Gaining weight in the winter is common. You may be navigating holiday meals and hectic schedules. Winter in many parts of the country can also make it harder to get outside and exercise.
Winter lifestyle changes often lead to adding on a few extra pounds. And you shouldn’t feel guilty. Enjoying yourself during the holidays has health benefits. However, it’s important to know your body. That’s because unexplained weight gain can signal that something else is going on.
If you’re feeling intolerant to the cold—finding it hard to get warm, have noticed unexplained weight gain, and you’re tired all the time, there’s a chance it could be a sign of an underactive thyroid, also referred to as hypothyroidism.
Thyroid function: a delicate balance
About 20 million Americans have some type of thyroid disease, which involves not having the right level of thyroid hormones. Women are up to eight times more likely to have problems and it’s usually because of low levels of thyroid hormones.
To understand what goes wrong, it helps to look at how the process is supposed to work. The thyroid is a butterfly-shaped gland located in the front of the neck. It’s called the body’s thermostat because it regulates how energy is used and sets body temperature. It controls cell metabolism by producing the hormones triiodothyronine (T3) and thyroxine (T4) which is converted into T3—the active form that acts on cells concentrated in the brain, bones, heart, intestine, and skin. The brain’s pituitary gland balances these hormones by releasing thyroid-stimulating hormone (TSH) when levels are low and dialing it back when levels are high.
The role of nutrients
Healthy thyroid function requires the trace minerals iodine, selenium, iron, magnesium, and zinc, along with vitamins A, D, E, B2, B3, B6, and B-12, and the amino acid tyrosine. These essential nutrients are required to create T4. Selenium, zinc, and a healthy gut microbiome are critical factors in converting T4 to T3. Once T3 makes it to cells, it has to be properly absorbed. Vitamins A, zinc, and exercise are all key to cell receptor sensitivity.
Of course, none of this is possible without macronutrients—the carbohydrates, proteins, and fats that fuel the body. For thyroid health, adequate protein levels are essential.
Signs of an underactive thyroid
Our bodies normally increase TSH levels during colder months to boost thyroid hormones and keep the body warmer. If you have an underactive thyroid, this increase may not happen—leaving you feeling chilled.  When body functions slow down due to low thyroid hormone levels, common symptoms include:
- Feeling cold (lower body temperature)
- Fatigue, lack of energy
- Weight gain (including water retention) 
- Constipation, slow bowel movements (slowed digestion)
- Muscle and joint aches (breakdown during catabolic state)
- Lower heart rate
- Brain fog, forgetfulness
- Depressed mood
- Dry skin, brittle nails (worse with winter dry air)
- Hair loss
- Irregular or heavy menstrual periods
A lot of people have mild or no symptoms. Yet low thyroid levels can still damage the body, increasing cholesterol and the risk of diseases like heart disease, diabetes, and metabolic syndrome. It can also advance to more overt hypothyroidism. That’s why it’s important to catch it early. [3, 4]
What causes hypothyroidism?
Worldwide, iodine deficiency is the most common cause of low thyroid levels. In the U.S., where salt is fortified with iodine, an autoimmune disease called Hashimoto’s thyroiditis is most often the cause—making up an estimated 90% of cases. In the case of Hashimoto’s disease, the body creates antibodies that attack the thyroid gland.
Autoimmune reactions during pregnancy can lead to low thyroid levels, endanger the unborn baby and increase the risk of miscarriage. Having another type of autoimmune disease or a family member with thyroid problems increases the risk.
While there is a genetic factor, lifestyle factors often play a role in thyroid disease. Stress, trauma, infection, inflammation, a low-calorie diet, liver or kidney problems, certain medications, an imbalance in the gut microbiome, and a lack of nutrients can all throw off the production of thyroid hormones. Various studies have linked deficiencies in selenium, vitamin D, and vitamin B12 to Hashimoto’s disease. 
Balancing Thyroid levels
Assessing your thyroid health involves blood tests to measure TSH, free T3, free T4, and antibodies. Because thyroid hormone levels often change with the seasons and due to illnesses, stress, or trauma, it’s important to track thyroid levels and watch for patterns.  If you’re pregnant, your healthcare provider will screen you and treat the condition if needed to avoid problems. [9, 10]
Optimizing thyroid function requires a broad approach. Tailoring thyroid medications with your healthcare provider may be required. Beyond this, lifestyle factors such as exercise, getting enough sleep, reducing stress and inflammation, and ensuring proper nutrition all play a role.
Diet and nutrients
Making sure you’re getting proper nutrients is essential for thyroid health. However, it’s important that these are balanced. For example, too much iodine can actually increase the risk of autoimmune-related thyroid problems. [11, 10]
Research suggests an anti-inflammatory diet such as the Mediterranean diet may be beneficial while extreme diets that involve high amounts of fat, limited carbohydrates, or fasting can increase levels of the stress hormone cortisol and negatively affect thyroid health. [11, 12, 13]
Some foods, medications, and supplements can affect the absorption of thyroid medications. If you’re being treated for thyroid problems, it’s important to discuss your diet and supplements you may be taking with your healthcare provider. 
The stress connection
Elevated levels of the stress hormone cortisol can suppress TSH, lowering the production of thyroid hormones. Cortisol can also reduce the enzyme activity that converts T4 into T3 and affect the ability of thyroid hormones to enter cells. [8,13]
Understanding your stress type can be a start to tamping it down if it’s a factor.
The Bottom Line
If you’re noticing unexpected weight gain combined with feeling unusually cold, tired, or achy this holiday season, it’s best to check with your healthcare provider.
Thyroid problems are very treatable. It just requires a simple blood test and tailored thyroid hormone treatment if needed.
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- “Hashimoto's Disease.” National Institute of Diabetes and Digestive and Kidney Diseases, U.S. Department of Health and Human Services, https://www.niddk.nih.gov/health-information/endocrine-diseases/hashimotos-disease
- Andersen, S. L., Carlé, A., Olsen, J., & Laurberg, P. (2016). Hypothyroidism incidence in and around pregnancy: a Danish nationwide study, European Journal of Endocrinology, 175(5), 387-393. Retrieved Dec 2, 2022, from https://eje.bioscientifica.com/view/journals/eje/175/
- Aktaş HŞ. Vitamin B12 and Vitamin D Levels in Patients with Autoimmune Hypothyroidism and Their Correlation with Anti-Thyroid Peroxidase Antibodies. Med Princ Pract. 2020;29(4):364-370. doi: 10.1159/000505094. Epub 2019 Nov 29. PMID: 31779003; PMCID: PMC7445676.
- Ganesan, K. & Wadud, K. “Euthyroid Sick Syndrome.” StatPearls, StatPearls Publishing, 27 Oct. 2022, https://www.statpearls.com/articlelibrary/viewarticle/21409/
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017 Mar;27(3):315-389. doi: 10.1089/thy.2016.0457. Erratum in: Thyroid. 2017 Sep;27(9):1212. PMID: 28056690.
- Chiovato L, Magri F, Carlé A. Hypothyroidism in Context: Where We've Been and Where We're Going. Adv Ther. 2019 Sep;36(Suppl 2):47-58. doi: 10.1007/s12325-019-01080-8. Epub 2019 Sep 4. PMID: 31485975; PMCID: PMC6822815.
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