More than nine million North Americans suffer from low thyroid function, but many people are not diagnosed or treated because they have never been tested. One million people take thyroid replacement pills, but some are not being treated properly because they are taking only one thyroid hormone when they need two to really feel better.

Common symptoms of low thyroid function include:
• Depression
• Tiredness
• Weight gain
• Muscle weakness
• Feeling cold
• Dry skin and hair
• Hair loss
• Poor appetite
• Memory loss
• Numbness in hands and feet or feelings of pins and needles
• Constipation
• Irregular periods
• Irritability
• Enlarged thyroid gland

If you have any of these symptoms, check with your doctor who will order blood tests for thyroid function, T3, T4, and TSH. The brain produces a hormone called TSH that causes the thyroid gland to make the thyroid hormone T4. As soon as blood levels of the thyroid hormones reach normal levels, the brain stops making TSH until T4 levels drop.

If your tests are abnormal, your doctor will start you off on a low dose of thyroid hormone and get another TSH test in a month. The first order of treatment is to get the TSH to a normal level because the total dose of thyroid should be determined by the TSH blood test, and not by blood levels of thyroid hormones T3 and T4. The doctor will keep adjusting the dose until the TSH is in the normal range between 0.5 and 3.

Note that high TSH means you are getting too little thyroid. Low TSH means that you are getting too much. If the TSH is below 0.5, your dose will be lowered and if it is above 3, your dose will be raised. The TSH test is very important because dropping the TSH too low, below 0.5, causes horrible side effects including emotional problems and severe osteoporosis. If the TSH is above 3, the patient is not getting enough thyroid replacement.

What If Your TSH Goes to Normal but You Still Feel Bad?
Most doctors prescribe just one thyroid hormone, T4, to treat patients with low thyroid function and the patients receive good control of their thyroid status. Therefore, if the TSH is normal and the patient feels normal, most doctors continue to keep their patients at that dose of thyroid replacement. However, some people being treated for low thyroid function continue to suffer the symptoms of hypothyroidism, even though their TSH blood test is normal. The thyroid gland makes T4 (levothyroxine ) that circulates in the bloodstream to the cells in their bodies. However, T4 does not affect the cells of your body. First your cells must convert the inactive T4 to a second thyroid hormone called T3, which is the active form of thyroid hormone.

More than 15 percent of hypothyroid patients taking T4 still feel rotten, even though their doctors tell them that their TSH test is now normal. These people often have cells that cannot convert T4 to T3 effectively and therefore do not feel better when they take just T4 pills (The Lancet, October 2, 2015:756-57). These people usually have low blood levels of T3. Their cells cannot efficiently convert T4 to the active form, T3, that stimulates cells. Studies in humans show that 12 percent to 36 percent of humans have a genetically-determined decreased ability to convert T4 to T3, and their thyroid symptoms and brain function can improve when they receive both T4 and T3 pills (Journal of Clinical Endocrinology and Metabolism, May 2009).

If you are one of the people who has been given T4 and you now have a normal TSH blood test but you do not feel better, your doctor should check your T3 level. You may have a genetic inability to convert the inactive T4 into the active T3 form. If your T3 is low and your TSH is normal, you should be given both thyroid hormones, T3 (liothyronine) and T4 (levothyroxine). The key is to get the TSH and T3 normal. A person who has normal TSH and T3 and a low T4 could still be at the correct dose.

Depression and Thyroid Hormones
Depression is common among people who have too much or too little thyroid hormone. Over the years, studies have shown that many people who are depressed also have low thyroid function and that giving these people thyroid hormones improves their depression. However, using thyroid hormones to treat depression is very controversial.

Many different studies show that giving the thyroid hormone, T3, can help treat depression. Drugs such as Paxil, Prozac and Zoloft that treat depression are supposed to raise brain levels of neurotransmitters. Brain levels of serotonin can be raised by taking T3. T3 may be given by itself or together with antidepressants. When T4 (Levothyroid) is given alone, some people become even more depressed. Some improve when they take both T3 and T4.

In a recent study, researchers removed thyroid glands from rats and gave them T4 thyroid hormone (Journal of Clinical Investigation, Jan 2, 2015 and Journal of Clinical Endocrinology & Metabolism, Jan 8, 2015). They found that many of the rats still had very low thyroid function, and also had impaired brain function with poor memory and learning abilities. The rats without thyroids were then given T3 and had their mental functions returned to normal.

My Recommendations
If you feel depressed, weak or tired, ask your doctor to draw the thyroid tests T3, T4, and TSH. If your TSH is above 3 or your T3 is below 100 ng/dL, you could be suffering from low thyroid function and will probably benefit from replacing your missing thyroid hormones.

Checked 7/23/18