Written on April 14, 2023 by Lori Mulligan, MPH. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.
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For understanding the connection between thyroid health and weight loss, we need to understand the function of the thyroid in the human body.
According to the Department of Health and Human Services, your thyroid is a small butterfly-shaped gland found at the base of your neck, just below your Adam’s apple. This gland makes the thyroid hormone that travels in your blood to all parts of your body. The thyroid hormone controls your body’s metabolism in many ways, including how fast you burn calories and how fast your heart beats. 
There are three conditions to explore the connection between thyroid health and weight loss. Two are thyroid diseases—hyperthyroidism, which leads to weight loss, and hypothyroidism, which leads to weight gain and makes weight loss difficult. The third is the use of non-prescription, over-the-counter thyroid hormones to stimulate weight loss. Let’s take a look at each condition to show the role of the thyroid and weight loss.
According to the Mayo Clinic, hyperthyroidism happens when the thyroid gland makes too much thyroid hormone. This condition also is called overactive thyroid. Hyperthyroidism speeds up the body’s metabolism. That can cause many symptoms, such as weight loss, hand tremors, and rapid or irregular heartbeat. 
According to Harvard, even though you may eat constantly, you could lose weight, usually between 5 to 10 pounds—even more in extreme cases. 
Reported in Clinical Obesity, available on WileyOnlineLibrary.com, its treatment usually leads to weight gain and some studies have reported an excess weight regain.
However, there is considerable inter-individual variability and a differential effect on body weight by different treatments, with some studies reporting more weight increase with radioiodine, and perhaps surgery, compared with anti-thyroid drugs. The excess weight regain may relate to treatment-induced hypothyroidism. 
According to the Cleveland Clinic, hypothyroidism is a common condition where the thyroid doesn’t create and release enough thyroid hormone into your bloodstream. This makes your metabolism slow down. Also called underactive thyroid, hypothyroidism can make you feel tired, gain weight, and be unable to tolerate cold temperatures. The main treatment for hypothyroidism is hormone replacement therapy.
If your hypothyroidism is not treated, you could gain weight. Once you are treating the condition, the weight should start to lower. However, you will still need to watch your calories and exercise to lose weight. Talk to your healthcare provider about weight loss and ways to develop a diet that works for you. 
According to the British Thyroid Foundation, your levothyroxine dose is often calculated according to your body weight. So, if you have any significant changes in your weight, it is advisable to have your blood tested to see whether you require a change in dose. 
According to the American Thyroid Organization, hormones have been used as a weight loss tool in the past. Starting or increasing thyroid hormone to cause thyroid hormone levels to be elevated is unlikely to dramatically change weight.
Studies have shown that excess thyroid hormone treatment can help produce more weight loss than can be achieved by dieting alone but includes the risk of major negative consequences, such as the loss of muscle protein, loss of bone, and/or heart problems. Furthermore, once the excess thyroid hormone is stopped, any weight loss is usually regained. 
According to research published in Thyroid, the majority of dietary thyroid supplements studied had clinically relevant amounts of thyroid hormones, T4 and T3, some of which exceeded common treatment doses for hypothyroidism.
These amounts of thyroid hormone, found in easily accessible dietary supplements, potentially expose patients to the risk of alterations in thyroid levels.
The current study results emphasize the importance of patient and provider education regarding the use of dietary supplements and highlight the need for greater regulation of these products, which hold a potential danger to public health. 
If you are concerned you may have thyroid complications, you have options.
Thyroid hormones regulate your body from head to toe. If you’re having symptoms like fatigue, restlessness, mood changes, cold sensitivity, or weight changes, book a visit with an Everlywell provider who can help you with the next steps.
Thyroid symptoms can be elusive. Your symptoms may vary depending on if you are underproducing or overproducing any of the thyroid hormones. A licensed clinician can help you better understand what you are experiencing.
If you take an Everlywell thyroid test and your results are abnormal, you’ll have the option to speak with a physician and discuss your treatment options.
Consider taking an at-home TSH blood test to check your levels of TSH plus free T3 and free T4, plus thyroid antibodies (TPO), to determine if you may have a thyroid condition. You may also be interested in discussing weight loss online with a healthcare provider.