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Dr. Anshul Gupta

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 we maintain them? In this article, we will unpack the function of thyroid hormones and factors that can influence our 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 three 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

Your optimal thyroid levels are set by your hypothalamus, and may change throughout your life. Optimal thyroid levels may vary between individuals, and are referred to as an optimal range of values. From taking the measurements of numerous euthyroid (healthy thyroid level) individuals, researchers have determined a reference range to use in clinical diagnoses. (Source)

The range of optimal thyroid levels may also vary according to the lab running the tests. According to the Cleveland Clinic, for example, optimal ranges are:

  • TSH: 0.40–4.50 mIU/mL 
  • T4: 5.0–11.0 ug/dL
  • FT4 (free T4): 0.9–1.7 ng/dL
  • T3: 100–200 ng/dL
  • FT3 (free T3): 2.3–4.1 pg/mL


How Quickly Can Thyroid Levels Change?

In most mammals, the hypothalamus is the center that controls circadian rhythms — the 24 hour period within which our bodies go through natural fluctuations in physiological processes and activity. Since the hypothalamus also controls our hormones, it makes sense that thyroid hormones will have natural highs and lows throughout the day. (Source)

To get accurate readings of thyroid hormone levels, health care providers typically conduct several blood tests a few months apart, always at the same time of day. It is well documented that TSH rises in the evening, peaks at night, and then decreases to remain low throughout the day, so this is taken into consideration when diagnosing a thyroid disorder. (Source)

Lifestyle Factors that Influence Thyroid Levels

Diet and Thyroid Levels

Our intestines are home to trillions of microorganisms that ensure healthy intestinal function, which is important for disease prevention. Recently, there has been considerable evidence that the state of our gut’s microbiota (all the microorganisms in the gut) can influence the thyroid gland, suggesting that an important thyroid–gut axis may exist.

An impaired microbiota — one where the good bacteria are unbalanced by the bad — has been shown to undermine maturation of immune cells, which could lead to the development of autoimmune thyroid disorders. The microbiota also influences the uptake of minerals essential to thyroid function, such as iron, idoine, and zinc. (Source)

The microbiota is dynamic and its composition can change depending on diet, among other factors. Research suggests a diet high in animal proteins, saturated fats, salt, and sugar can lead to an increase of pathogenic bacteria. On the other hand, a diet rich in plant protein, omega-3, and polyphenols (nutrients from plants) can boost the positive benefits of a healthy gut. (Source

Dietary Recommendations for Optimal Thyroid Levels

According to the most recent dietary guidelines released by the U.S. Department of Agriculture, women between the ages of 31 and 59 should consume 2 to 3 cups of vegetables per day, and men should consume 3 to 4 cups. Similarly, women should typically aim for 1.5 to 2 cups of fruit per day, and men 2 to 2.5 cups. (Source)

Exercise and Thyroid Levels

Exercise requires an expenditure of energy, and thyroid hormones play a critical role in making that energy available. Several studies over the years have been done to determine how exercise affects thyroid hormone levels, but the results have been inconsistent.

In some studies, TSH, T3, and T4 levels were seen to either rise or fall, depending on the study. The length and intensity of exercise and the nutritional status of the study participants seemed to have some bearing on results. In all cases, changes in hormone levels were transient.

The bottom line is that moderate exercise probably doesn’t have any lasting effects on thyroid levels, but is recommended for the many other life-enhancing health benefits it offers. (Source, Source)

Exercise Recommendations for Optimal Thyroid Levels

Adults are recommended to engage in a minimum of 150 minutes of moderately intense exercise per week. While exercise won’t cure thyroid disorders, regular moderate exercise may help manage some of the symptoms. In hypothyroidism, for example, moderate exercise can help maintain a healthy weight, decrease joint inflammation, and provide psychological benefits such as improved mood. If you are currently diagnosed with a thyroid disorder, it is best to consult with your primary healthcare practitioner before beginning a new exercise program. (Source)

Stress and Thyroid Levels

When our bodies are stressed, we enter a state of fight-or-flight, where the majority of our blood flow is redirected to processes that will help us escape or overcome the harmful situation. During times of high stress, physiological resources may be directed away from regulation of thyroid hormone levels and toward more urgent concerns.

In a controlled lab study, rats who had been exposed to a stress-inducing environment had lower levels of TSH, T3, and T4 in the blood. This suggests that exposure to stress may inhibit thyroid hormone levels. Nonetheless, the exact mechanisms and relationship between our body’s stress and thyroid regulatory systems are not fully understood. (Source)

Stress Control Recommendations for Optimal Thyroid Levels

The evidence does not suggest that stress can cause thyroid disorders. However, managing stress can provide huge benefits for our overall health, as well as help alleviate certain thyroid disorder symptoms. Some general coping strategies include making sure you eat a balanced diet, get enough hours of quality sleep, and make time to connect with others and share how you are feeling. (Source)

The Bottom Line on Optimal Thyroid Levels

Thyroid hormones play several important roles in our bodies, from regulating our metabolism to ensuring proper neurodevelopment. This is why maintaining optimal thyroid levels is crucial for good health. Due to individual differences in metabolism and physiology, optimal thyroid levels are expressed as a range of values. 

Some of the best ways for maintaining optimal thyroid levels include consuming a gut-friendly diet, engaging in regular moderately intense exercise, and learning to manage stress.

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