Decades of research have demonstrated that the thyroid plays a key role in regulating the body’s metabolism, affecting weight gain and related metabolic problems like diabetes, high cholesterol and fatty liver disease.
But scientists are still exploring the way thyroid hormone interacts with the cells of the body. In doing so, they have uncovered a fascinating aspect of the thyroid’s mechanism that until now hasn’t been clearly understood…
The subtlety of thyroid function
Researchers at the University of Pennsylvania have discovered that the thyroid operates less like an on/off switch and more like the dimmer that gradually adjusts the brightness of your light fixture.
“We were able in this study to show that thyroid hormone doesn’t just turn things on or off, as the canonical model suggests, but instead more subtly shifts the balance between the repression and enhancement of gene activity,” says principal investigator and Penn Medicine professor Dr. Mitchell Lazar. “Yet, as people with hypothyroidism know, the lack of thyroid hormone can have profound effects on the body.”
The study focused on TR-beta, the main receptor in the liver. TR-beta regulates some of the thyroid hormone’s most important metabolic effects, including the lowering of cholesterol levels.
When thyroid hormone binds to TR-beta, it can function as a sort of dimmer switch, shifting the balance in favor of more gene activation at some sites and more gene repression at others — instead of simply turning the entire network of genes on or off, as once thought.
The researchers say more work needs to be done to clarify why thyroid hormone acts in this more subtle way when binding to TR-beta. But they believe the findings are a significant advance in understanding thyroid biology, and that they can be used to develop more precise medicines targeting a number of metabolic diseases.
What’s the right level for thyroid treatment?
If your doctor suspects your thyroid may not be working properly, they will likely start by testing your level of thyroid-stimulating hormone (TSH). According to the American Thyroid Association, adults with a TSH level of 0.4 to 4.0 milli-international units per liter (mlU/L) are considered to have normal thyroid function.
TSH levels under 0.4 mlU/L indicate you could have hyperthyroidism, a condition where your thyroid is producing too much thyroid hormone. Symptoms include heart palpitations, tremors, disrupted sleep, frequent bowel movements and unintentional weight loss. Untreated hyperthyroidism can contribute to abnormal heart rhythm and worsen osteoporosis.
When your TSH levels measure above 4 mlU/L, you likely have hypothyroidism, or underactive thyroid. Your TSH levels are abnormally high because your body is producing more TSH in an effort to get your thyroid to release more thyroid hormone.
Signs your thyroid is underactive include weight gain, fatigue, thinning hair, stiff or painful joints, memory issues and increased sensitivity to cold. Left untreated, hypothyroidism can lead to obesity, infertility and heart disease, so doctors will usually prescribe a thyroid hormone replacement medication like levothyroxine.
Most physicians won’t prescribe thyroid medication unless TSH levels surpass the 4 mlU/L mark. However, the National Academy of Clinical Biochemistry has recommended beginning treatment at 2.5 mlU/L or higher, especially if the patient has cardiovascular risk factors. One study recommends clinicians consider gender, age, pre-existing conditions, symptoms and quality of life when deciding whether to prescribe thyroid medication to patients whose TSH is above 2.5.
If your TSH levels are in the 2.5-to-4.0 range, and you’re at higher cardiovascular risk and/or experiencing some or all of the symptoms of an underactive thyroid, you may want to talk to your doctor about whether you should begin thyroid hormone replacement.
It’s also a good idea to support good thyroid function with nutrients like iodine, copper, selenium and zinc. The thyroid needs iodine to produce thyroid hormone, so it’s important that your diet includes plenty of iodine-rich foods. Some good sources of iodine include organic yogurt, cranberries, iodized salt, navy beans and sea vegetables like kelp and wakame.
Be aware that it gets more difficult for your body to absorb iodine as you get older, so you may need an iodine supplement to ensure you’re maintaining healthy levels of the nutrient. Also, combining iodine with the amino acid L-Tyrosine can increase thyroid hormone efficiency.
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Discovery Illuminates How Thyroid Hormone ‘Dims’ Metabolism — Penn Medicine News
Interpreting Thyroid Levels Tests — Healthline
The Normal TSH Reference Range: What Has Changed in the Last Decade? — The Journal of Clinical Endocrinology & Metabolism
What do different TSH levels mean? — Medical News Today
Hypothyroidism (underactive thyroid) — Mayo Clinic
What Your Hypothyroidism Test Results Mean — Everyday Health