Normal TSH levels are an important consideration for thyroid health, though they give an incomplete picture on their own. When you assess thyroid function, other tests such as T4, T3, reverse T3, and TPO antibodies provide a fuller picture.

Be that as it may, having high or low TSH often functions as an early warning sign of things to come. Normal TSH levels are generally considered to be 0.5-5.0mIU/L.

Interestingly, there’s been a lot of banter between different endocrine organizations to narrow the healthy range of TSH. Most recommendations typically recommend lowering the upper limit to 2.5mIU/L. This puts the healthy range between 0.5-2.5mIU/L.

So, that do you do if your TSH levels are out of whack? The simple and most often applied answer is using a pharmaceutical version of the hormone such as Armour Thyroid or Synthroid.

There is something fundamentally missing from the thyroid discussion with strong relevance. What role does our behavior?

As with most other hormones in the body, circadian rhythms regulate TSH levels. In today’s blog we cover how our behavior affects thyroid function, with a focus on TSH.

What thyroid hormone does

Thyroid hormone is incredibly important for our health. An essential regulator of metabolism, thyroid hormone stimulates cells to make more ATP.

The thyroid gland is a small, butterfly-shaped gland located in the front of the neck, just below the Adam’s apple. Upon stimulation, it releases the thyroid hormones T4, T3, and tiny amounts of reverse T3(rT3).

Both T4 and T3 are considered the active thyroid hormones, with T3 being the most active. While the thyroid gland produces much more T4 than T3(90% vs 10%), deiodinases in target tissues convert T4 into T3 and rT3.

Greater T3 in tissues increases metabolic rate. But reverse T3 binds to the same receptors as T3 and has no biological response. In this way, rT3 blocks the function of T3.

Regulation of thyroid function for normal TSH levels

Regulation of thyroid function is a pretty simple concept that begins in the hypothalamus. When the body requires more thyroid hormone, the hypothalamus secretes the hormone thyrotropin releasing hormone(TRH).

TRH travels to the pituitary gland and signals it to secrete thyroid stimulating hormone. TSH then makes its way to the thyroid gland and stimulates the secretion of the thyroid hormones, primarily T4 and T3.

These thyroid hormones then circulate throughout the body, entering cells and stimulating metabolism. Under healthy conditions, deiodinases within cells of a tissue convert T4 to T3, with very little conversion to rT3. T3 then binds to thyroid hormone receptors and stimulates metabolism.

As production of T4 and T3 meet our cellular needs, the excess T4 and T3 in the blood circulate back to the hypothalamus and pituitary to inhibit secretion of TRH and TSH, respectively. Additionally, TSH can also inhibit TRH secretion.

What normal TSH levels tell us

Having TSH levels within normal limits tells us that the above process is in working order. Being outside of this range tells us that the system that regulates thyroid hormone isn’t working properly.

Elevated TSH levels generally tell us that the thyroid gland is under-producing T4 and T3, also known as hypothyroidism. Conversely, low TSH levels generally mean that the thyroid gland is overproducing thyroid hormone, also known as hyperthyroidism.

Now, we can get to poor thyroid function in a few ways. An autoimmune attack can cause either hyperthyroidism or hypothyroidism.

In Graves’ disease, antibodies bind to the TSH receptor in the thyroid, causing OVER-stimulation of thyroid hormone production. In Hashimoto’s thyroiditis, antibodies attack different aspects of the thyroid gland, causing impaired secretion of thyroid hormones.

The most common cause of hypothyroidism in the world is inadequate iodine intake. Iodine is a central component of T4 and T3, as well as DIT and MIT, which are precursors to T4 and T3. But in areas with adequate iodine intake, Hashimoto’s thyroiditis is the most common cause.

Graves’ disease is the most common cause of hyperthyroidism in the US. Other factors that can cause hyperthyroidism include excess iodine intake or thyroid nodules. Furthermore, a tumor of the pituitary can cause hyperthyroidism, though it’s fairly uncommon.

The role of lifestyle in thyroid function

A healthy lifestyle plays an important role in proper thyroid function. Circadian rhythms and sleep both have important regulatory effects on thyroid function.

Circadian rhythms and normal TSH levels

TRH and TSH

Like most other hormones central to human health, the thyroid hormones are under circadian control, at least to some extent. Data consistently shows a circadian rhythm of TSH secretion, with strong evidence that free T3 also follows a circadian rhythm.

There is a direct link between the master circadian clock in the brain and TRH-secreting neurons hypothhalamus. This implies an important role for light in regulating TRH secretion, which may drive rhythms in TSH secretion.

T4 and T3

Circadian rhythms not only affect the secretion of thyroid hormones, but also the sensitivity of tissues to their effects. Circadian rhythms regulate the expression of TSH receptors in thyroid hormone-producing cells.

On top of this, the expression of thyroid hormone receptors follows a circadian rhythm in target tissues. Finally, deiodinase activity in target tissues also follow a circadian rhythm.

This means that circadian rhythms regulate:

• Sensitivity of the pituitary to TRH, and thus, TSH secretion
• Sensitivity of the thyroid gland to TSH, and thus T4 and T3 secretion
• The conversion of inactive T4 to active T3 in target tissues
• The feedback system that halts TRH and TSH production

This is an example of the importance of circadian synchronization between different tissues for optimal function. Even if you produce adequate TSH, low TSH receptor expression in the thyroid will lead to impaired thyroid hormone production.

As a result, TSH levels increase due to inadequate T4 and T3 feedback on the hypothalamus and pituitary. Additionally, even if you produce adequate T4, low deiodinase expression in tissues will decrease conversion of T4 to the more active T3.

Thus, circadian rhythms regulate multiple steps in the chain from thyroid hormone production to the effects of thyroid hormones on target tissues.

Sleep and normal TSH levels

There is a bi-directional relationship between sleep and thyroid hormone secretion.

As you can see, TSH has a fairly strong circadian rhythm, with the thyroid hormones following a similar pattern. In a healthy person who sleeps well, TSH increases to its peak while we sleep, with T3 following suit 90 minutes later.

Poor sleep blunts the rise in TSH, and TSH has an inverse relationship to slow wave sleep. In other words, when TSH is high, slow wave sleep decreases. Correcting TSH levels with synthetic T4 improves sleep, indicating that sleep depends on proper thyroid function.

The overall impact of poor sleep on the thyroid depends on the length of sleep deprivation. In the short term, impaired sleep increases thyroid hormone production, likely via increased sympathetic nervous system activity. Chronic sleep deprivation decreases thyroid hormone production.

Thus, poor sleep has an independent effect on thyroid function that depends on the length of sleep deprivation.

Conclusion

On paper, it’s fairly logical to see why circadian rhythms play an important role in thyroid function. Since circadian rhythms optimize our physiology to the environment, they play an important role in our metabolism.

On top of that, sleep and thyroid function maintain a bi-directional relationship with one another. Poor thyroid function impairs sleep, and poor sleep impairs thyroid function.

The current approach to addressing thyroid problems is to throw synthetic T4, T3, or combinations of the 2 at the problem. This improves symptoms, but does it get at the root cause of the problem?

Furthermore, since circadian rhythms regulate receptor expression, increased hormone levels may not lead to increased hormone activity. You can lead the horse to water, but you can’t make him drink.

Normal TSH levels are important to overall health, but only if it leads to improved thyroid function AND tissue sensitivity to thyroid hormones. The evidence presented above indicates that behaviors centered around circadian rhythms and sleep are critical to both processes.

For more info on how to get started on improving your circadian rhythms and sleep, check out the homepage at https://hackyourgut.com/