registered Nutritionists®. BANT members combine a network approach to complex systems, incorporating the latest science from genetic, epigenetic, diet and nutrition research to inform individualised recommendations. BANT oversees the activities, training and Continuing Professional Development (CPD) of its members.
Registered Nutritional Therapists are regulated by the Complementary and Natural Healthcare Council (CNHC) that holds an Accredited Voluntary Register (AVR) for the Professional Standards Authority for Health and Social Care (PSA). A report by the Royal Society for Public Health and the Professional Standards Agency made a key recommendation that AVR practitioners have the authority to make direct NHS referrals, in appropriate cases, to ease the administrative burden on GP surgeries. BANT nutrition practitioners are the key workforce asset to harness 21st-century lifestyle medicine to tackle the rising tide of stress-related fatigue, obesity, Type 2 Diabetes, dementia and other chronic diseases.
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