Dr Nick Fuller
Leading Obesity Expert at the University of Sydney and founder of Interval Weight Loss.
When the scale keeps ticking up despite your best efforts, your thyroid might be the culprit. Hypothyroidism, or an underactive thyroid, slows metabolism, meaning your body burns fewer calories at rest. Studies show that untreated hypothyroidism can lead to an average weight gain of up to 5 kg, mostly from fluid retention and fat accumulation.
But it’s not just about a sluggish metabolism—low thyroid hormones can also increase appetite and cause fatigue, making it harder to stay active. So, how exactly does this condition affect your weight, and what can you do about it?
Your thyroid gland regulates metabolism or the process that converts food into energy. When it produces too few hormones (hypothyroidism), metabolism slows down, making weight gain more likely. On the other hand, an overactive thyroid (hyperthyroidism) speeds up metabolism, often causing weight loss.
However, research suggests that untreated thyroid dysfunction doesn’t always directly impact BMI. Instead, weight changes are more noticeable once thyroid hormone levels are stabilised through treatment.
There are different types of hypothyroidism, each with its own causes and effects on the body.
This is the most common type, where the thyroid gland itself isn’t making enough hormones. The leading cause is Hashimoto’s thyroiditis, an autoimmune condition that affects up to 7.5% of the population.
Here, the problem starts in the pituitary gland, which fails to send the right signals to the thyroid. This can result from tumours, surgery, or certain medications.
This occurs when the hypothalamus (the brain region that regulates hormones) doesn’t produce enough thyrotropin-releasing hormone (TRH), leading to reduced thyroid hormone production.
This is a milder form, where thyroid hormone levels are slightly low, but symptoms may not be obvious. It affects around 3–15% of adults, particularly older individuals.
Hypothyroidism can sneak up on you, developing slowly over time. Since the thyroid plays a big role in metabolism, low hormone levels can leave you feeling sluggish, affect your weight, and impact overall well-being. Spotting the signs early makes it easier to get the right diagnosis and treatment.
Fatigue – Feeling constantly drained, even with enough sleep.
Increased sensitivity to cold – Always reaching for a jumper while others feel fine.
Constipation – Slow digestion due to reduced gut motility.
Dry skin – Lack of thyroid hormones can affect oil production.
Weight gain – A common symptom due to a sluggish metabolism.
Puffiness in the face – Fluid retention can cause swelling, especially around the eyes.
Weight gain is a common effect of hypothyroidism, but it’s not just about burning fewer calories. Understanding the ways hypothyroidism contributes to weight gain can help in managing it more effectively.
Thyroid hormones control your basal metabolic rate (BMR). With lower hormone levels, your body burns fewer calories, leading to weight gain even without increased food intake.
Fatigue from hypothyroidism makes it harder to stay active, reducing calorie burn. A study found that people with hypothyroidism were less physically active than those with normal thyroid function.
Weight gain in hypothyroidism isn’t just fat—fluid retention plays a big role. This can lead to bloating and puffiness, especially in the face and legs.
Hypothyroidism affects how your body processes fats, often leading to higher cholesterol and fat accumulation. If untreated, this can increase the risk of heart disease.
Low thyroid hormones disrupt other hormones like insulin and leptin, which regulate appetite and fat storage. This can make it harder to lose weight, even with diet and exercise.
Thermogenesis is the body's process of generating heat. Since thyroid hormones help regulate this, lower levels mean reduced calorie burning, especially in colder environments.
Emerging research suggests hypothyroidism alters gut bacteria, which may contribute to weight gain and digestion issues. A 2021 study found a link between gut microbiome imbalances and metabolic changes in hypothyroid patients.
Managing hypothyroidism typically involves restoring proper thyroid hormone levels to support metabolism and overall health. Below are some common approaches to treating hypothyroidism.
The most effective treatment is levothyroxine, a synthetic thyroid hormone that restores normal hormone levels. Proper dosing is crucial and requires regular monitoring.
Thyroid-stimulating hormone (TSH) levels should be checked periodically to ensure proper treatment. The frequency of testing depends on individual needs.
A nutrient-rich diet can support thyroid function. Foods high in iodine (seafood, dairy), selenium (Brazil nuts, eggs), and zinc (meat, legumes) are particularly beneficial.
Iron, vitamin D, and B vitamins play a role in thyroid function. A deficiency in these nutrients can worsen symptoms, so supplementation may help.
Managing stress, getting enough sleep, and avoiding excessive processed foods can help improve thyroid health. Chronic stress increases cortisol levels, which can interfere with thyroid hormone production.
Engaging in regular physical activity is essential for managing hypothyroidism, as it can help alleviate symptoms such as weight gain and fatigue. A personalised exercise program tailored to individual capabilities and preferences can enhance overall well-being.
If you're looking for a program to support weight loss, manage hypothyroidism, and overall health, the Interval Weight Loss (IWL) program may be a great option. A recent study found that IWL outperforms CSIRO and Weight Watchers (WW) as a personalised approach to weight management, making it a strong choice for those seeking sustainable results.
Yes, hypothyroidism can cause weight gain, but it’s not just about calories in versus calories out. A sluggish metabolism, fluid retention, and hormonal imbalances all play a role.
The good news? With the right treatment and lifestyle adjustments, managing your weight and overall well-being is absolutely possible.
Dr Nick Fuller is the founder of Interval Weight Loss and is a leading obesity expert at the University of Sydney with a Ph.D. in Obesity Treatment. Dr Fuller is also the author of three best-selling books and his work been published in top ranked journals in the medical field, including JAMA, Lancet and American Journal of Clinical Nutrition.