On the previous thyroid article, we discussed about the importance of a healthy thyroid, how it works, and what symptoms you might experience if your thyroid is not working properly. Your thyroid is like the engine in your car. It dictates how fast or slow you are moving. In your body, it also determines how many calories you are burning and how fast your body produces energy. The following symptoms were found to be associated with low thyroid function:
Dry skin, fatigue, cold sensitivity, constipation and muscle cramps 
Increase in cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides. 
Subclinical Hypothyroidism was found to be associated with ischemic heart disease in people of 65 years and younger. 
Lower sexual function, Depression. 
Slow bowel movement or constipation. 
Dysfunction of sugar metabolism as was noticed by higher HbA1C. 
Most doctors only measure the thyroid stimulating hormone (TSH), which does not provide us with enough information about your thyroid function. Over the years we have been in practice, we noticed many patients with thyroid symptoms after their doctor told them that their thyroid is fine. Unfortunately, they were misdiagnosed and were still suffering. This article will explain about the important tests to preform to make sure that your thyroid is working properly.
Why is it important to measure T3 and T4 and not just TSH?
The thyroid makes two hormones: T3 and T4. Measuring the hormones that the thyroid produce will show us how much is your thyroid gland is working. Take the hormone T3 for example. A study published at the Department of Medicine of University of California, found that normal levels of TSH may not be correlating with normal levels of T3.  This means that measuring your TSH hormones, without measuring T3, will not give us enough information about how much thyroid hormone T3 you have in your blood. You might have normal levels of THS, but low levels of T3. Remember that T4 is converted to T3, the most active hormone that stimulates your cells and metabolism. If your levels of TSH and T4 are normal, but the T3 is low, all the cells in your body won’t be stimulated enough to work properly.
If your TSH, T3, and T4 levels are normal, could you still have symptoms of low hypothyroidism?
The short answer is, yes. Measuring your TSH, T3, and T4 is a good starting point, however, to get a complete picture of the thyroid-related function, we must also measure reverse T3 (rT3) and thyroid antibodies. Reverse T3 is a biologically inactive form of the active hormone T3. In other words, it does not activate the cells as T3 hormone does. It simply ‘floats’ in your blood without influencing your metabolism and energy production. Another study found that rT3 is not just an inactive metabolite, but a powerful inhibitor of the conversion of T4 to T3. It is so powerful, that it is estimated to be 100 times stronger than propylthiouracil, a medication used to decrease thyroid function. 
Factors that increase conversion of T4 to RT3 are stress, trauma, low-calorie diet, inflammation (cytokines, etc.), toxins, infections, liver/kidney dysfunction, and certain medications.  That is why it is important to measure T4 and T3.
Another factor to consider is thyroid antibodies, Anti-thyroid peroxidase (anti-TPO), and Thyroglobulin Antibodies (TgAb), which impair the proper function of the thyroid. The present of these antibodies indicate that an inflammatory condition in your thyroid gland.  It is important to measure these antibodies, since your immune system might be ‘attacking’ your thyroid and you might not know it.
The last thing to mention about you’re the connection between your thyroid hormones and the rest of your body is T3 receptor issues. In certain chronic conditions, your body might not respond to the hormone T3. In this case, even proper and healthy levels of TSH, T4, and T3 will not be enough, since your cells will not respond to them. From my experience, high levels of T3 or rT3 with symptoms of low thyroid function, might indicate that your cells do not respond to these hormones.
To find out what might be causing your symptoms and is ‘holding your body’ from getting healthier, we must start with a comprehensive testing of your thyroid. This will give us a complete picture of your thyroid function, related hormones, and your body reaction to these hormones. Most doctors won't do this test, so find a functional medicine doctor that will properly evaluate the function of your thyroid and will look for thyroid symptoms. If you are not testing, you're guessing.
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Chopra, I. J. (1977). A Study of Extrathyroidal Conversion of Thyroxine (T4) to 3,3′,5-Triiodothyronine (T3) in Vitro*. Endocrinology, 101(2), 453-463. doi:10.1210/endo-101-2-453