Understanding Thyroid Testing: Why Hypothyroidism is Often Missed Through Conventional Testing
BY: DR. JULIE MILLER
The thyroid is a butterfly-shaped gland that sits low on the front of the neck and is the body’s major metabolic regulator. When your thyroid is underactive, called hypothyroidism, everything slows down leading to weight gain, fatigue, poor concentration, depression, constipation, infertility, low libido, PMS, muscle and joint pain, dry skin, excessive hair loss, and low body temperature. Conversely, when it is overactive, called hyperthyroidism, your metabolism speeds up, leading to weight loss, anxiety, insomnia, fast heartbeat, and loose stool. According to the Thyroid Foundation of Canada, approximately 1 in 10 Canadians have a thyroid condition and as many as 50% of those cases remain undiagnosed. 
It is very common in my practice for my patients to present with symptoms of a low functioning thyroid but have been told by their MD that their thyroid looks “normal” on their blood work. This is because the only hormone that is generally tested, called Thyroid Stimulating Hormone (TSH), doesn’t give a complete picture of thyroid function and isn’t as sensitive a marker of peripheral thyroid hormone as once thought. The hypothalamus, which lies deep in the brain, is responsible for managing numerous important functions including hunger, sleep, body temperature and hormone balance, and is constantly monitoring the levels of thyroid hormone in the bloodstream. When low levels of thyroid hormone are detected it releases Thyroid Releasing Hormone (TRH), which then tells the pituitary to release Thyroid Stimulating Hormones (TSH). TSH then travels in the bloodstream to the thyroid where it stimulates production and release of thyroid hormone. The pituitary itself is also sensitive to levels of thyroid hormone and will adjust output of TSH accordingly and as blood concentrations of thyroid hormone increase, the release of both TRH and TSH are inhibited. Consequently, TSH levels are a representation of this negative feedback loop between the thyroid, hypothalamus, and pituitary and high levels are indicative of low production of thyroid hormones (hypothyroidism) and low levels reflect higher production of thyroid hormones (hyperthyroidism). However, a “normal” TSH that falls within the reference range (0.2-4.0 mIU/L) does not rule out thyroid dysfunction.
About 85% of the thyroid hormone produced by the thyroid is called thyroxine (T4) and is the storage form of thyroid hormone. Although a small amount of the active thyroid hormone triiodothyronine (T3) is produced in the thyroid itself, most T3 is made from T4 in the peripheral tissues and organs such as the liver, kidneys, and digestive tract. T3 is approximately 5 times more potent than T4 and it acts like a metabolic gas pedal. However, the body can also convert T4 to reverse T3, an inactive form that can block active T3 from binding to cellular receptors. Conditions that increase reverse T3 (the “breaks”) include: inflammation, chronic infections, chronic illness, emotional and physiological stressors, calorie restricted dieting, and certain medications. Therefore, to gain a complete understanding of thyroid function it is important to measure the actual production of thyroid hormones: free T4, free T3, and reverse T3. One pattern that I see frequently in my practice that is missed by conventional TSH-only testing is a normal TSH and free T4 but low free T3. This is due to poor conversion of T4 to T3 which can result from nutrient deficiencies, such as selenium and zinc, which are needed for the deiodinase enzymes that run the conversion or the above-mentioned factors that increase the production of reverse T3.
The vast majority of thyroid conditions are due to an autoimmune process where the immune system attacks the thyroid gland. Hashimoto’s thyroiditis is the hypothyroid manifestation and Graves’ disease is the hyperthyroid form. In order to determine the presence of an autoimmune component we can test for elevations in the two main types of thyroid antibodies: thyroid peroxidase antibodies and thyroglobulin antibodies. Often the antibodies are not tested in a conventional setting because their presence does not change how hypothyroidism is treated using thyroid replacement. In the case hyperthyroidism, Thyroid Stimulating Immunoglobulin (TSI) is the most specific antibody test to diagnose Graves’. In my opinion, it is incredibly important to know if there is an autoimmune process occurring because having one autoimmune condition increases the risk of developing another and we can address the underlying causes that contributed to the development of autoimmunity in the first place.
Most conventional medical doctors only check TSH levels and very occasionally the storage form of thyroid hormone, free T4. However, there are many factors involved in optimal thyroid function and to get a complete picture of a patient’s thyroid health and medication needs, I recommend ordering the following thyroid tests:
- TSH: ideally less than 2.0 mIU/L (but a TSH < 2.0 does not rule out thyroid dysfunction)
- Free T4: optimal levels are usually in the upper half of the reference range (17.0-25.0 pmol/L)
- Free T3: optimal levels are usually in the upper half of the reference range (5.0-6.5pmol/L)
- Reverse T3: ideally less than 15, greater than 15 indicates thyroid resistance; free T3 to reverse T3 ratio should be greater than 0.20
- Thyroid Peroxidase Antibodies (TPOAb) and Thyroglobulin Antibodies (TgAb): optimal levels are negative or as low as possible
The thyroid is greatly affected by other nutrients and hormones and additional labs to consider include vitamin D, ferritin (iron stores), estrogen, and cortisol. If you suspect your thyroid isn’t functioning properly or are currently on thyroid medication but still feel less than optimal, a full thyroid panel can help to reveal the underlying imbalance and guide the ideal treatment approach.