What does a blood test reveal about your health and thyroid?

Photo: George Hodan. Design: BOOST Thyriod.

Photo: George Hodan. Design: BOOST Thyriod.

Blood tests are the easiest and the fastest way for the doctor to assess your hormonal balance and health.

Thyroid function tests are one of the most common lab tests—most of the time they will clearly show if your thyroid is underactive. However, sometimes the test results can be confusing. How can the same “normal” range fit almost any person of any age, gender, and physical state?

What is considered “normal”?

Normal ranges were created a long time ago in order to help understand and classify health and illnesses. It was of a tremendous help at that point, when some cut-offs needed to be made and diseases were studied mostly in their most severe (deadly) forms.

Today is different, we’re heading towards personalization in every aspect of life—and health is a major part of component. We need to step into the gray zone.

Is a “normal” value the same for everyone?


What’s normal for you might not be normal for the person next to you. What’s normal for you today will likely not be normal for you in the future. Your weight, stress levels, or dietary habits might change—you might get pregnant, relocate, and more. That and your genes will determine your normal values for any given moment in your life.

What does it mean to have a blood value out of range?

Medically accepted averages vary from country to country. If you’ve moved around a lot it can be challenging to determine which country’s averages are applicable to you—do the averages from your country of origin apply to you or has your averages changed to fit your new environment’s averages? Keep this in your mind if you are relocating.

One measurement at one point in time will not tell you much. You should re-test one or two times to make sure your blood values are true. This way you and your healthcare practitioner can actually get a good picture of your health.

TSH test is a first step towards the diagnosis of hypothyroidism. However it can be very misleading, it is shown to increase with aging, and is not an immediate sign of a disease (1).

The same is known for babies, when born and growing up they have high TSH values, which drop as they reach puberty (1).

Are you safe if you have a value inside the average range?

In general, yes, but not always.

You should observe if the value is shifting from one end of the “normal” range to the other. Even shifts as this might be normal, caused by stress, seasonal changes, other illness, changes in the lifestyle, change in your current medications. Some patients at the first stage of Hashimoto’s have TSH levels which are by some lab’s considered normal 2.5–4.5 mIU/L (1).

The best advice? Know yourself and know your body.

What might tamper with test results?

  • The time of the day, the time of the year
    TSH is not produced by the thyroid at the equal rate during the entire day. It follows a circadian rhythm and is mostly produced during our sleep. Presently a normal reference range of serum TSH for adults is 0.4–4.5mIU/L. In some countries that upper limit has been disputed, and reduced to 2.5mIU/L (2).

  • Type of the test used

  • Eating too little or overeating

    If you are under nourished you might experience the decrease in fT3 and total T3. Overeating will have an opposite effect and increase fT3 and T3.

  • Stress
    Stress is a big factor in thyroid health, and it decreases T3.

  • Certain medications

    Medications can increase fT3 and fT4, but not really in a good way. These medications block the activity of T3 and T4, so it does not matter if more of T3 and T4 can be found in the blood. They are unable to fulfil their role.

  • Exercise
    Basically, if you have just run a marathon, or did any type of long, excessive or high-intensity exercise, wait for a couple of days before going to be tested. And, in general it is good not to overexercise yourself if you have a thyroid condition.

  • Sometimes labs make mistakes too
    It is not often, but people work in labs, and us people make mistakes. Sometimes the chemicals used for testing might be old or spoiled, I worked in the lab, it can happen. Luckily, usually people in the lab realize that very fast and can re-test with the same blood sample and freshly prepared chemicals.

What do thyroid antibodies tell us?

Three antibodies related to autoimmune thyroid disease can tell what type of autoimmune disease one has, and how severe or advanced it might be.

Anti-thyroglobulin (TgAb), anti-thyroperoxidase (TPOAb) and anti-TSH receptor (TRAb)

High levels of TgAb are present in 8 in 10 patients with Hashimoto’s and in 4 in 10 of people being diagnosed with Grave’s disease.

High levels of TPOAb are found in 9 in 10 patients with Hashimoto’s and 8 in 10 of patients with Grave’s disease.

High levels of either TgAb or TPOAb can be found in 1 in 10 of people without an autoimmune thyroid disease, as well as in 2 in 10 of people that are deemed healthy.

High levels of TRAb are found in 9 in 10 patients with Grave’s disease and in less than 2 in 10 patients with Hashimoto’s.

Equally, it seems that in a small group of people with autoimmune diseases no antibodies are detected.

TPOAb testing in early pregnancy can predict if a person is under a risk of developing postpartum thyroiditis. Postpartum thyroiditis occurs in about 1 in 10 people after giving birth. About 5 in 10 of pregnant people with high TPOAb will develop postpartum thyroiditis (10).

TgAb or TPOAb were found in 1 in 10 healthy people, more in females than males, and more in elderly (3, 4).

How do TSH values change with the age?

  • Newborns and young children
    TSH and T4 levels are high in newborn babies, as the baby grows the levels start dropping. T3 increases during the first year of life, but then it starts dropping (8).

  • Adolescents
    TSH and T4 are decreasing and will reach expected adult levels at around 18 years old (8).

  • Adults
    Several studies done on more than 10,000 non-diagnosed people show that for a healthy population TSH value is close to 1.5 mIU/L, and is higher in females than in males. 1 in 10 adults under 30 years has TSH higher than 2.5mIU/liter (5).

  • Pregnancy
    TSH levels rapidly grow during pregnancy. They increase from the average 1.20 mIU/L in the first trimester to 2.12 mIU/L in the second and 3.30 mIU/L in the third trimester. The range of T3 is 1.7–4.3 nmol/L in second trimester and 0.4–3.9 nmol/L in third trimester. T4 range is 92.2–252.8 nmol/L in second trimester and 108.2–219.0 nmol/L in third trimester (7).

  • 70+
    4 in 10 people over 80 years have TSH higher than 2.5mIU/liter, and 1 in 10 has TSH higher than 4.5mIU/liter (5).

Some research suggests that the upper normal TSH reference range in people over 70 years old should be extended to 6.0 mIU/L (6). A study on more than 15,000 people in Scotland has confirmed that the TSH values should be reconsidered for different age intervals, and especially in people older than 70 years should not be considered out of normal ranges [10].

Is TSH over 2.5 indicative of a disease?


1 in 10 of people with TSH values above 3.0 mIU/L has Hashimoto’s (9).

TSH itself, as a marker of disease is not enough. If you have any symptoms connected to thyroid, talk to your health provider and get the full thyroid blood work done. This is the best first step towards understanding the health of your thyroid.

Mildly hypothyroid, subclinical hypothyroid, and hypothyroid—what’s the difference?

There is a big difference between the subclinical, mildly hypothyroid and (overtly) hypothyroid. It will determine the treatment options you will be offered.

In most cases doctors will wait with prescribing thyroid therapy until the patient is overtly hypothyroid. Symptoms will in most of the cases become stronger as hypothyroidism intensifies.

However, even the mild hypothyroidism is significant, if you see the trend of TSH levels increasing and you progressively feel the increasing number of symptoms. Talk to your doctor to determine what is the best therapy approach for you, and what is your, personal “normal” where you feel good.

Log your blood values and observe trends in the free to use BOOST Thyroid app.

Article was originally published on April 5, 2017.


1. Carvalho GA, et al. The clinical use of thyroid function tests, 2013

2. Wartofsky L, et al. The evidence for a narrower thyrotropin reference range is compelling, 2005

3. Hollowell JG, et al. Serum TSH, T(4), and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III), 2002

4. Dunn JT. Guarding our nation’s thyroid health, 2002

5. Surks MI, et al. Age-specific distribution of serum thyrotropin and antithyroid antibodies in the US population: implications for the prevalence of subclinical hypothyroidism, 2007

6. Fontes R, et al. Reference interval of thyroid stimulating hormone and free thyroxine in a reference population over 60 years old and in very old subjects (over 80 years): comparison to young subjects, 2013

7. Kumar A, et al. Thyroid function tests in pregnancy, 2013

8. Hubner U, et al. Continuous age-dependent reference ranges for thyroid hormones in neonates, infants, children and adolescents established using the ADVIA Centaur Analyzer, 2002

9. Lewandowsky K. Reference ranges for TSH and thyroid hormones, 2015

10. Vadiveloo T, et al. Age- and gender-specific TSH reference intervals in people with no obvious thyroid disease in Tayside, Scotland: the Thyroid Epidemiology, Audit, and Research Study (TEARS), 2013

11. http://apps.who.int/iris/bitstream/10665/66342/1/WHO_DIL_00.4_eng.pdf