The thyroid gets blamed for a lot of things—especially weight gain—and a quick internet search will turn up all sorts of information about foods to include—or exclude—for a healthy thyroid. But, what exactly does your thyroid do, and are these nutrition claims for real?
What Does the Thyroid Do?
The thyroid is a two-inch long butterfly-shaped gland at the front of your neck. The thyroid uses iodine from food to make the hormones triiodothyronine (T3) and thyroxine (T4). The thyroid produces mostly T4, the less active hormone, but it can be converted to T3 by the pituitary gland, the liver, and kidneys.
T3 controls the speed of your metabolism—your body’s use of energy—and helps regulate several key body functions. It can also control how some of our genes are expressed by turning them on or off, says Antonio Bianco, MD, PhD, a researcher at Rush University, speaking at the annual meeting of the Academy of Nutrition and Dietetics last October. “By changing the expression of these genes, thyroid hormone has well-known effects on development, growth, metabolism, and cognition.”
Common Thyroid Conditions. Two glands in your brain, the hypothalamus and the pituitary gland, work with the thyroid to keep hormone levels balanced. Thyroid-stimulating hormone (TSH) stimulates the thyroid to produce more hormones but it is the hypothalamus that produces TSH-releasing hormone (TRH), which prompts the pituitary gland to release TSH. “Both T4 and T3 are monitored by the hypothalamus and pituitary gland, and according to the levels of T4 and T3, TRH and TSH are adjusted so the thyroid can maintain normal levels,” Bianco says.
If T4 and T3 levels stay too high, it can cause hyperthyroidism. Symptoms of hyperthyroidism can include rapid heartbeat, increased frequency of bowel movements, weight loss, nervousness or anxiety, hair loss, light or skipped menstrual periods, irritability, and hand trembling. Conversely, low levels of T4 and T3 lead to hypothyroidism, which is much more common. Symptoms can include slow heart rate, constipation, weight gain, fatigue, weakness, depression, frequent and heavy menstrual periods, decreased libido, memory loss or trouble concentrating, sensitivity to cold temperatures, and muscle aches and pains.
Causes of Thyroid Disease
The most common cause of hyperthyroidism is the autoimmune disorder Graves’ disease, in which an antibody (thyroid-stimulating immunoglobulin, or TSI) causes the thyroid gland to make too much thyroid hormone. Graves’ disease is more common in women and tends to run in families. Hyperthyroidism also may be caused by goiter, lumps, or nodules in the thyroid gland that cause excessive hormone production. Excessive intake of iodine from foods, supplements, or medications can also cause the thyroid gland to overproduce thyroid hormones.
Thyroiditis, inflammation of the thyroid gland, can cause temporary hyperthyroidism symptoms, but the most common form of thyroiditis, Hashimoto’s thyroiditis, is an autoimmune disorder and the major cause of hypothyroidism. Hypothyroidism is also a side-effect of surgical or radiation therapy to treat hyperthyroidism or thyroid cancer. While iodine deficiency is a frequent cause of hypothyroidism globally, iodine intake tends to be adequate in the U.S., with rare exceptions in individuals who avoid iodized table salt.
Diagnosis and Treatment
Hyper- and hypothyroidism are diagnosed based on symptoms, a physical exam, and blood tests to measure T4, T3 and TSH levels. Your doctor may also decide to order either an ultrasound or a nuclear medicine scan of your thyroid. Hyperthyroidism can be treated with medications that interfere with the production of thyroid hormones. Another option is radioactive iodine therapy to damage the cells that make thyroid hormones. In rare cases, surgery to remove all or part of the thyroid may be necessary. Replacement of thyroid hormones is the primary treatment for hypothyroidism due to Hashimoto’s thyroiditis or other damage to the thyroid. Synthetic T4, levothyroxine, is a commonly prescribed medication.
Feeding Your Thyroid
Sarah Peterson, PhD, RD, assistant professor of nutrition at Rush University, who spoke with Bianco, says “There’s lots of claims out there, but not a lot of data” about how to eat for thyroid health. What about internet advice to avoid cruciferous vegetables—broccoli, cauliflower, cabbage, Brussels sprouts, kale, and bok choy? Peterson says that consuming a very high intake of cruciferous vegetables could interfere with thyroid hormone synthesis but few people eat that much. “Raw is the biggest problem—when you start to cook the vegetable, this problem dissipates,” she says.
What is clear is the importance of taking levothyroxine while fasting. “The recommendation is that you take levothyroxine 60 minutes before breakfast or at least three hours after an evening meal for optimal, consistent absorption,” Peterson said. If someone has no choice but to take this medication with or near a meal, she recommends avoiding fiber, soy products, coffee, or lactose (the natural sugar in milk and dairy foods) in that meal, as research suggests that they interfere with absorption.
—Carrie Dennett, MPH, RDN