Thyroid function test are a series of blood tests used to measure how well your thyroid gland is working. The thyroid is a small gland located in the lower-front part of your neck. It’s responsible for helping to regulate many of the body’s processes, such as metabolism, energy generation, and mood.
The thyroid is known as your metabolic master because it controls every single cell in the body. Without enough of the crucial thyroid hormone, every system in the body slows down, resulting in fatigue, weight gain, constipation, hair loss, dry skin and more.
The thyroid produces two major hormones: triiodothyronine (T3) and thyroxine (T4). If your thyroid gland doesn’t produce enough of these hormones, you may experience symptoms such as weight gain, lack of energy, and depression.
This condition is called hypothyroidism. If your thyroid gland produces too many hormones, you may experience weight loss, high levels of anxiety, tremors, and a sense of being on a high. This is called hyperthyroidism.
The usual blood tests done for thyroid function are thyroid stimulating hormone (TSH), T4 and sometimes T3. A blood sample is taken from a vein in the arm and sent off to the laboratory for analysis. Usually the ‘free’ or active portion of T4 and T3 is measured (i.e., FT4 and FT3). Laboratories use reference ranges to compare blood test results with results in the normal healthy population.
Typical reference ranges for healthy adults are;
- TSH : 0.4 to 4.0 mU/l (milliunits per litre).
- FT4 : 9.0 to 25.0 pmol/l (picomoles per litre).
- FT3 : 3.5 to 7.8 pmol/l (picomoles per litre).
In pregnancy, the serum TSH reference range is different from the general population and should ideally be based on reference ranges derived from healthy pregnant women in the same population. Where such pregnancy reference ranges are unavailable, a TSH range of 0.4–2.5 mU/l in the first trimester and 0.4–3.0 mU/l in the second and third trimesters can be used. These ranges are only a guide and will vary according to the laboratory.
There are also different reference ranges for testing babies and young children. Both the T4 and TSH tests are routinely performed on newborn babies to identify a low-functioning thyroid gland. If left untreated, this condition, called congenital hypothyroidism, can lead to developmental disabilities.
What the results mean
The T4 test and the TSH test are the two most common thyroid function tests and they’re usually ordered together. T4 test is known as the thyroxine test. Most of the T4 in your body is bound to protein but a small portion of T4 is not and this is called free T4.
Free T4 is the form that is readily available for your body to use. Sometimes a free T4 level is also checked along with the T4 test. A high level of T4 indicates an overactive thyroid (hyperthyroidism). Symptoms include anxiety, unplanned weight loss, tremors, and diarrhea.
TSH test measures the level of thyroid-stimulating hormone in your blood. The TSH has a normal test range between 0.4 and 4.0 milli-international units of hormone per liter of blood (mIU/L). If you show signs of hypothyroidism and have a TSH reading above 2.0 mIU/L, you’re at risk for progressing to hypothyroidism.
Symptoms include weight gain, fatigue, depression, and brittle hair and fingernails. Your doctor will likely want to perform thyroid function tests at least every other year going forward. Your doctor may also decide to begin treating you with medications, such as levothyroxine, to ease your symptoms.
T3 test checks for levels of the hormone triiodothyronine. It’s usually ordered if T4 tests and TSH tests suggest hyperthyroidism. The T3 test may also be ordered if you’re showing signs of an overactive thyroid gland and your T4 and TSH aren’t elevated.
The normal range for the T3 is 100–200 nanograms of hormone per deciliter of blood (ng/dL). Abnormally high levels most commonly indicate a condition called Grave’s disease . This is an autoimmune disorder associated with hyperthyroidism.
A T3 resin uptake, also known as a T3RU, is a blood test that measures the binding capacity of a hormone called thyroxin-binding globulin (TBG) . If your T3 level is elevated, your TBG binding capacity should be low.
Abnormally low levels of TBG often indicate a problem with the kidneys or with the body not getting enough protein. Abnormally high levels of TBG suggest high levels of estrogen in the body. High estrogen levels may be caused by pregnancy, eating estrogen-rich foods, obesity, or hormone replacement therapy.
If your blood work suggests that your thyroid gland is overactive or underactive, your doctor may order a thyroid uptake test or an ultrasound test. These tests will check for structural problems with the thyroid gland, thyroid gland activity, and any tumors that may be causing problems.
Based on these findings, your doctor may want to sample tissue from the thyroid to check for cancer. If the scan is normal, your doctor will likely prescribe medication to regulate your thyroid activity. They will follow up with additional thyroid function tests to make sure the medication is working.
When should I have a thyroid function blood test?
Symptoms of an over- or under-active thyroid.
- Swelling or thickening in the neck.
- An irregular or fast heart rate.
- High cholesterol (which causes atherosclerosis – a build-up of fat in the arteries).
- Osteoporosis (fragile or thinning bones).
- Fertility problems, abnormal menstrual cycles, recurrent miscarriage, low libido.
- Family history of autoimmune disorders, e.g., type 1 diabetes, vitiligo, etc.
- Feeling unwell after having a baby.
- Planning for pregnancy or in early pregnancy (and you have a family history or personal history of thyroid disorders, a past history of postpartum thyroiditis, or type 1 diabetes).
How you prevent thyroid diseases
1. Steer clear of starvation diets;
The Journal of Clinical Endocrinology & Metabolism discovered that fasting resulted in a 53 percent reduction in serum T3 levels (your active thyroid hormone that increases metabolism) and a 58 percent increase in reverse T3 (RT3) levels, which block thyroid hormone.
2. Say no to excessive endurance exercise;
Long distance runs and spinning classes may be doing your thyroid a disservice. Similarly to starving yourself, excessive exercise sends your cortisol levels through the roof inhibiting the conversion of the less active thyroid hormone T4 to the metabolically-potent hormone T3. This also raises levels of RT3, which act as the defensive team blocking your thyroid hormone from getting into your cell.
3. Protect yourself against X-rays;
The thyroid gland is one of the organs most sensitive to the risk of radiation – whether it’s from a dental X-ray, mammogram, MRI or general background radiation. A study from National Cancer Institute compared the number of dental X-rays received by a group of thyroid cancer patients prior to their diagnosis with the number received by a group of similar individuals without thyroid cancer.
Overall, those who had dental X-rays were twice as likely to develop thyroid cancer. The patients who received more than 10 X-rays had more than five times the risk of developing cancer than someone who had not had any dental X-rays.
4. Stop smoking;
It’s well known that smoking is bad for your health, but for those with a predisposition for thyroid disease, it can put you at greater risk. Many components of smoke, ranging from thiocyanate to nicotine can lead to increased excretion of iodine and in turn, affect your thyroid’s performance. Scientists also suggest that smoking may influence thyroid hormone levels by affecting the enzyme which converts the active form of thyroid hormone to an inactive form.
5. Keep an eye on blood-work;
Long before your thyroid can be diagnosed, you may have a warning sign: thyroid antibodies, which are prevalent in Hashimoto’s. If you have a family history of thyroid disease it’s wise to test for these. Their presence predicts a propensity towards hypothyroidism, and should be monitored on a yearly basis.
6. Detox to save your thyroid;
There are thyroid disruptors all around us — in plastic water bottles, pop cans and even lurking in your shampoo bottles. A connection between common chemicals called phthalates and thyroid hormone levels was confirmed by the University of Michigan in a large-scale study at the University of California also linked canned soups to changes in thyroid hormone levels. They discovered that as BPA levels doubled, participants experienced a decrease in T4 levels.