Thyroid disorders are common around the world. A 2019 study found roughly 25% of older adults in the United States experience some form of thyroid disorder — either hypothyroidism (low thyroid levels) or hyperthyroidism (high thyroid levels). But what are optimal thyroid levels, and how can you maintain them? In this article, we will unpack the function of thyroid hormones and factors that can influence your thyroid levels. (Source)
What Are Thyroid Hormones?
Hormones are messenger proteins that are produced in one area of the body and transported to another area via blood. The thyroid gland, found at the front of the neck, produces thyroid hormones in two forms: triiodothyronine (T3) and thyroxine (T4). The main difference between the two is that T3 is the active form, while T4 can be converted into T3 outside of the thyroid when needed.
Only about 20% of the thyroid hormones produced by the thyroid gland is T3, while the remaining 80% is T4. This gives the body greater flexibility in controlling optimal thyroid levels in the blood. (Source, Source)
Thyroid hormones serve a number of different purposes throughout the body.
Thyroid Hormones Are Required for Sensory Development
Optimal thyroid levels are critical for the development of the central nervous system (CNS). Fetuses receive maternal T4 hormones through the placenta, which are then converted to the active T3 form in the fetal thyroid gland. Individuals who experience fetal hypothyroidism have been shown to have difficulty achieving neural and sensorimotor milestones. (Source)
Thyroid Hormones Regulate Our Metabolism
Metabolism refers to all the processes our body conducts in order to produce energy from the food we consume. Thyroid hormones play a critical role in regulating these processes.
Hyperthyroidism, an overproduction of thyroid hormone, is linked with increased weight loss and increased resting energy expenditure (the amount of energy that is spent when your body is at rest). Hypothyroidism, or underproduction of thyroid hormone, on the other hand, has the opposite effect of decreasing energy expenditure. (Source)
Thyroid Hormones Influence the Menstrual Cycle
While the exact pathways are still being researched, it is well known that women diagnosed with thyroid disorders also experience disruptions in their menstrual cycles. This may be influenced by the fact that thyroid hormones are known to interact with steroid hormones, such as estrogen and progesterone. One long-term study found that hyperthyroid women with elevated T4 levels also had higher levels of estrogen, compared to euthyroid (normal thyroid levels) women. (Source)
How Are Thyroid Hormones Regulated?
In order to maintain optimal thyroid levels, the thyroid gland is regulated by the pituitary, a small gland located at the base of the brain, which in turn is controlled by the hypothalamus, an area in the center of the brain. The interplay of the 3 is known as the hypothalamic-pituitary-thyroid (HPT) axis. (Source)
The hypothalamus plays a key role in maintaining homeostasis in the body, releasing regulatory hormones to control functions such as heart rate and metabolism. Low thyroid hormone levels in the blood alert the hypothalamus that more thyroid hormones need to be produced.
Once the hypothalamus receives this signal, it releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid stimulating hormone (TSH). Thyroid stimulating hormone prompts the thyroid gland to begin producing more thyroid hormones. (Source)
In addition to regulation by the HPT axis, release of TRH and TSH can also be influenced by factors such as overall nutritional level and the presence or absence of other hormones, such as adrenaline and noradrenaline. (Source)
What Are Optimal Thyroid Levels?
If you suspect you may have a thyroid disorder, the first thing your health care provider will probably do is measure your TSH levels. TSH is the most sensitive biomarker for thyroid disorders, due to its role in the HPT axis.
TSH is released by the pituitary gland when there aren’t enough thyroid hormones circulating in the blood. When test TSH levels are high, this suggests the thyroid gland isn’t producing enough hormones and the pituitary is trying to prompt it to make more. On the other hand, if TSH levels are low, this may mean the thyroid gland is over-producing thyroid hormones and the pituitary is trying to slow it down. (Source)
Other than your TSH levels, a health care provider may also conduct a test to measure your direct thyroid hormone levels. Both T3 and T4 exist in the blood in two forms: bound to proteins or circulating freely. Measuring free T4 in particular can be more accurate in determining thyroid function than measuring bound T4. A combination of these different tests will provide the most accurate results to help diagnose any potential thyroid diseases. (Source)
Optimal Thyroid Levels Vary
What “optimal thyroid levels” are is something of a controversy in the medical community. Some practitioners believe the reference ranges listed with blood test results are adequate for comparison and diagnosis. These are the median of hormone levels found in a sample of healthy individuals. Others find these measurements to be arbitrary and therefore ineffective comparison points when it comes to identifying and diagnosing a thyroid condition.
If you're curious about your own thyroid levels, it's best to consult a functional medicine practitioner or your WellTheory Certified Nutritionist and Registered Dietitian. They will take into consideration your age, lifestyle, and health conditions to assess whether or not your thyroid is in its best shape and, if needed, guide you through the process of achieving more optimal levels. (Source)