Despite these successes, authors have questioned the efficacy of l-thyroxine monotherapy because about 10% to 15% of patients are dissatisfied as a result of residual symptoms of hypothyroidism (1, 2), including neurocognitive impairment (3), and about 15% of patients do not achieve normal serum triiodothyronine (T3) levels (4). Studies of several animal models indicate that maintaining normal serum T3 levels is a biological priority (5). Although the clinical significance of relatively low serum T3 in humans is not well-defined (1), evidence shows that elevating serum T3 through the administration of both l-thyroxine and l-triiodothyronine has benefited some patients (6, 7). However, this has not been consistently demonstrated across trials (1). Novel findings highlight the molecular mechanisms underlying the inability of l-thyroxine monotherapy to universally normalize measures of thyroid hormone signaling (8, 9), and new evidence may lay the foundation for a role of personalized medicine (10). Understanding the historical rationale for the trend toward l-thyroxine monotherapy allows us to identify scientific and clinical targets for future trials.
Also available on the market are combination medications that contain both synthetic T4 and T3 hormones, but such medications aren’t usually recommended. For one thing, most patients see their condition improve with synthetic T4 alone because of the ability of the thyroid to convert these hormones to T3 when needed. Also, synthetic T3-T4 combination drugs can cause anxiety — if you have a preexisting mental health disability, such side effects may be even greater. (3)
Goitrogens are substances found naturally in certain foods that can slow down the production of thyroid hormone—keep in mind, though, this phenomenon occurs typically in people with an underlying iodine deficiency (which is rare in the United States). Still, even for people without iodine deficiency, experts recommend not over-consuming goitrogenic foods.
It’s more of a problem for people with iodine deficiency or those who eat huge amounts of goitrogens. “Goitrogens are not as active when they’re cooked, so eat them cooked,” says Fiorella DiCarlo RDN, CDN, with both clinical and research experience in medical nutrition therapy in New York City. “The last thing I want to do as a dietitian is to tell people to not eat vegetables!,” she adds. It sounds weird, but if you have a precondition, eating these vegetables raw and in large quantities could affect your thyroid.
Hypothyroidism is a very common condition. Approximately 3% to 4% of the U.S. population has some form of hypothyroidism. This type of thyroid disorder is more common in women than in men, and its incidence increases with age. Examples of common causes of hypothyroidism in adults include Hashimoto's thyroiditis, an autoimmune form of overactive thyroid, lymphocytic thyroiditis, which may occur after hyperthyroidism (underactive thyroid), thyroid destruction from radioactive iodine or surgery, pituitary or hypothalamic disease, medications, and severe iodine deficiency.
Initial strategies for thyroid hormone replacement included thyroid transplantation, but efficacious pharmacologic strategies soon won favor. Natural thyroid preparations containing T4 and T3, such as desiccated thyroid, thyroid extracts, or thyroglobulin, were the initial pharmacologic agents. Synthetic agents were synthesized later. Early clinical trials demonstrated the efficacy of synthetic and natural agents, but concerns arose regarding consistency of natural thyroid preparations and adverse effects associated with T3-containing preparations (natural or synthetic). With the demonstration of peripheral T4-to-T3 conversion and the availability of the serum TSH radioimmunoassay in the early 1970s, there was a major trend in prescribing preference toward l-thyroxine monotherapy. BMR = basal metabolic rate; DT = desiccated thyroid; IV = intravenous; RIA = radioim-munoassay; T3 = triiodothyronine; T4 = thyroxine; TG = thyroglobulin; TSH = thyroid-stimulating hormone.
Cruciferous vegetables, such as broccoli and cabbage, are full of fiber and other nutrients, but they may interfere with the production of thyroid hormone if you have an iodine deficiency. So if you do, it’s a good idea to limit your intake of Brussels sprouts, cabbage, cauliflower, kale, turnips, and bok choy, because research suggests digesting these vegetables may block the thyroid's ability to utilize iodine, which is essential for normal thyroid function. 

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