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Normal TSH Levels: What’s Normal & Why?

Distribution of health TSH levels

Normal TSH Levels: What’s Normal & Why?

Anil Kumar, PhD  | Last updated: Nov 2, 2018

Introduction

Thyroid hormones are key to everyday health. Abnormal thyroid levels can have serious consequences on metabolism, growth, and development.

Overt hypothyroidism and hyperthyroidism are common thyroid disorders that occur due to abnormal levels of TSH (also called thyroid-stimulating hormone or thyrotropin).

The risk of these disorders during lifetime are quite high, especially in senior population.

One study for Denmark shows the overall risk of overt hypothyroidism or hyperthyroidism diagnosis–over the lifetime–was 9.2% (Carle’ 2006).

The risk among women was much higher at 14.6% in comparison to the risk in men at 3.7%. The risk for US, Europe, and countries with no iodine deficiency in the population should be similar.

 

TSH levels and risk of thyroid disorder


 

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What are normal TSH levels?

The distribution of TSH levels in healthy population is an almost bell curve (but non-Gaussian distribution) and skewed towards the higher TSH levels. The data for 1,671 healthy Danish women are plotted in the figure below.

Healthy limits are between 2.5th and 97.5th percentile (±3 std dev). These values fall approximately between 0.1 mIU/L and 4.5 mIU/L. However, variations occur from known modulators that include age, gender, race, and iodine consumption.

 

Distribution of health TSH levels


What are the reasons for high TSH levels?

Several factors affect TSH levels. Problems with the hypothalamus, pituitary, or thyroid gland can cause abnormal TSH levels. Here are few other common factors that can also affect TSH levels:

  • Sleep
  • Pregnancy
  • Illness
  • Medication
  • Age

How sleep affects TSH levels?

  • TSH levels (T3 and even T4 to a less extent) vary in two different cyclical patterns as seen in the plot below (from Weeke and Gundersen 1978)
  • In the relatively stable diurnal rhythm–defined as a period of 24 hours–TSH increases by almost 150% in the night during sleep, compared to day time; the levels slowly decrease after waking up and results can vary significantly depending on the time of sample collection
  • Additionally, the pituitary gland releases TSH in short pulses and levels rise by about 10-15% every 30 minutes; the peaks of these pulses are higher during sleep in comparison to day time
  • Insufficient sleep may disrupt this natural rhythm resulting in above normal TSH levels in the morning
  • Therefore, night shift workers and those with abnormal circadian rhythm may observe unexpected TSH results from their tests

 

Effect of sleep on TSH levels


How pregnancy affects TSH levels?

  • The pregnancy hormone hCG (human chorio-gonadotropin) weakly stimulates the thyroid (as hCG is structurally similar to TSH)
  • As hCG levels rise, the TSH levels drop especially in very early stage of pregnancy
  • Therefore, trimester specific ranges of TSH (and T3, T4) should be used by the lab to test pregnant women (Laurberg 2011)
  • Adequate TSH is important for fetal development and normal pregnancy

 

Thyroid levels during pregnancy chart


How illness and medication affect TSH levels?

  • Illness may affect functions of hypothalamus, pituitary and thyroid glands which may result in inadequate TSH in blood, generally on the low side
  • Medication to illness might also affect TSH
  • During recovery from illness, levels might temporarily spike as the thyroid stabilizes
  • Obesity and overfeeding increases pituitary and thyroid function resulting in higher TSH (Laurberg 2011)
  • Ask the lab for healthy range for your specific test; that’s the range you should monitor since values vary as part of the normal sample processing

How age affects TSH levels

  • TSH rises with age; there is an almost linear correlation between age and TSH (Hollowell 2002)
  • Although gender, race, and geography affect TSH levels, the risk of thyroid problems among 80+ year old is almost 5x in comparison to those below 50
  • In almost every age group, women tend to have higher TSH levels than men
  • The risk of abnormal TSH levels is highest in older women and lowest among men

 

TSH levels by age


What happens if TSH level is high?

  • High TSH is likely sign of an under-active thyroid gland; it is forcing the pituitary gland to release more TSH, a condition known as hypothyroidism
  • TSH values over 10 mIU/L is overt hypothyroidism; studies show it results in higher levels of cholesterol, both total and low-density cholesterol (Mason 1930)
  • Mildly elevated TSH levels (above 4.5 mIU/L) are a known risk factor for future overt hypothyroidism
  • There is a general belief that the slow increase in TSH with age is not harmful and might help in longevity despite some cardiovascular risk (Atzmon 2009, Rozing 2010, Gussekloo 2004)

What happens if TSH level is low?

  • Low TSH is an indication of hyperthyroidism; it is likely sign of an overactive thyroid gland that is producing high levels of thyroid hormones, T4 and T3 (Pantalone and Nasr 2010)
  • Abnormally low (<0.1 mIU/L) TSH levels are almost always an indication of overt hyperthyroidism (Helfand 2004)
  • Elevated T4 and T3 with abnormally low TSH levels may indicate nodular thyroid disease or an autoimmune Grave’s disease
  • There is good evidence that undetectable low TSH levels (<0.1 mIU/L) increase the risk of atrial fibrillation later in life (Helfand 2004), which is associated with higher probability of stroke; in one study of those older than 60 years, the risk of atrial fibrillation over 10 years was around 32% in comparing to 8% in people with healthy TSH levels (Sawin 1994)
  • Some studies suggest low TSH levels result in loss of bone mineral density and bone fracture; however, this seems true only for vertebral fracture and data do not suggest any correlation to hip fracture (Helfand 2004)
  • The Cleveland Clinic Journal of Medicine recommends that low TSH levels (0.1 – 0.4 mIU/L) should always follow a measurement of T4 and T3; the levels should be re-checked within few weeks to unambiguously confirm the results and avoid potential variation due to one-off event

What are the symptoms of high TSH levels?

Well known symptoms of high TSH levels include:

  • Cold intolerance (compared to those around you)
  • Weight gain
  • Loss of appetite
  • Low energy and weakness
  • Coarse dry hair
  • Dry rough skin
  • Fatigue (which can happen with both low and high TSH levels)

See detailed list of symptoms for high TSH levels at All About Thyroid.


What are the symptoms of low TSH levels?

Common symptoms of low TSH levels include:

  • Heat intolerance (compared to those around you)
  • Palpitations
  • Anxiety
  • Tremors
  • Difficult sleeping or insomnia
  • Unusually high appetite and weight loss
  • Fatigue (which can happen with both low and high TSH levels)

You can read the detailed list of symptoms for low TSH levels at All About Thyroid.


Why a TSH test alone is not enough?

Thyroid health is determined by the hypothalamus-pituitary-thyroid (HPT) axis. One gland might compensate for any abnormality in another one to maintain a balance.

For example, if thyroid gland cannot release enough T4, the pituitary gland will release more TSH to compensate and balance T4 levels. Therefore, knowing TSH values alone might not be sufficient for a full diagnosis.

There is well known age correlation to TSH, with levels increasing over age. The T4 values do not show age correlation. With advancing age, there is a lower pituitary response to thyroid function which a TSH test alone might not capture.

If TSH levels are abnormal, one should check T4 and T3 for thyroid dysfunction. Also, checking for TPO antibodies can help assess the presence of autoimmune thyroid disease (Glinoer and Spencer 2010)

Autoimmune diseases–and presence of TPO antibodies–in certain groups can be very high, e.g., elderly white women have a rate of almost 50% positivity (Laurberg 2011) and those aged 55+ year with TSH above 4.5 mIU/L have a rate of 36-76% (Boelaert 2013).

In certain cases, e.g., in pregnancy, a combination of thyroid dysfunction and TPO antibodies positivity may predict pregnancy complications and maternal morbidity in in later life (Männistö’ 2010)


What to know about repeat TSH testing?

  • If TSH levels are outside normal range, a repeat test is very useful; in one study, repeat of 400k patients after 5 years suggested only 2% developed abnormal levels, and 50% of those with abnormal levels tested within healthy levels (Meyerovitch 2007)
  • How do you know the measured high TSH levels were not a random event? Analytical studies suggest a deviation of over 40% from your last measurement is not a random event (Karmisholt 2010)
  • Mean values of healthy TSH levels in a person are mostly determined by their genetics; however, environmental factors can affect them considerably (e.g., iodine content in food)
  • The TSH variation is much narrower for individual person in comparison to population variation (50% tighter variation compared to data for large population)
  • TSH is the most sensitive indicator of thyroid gland function when pituitary-hypothalamus function is healthy; e.g., a 30% reduction in T4 results in a 6-x increase in TSH (Eisenberg 2009)

 

Reference:

  1. The TSH upper reference limit: where are we at? by P. Lauerber et al., Nature Reviews Endocrinology 7, p.232-239 (2011)
  2. Approach to a low TSH level: Patience is a virtue by K. Pantalone and C. Nasr, Cleveland Clinic Journal of Medicine, 77 (2010)
  3. Serum TSH determinations in pregnancy: how, when and why? by D. Glinoer and C. Spencer, Nature Reviews Endocrinology 6, p.526–529 (2010)
  4. Circadian and 30 minutes variations in serum TSH and thyroid hormones in normal subjects by J. Weeke and H. Gundersen, European Journal of Endocrinology 89(3), p.659-672 (1978)
  5. Serum Thyrotropin Measurements in the Community by Meyerovitch et al., Arch Intern Med. 167(14), p.1533-1538 (2007)
  6. Screening for Subclinical Thyroid Dysfunction in Nonpregnant Adults: A Summary of the Evidence for the U.S. Preventive Services Task Force by M. Helfand, Ann Intern Med. 140, p.128-141 (2004)

 


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