In my last blog, I wrote about some thyroid disorders and their symptoms.

So if you think you have a thyroid disorder, what is your next step?

Thyroid testing.

There are many tests out there, but not all of them actually tell you anything much. These are the tests most likely to give you good information about your thyroid.

The main thyroid screening test is the thyroid stimulating hormone test (TSH). TSH is the messenger hormone that the pituitary sends down to the thyroid to tell it to make the inactive thyroid hormone, known as T4. When enough T4 is made, this informs the pituitary, via a negative feedback loop, that the pituitary does not have to keep sending down a lot of TSH. So the higher the T4, the lower the TSH, indicating high(er) thyroid function and vice versa--the higher the TSH, the lower the T4, indicating lower thyroid function.

T4 can be converted into the active thyroid hormone, T3 in the thyroid, but most of the conversion takes place elsewhere in the body, especially in the liver.

Since T3 is the active hormone, it's the one that's creating the effects of healthy thyroid function. But since most of it is made outside the thyroid, the TSH test is more of a reliable marker for T4, the inactive thyroid hormone. This is important because some people have difficulty converting T4 to T3. Therefore, a person can have a normal TSH indicating a normal T4, but whether their T3 is normal isn't really known. This is the downside of using only the TSH as a screening and monitoring test. Even adding in a T4 as we've seen does not give us the whole picture.

Also, hormones are typically bound to other molecules until they are needed. When we look at T4 and T3, we can look at the Total T4 and Total T3, which includes the bound hormones, or the Free T4 and Free T3 which let us know how much hormone is available for use. While total T4 and especially Total T3 can be helpful, the Free T4 and Free T3 usually give us more information.

So for basic thyroid screening and monitoring of thyroid medications, I would recommend TSH, FT4 and FT3.

As mentioned in my last blog, auto-immune thyroiditis (Hashimoto's disease)is the most common cause of hypothyroidism in the US currently. The tests for this are the thyroglobulin antibody and the thyroid peroxidase antibody tests (also known as TG antibodies and TPO antibodies). If a person has "normal" TSH, FT4 and FT3 levels, but still has hypothyroid symptoms, these should be tested. Some physicians are ordering these tests now with every thyroid screen because of the high incidence of auto-immune thyroiditis.

The last test I'll discuss is the reverse T3 test. Reverse T3 is like "anti" T3. It can look like FT3 on paper, but in your body it does NOT work as FT3. Why would the body make RT3? Well because the thyroid regulates the body's metabolic rate, it needs to adjust metabolism if circumstances in the body change. For example if a woman becomes pregnant, then the thyroid has to adjust metabolism to account for the woman and the growing baby.  Or if a person greatly decreases their food intake, especially over a long time, as in a famine, the thyroid will decrease metabolism. Making Reverse T3 is a way for the thyroid to decrease metabolism. Elevated reverse T3 can also be due to low levels of certain vitamins and minerals, hormonal imbalances, major stress events, or in Grave's Disease. ReverseT3 is best evaluated in conjunction with FT3. 

In the next blog, I will write about what to do if you are on thyroid medication, but still have hypothyroid symptoms.