Thyroid hormone is a metabolic hormone secreted by the thyroid gland. It is also one of the most poorly understood by both patients and, unfortunately, clinicians.
Thyroid affects every cell in your body. It regulates your metabolism, temperature, brain and cardiovascular function, to name a few.
People with low thyroid tend to feel tired, lack energy, difficult to focus, are forgetful, sometimes confused or even depressed. They may have some constipation and joint pains.
They tend to be more susceptible to colds and other illnesses. They are not happy. The lower the thyroid, the higher the cholesterol which causes an increase in cardiovascular problems.
The brain senses the level of thyroid hormone and, if it is low, the brain sends a signal to your pituitary gland to increase production of Thyroid Stimulating Hormone (TSH). This enters the blood and eventually travels to the thyroid gland to increase the production of thyroid hormone. If the thyroid hormone level is high, less TSH is made.
There are 2 main thyroid hormones made, T4 and T3. The active form of the thyroid is T3. The T4 is considered a pro-hormone (not much activity) and is the source of most of the T3 in the body.
Many of our cells will pick up T4 and convert it to T3, when needed. Special enzymes called deiodinases do the conversion.
The thyroid gland makes different thyroid hormones: T4 (80%) and T3 (20%). The body, being smart, is efficient and frugal. When we sleep, the body shuts a lot of the systems down for rest and repair. This is one of the reasons for sleeping. During sleep, we do not need much thyroid, so the 20% that the thyroid makes is usually more than enough for proper function. Remember, T4 lasts weeks, while the T3 only lasts ~8 hours, so whatever T3 is made during sleep is wasted, and the body does not want to waste anything.
For underactive thyroid, clinicians would measure the TSH to screen and sometimes the T4. This was done for years. The assumption was (and still is by many today) that, that if the body needed T3, it would make it. This was the case for millions of years until about 15-20 years ago with our exposure to many chemicals in our foods and plastics. They are called endocrine disruptors. A recent study revealed that 70% of the drinking water on the East Coast is contaminated (even well water) with “forever chemicals,” which causes a lowering of T3. If you run routine TSH and T4, the problem will not be detected. Another test called the Free T3 will determine if you are converting enough T4 to the active and needed Free T3.
Summary:
Thyroid gland makes thyroid hormone: T4 (80%) and T3 (20%)
When more thyroid hormone is needed, the cells will convert the T4 to T3, if the system is working well.
The activity of deiodinase is reduced by several chemicals that we are all exposed to, thereby reducing our production of Free T3
We encourage patients to make sure they have the Free T3 measured. For most adults, it should be around 4.0
A word about cholesterol. About 95 % of the cholesterol in our body is made by the liver. Insulin increases the production of an enzyme called HMGCoA-R (3-Hydroxy-3-Methy-glutaryl- CoA Reductase, for those interested) which is the critical “rate limiting” step in the production of cholesterol. The more HMGCoA-R produced, the more the cholesterol. This is where most Statin drugs work. Statins reduce the production of HMGCoA-R, and insulin increases it.
Your liver makes cholesterol and it also removes it. The Free T3 we mentioned above is responsible for the production of the receptors on the liver cell that grabs LDL cholesterol and pulls it out of circulation. Then the liver removes it from the body, and the LDL cholesterol level goes down.
So, if your insulin is elevated (insulin resistance, adult onset diabetes) and/or the Free T3 level is low, the LDL cholesterol will go up.
This is why we want low insulin levels and we check the Free T3 (not the TSH or T4).