Thyroid hormones are extremely important for our health.
In fact, abnormal thyroid levels have serious consequences to a child's growth and development.
Hypothyroidism and hyperthyroidism are common disorders resulting from abnormal thyroid-stimulating hormone (also called thyrotropin).
The risk of thyroid disorders during our lifetime is quite high. It increases with age and senior population is most affected.
A study from Denmark suggests overall risk of overt (i.e., clinical stage) hypothyroidism or hyperthyroidism diagnosis, during a person's lifetime, is close to 10% (Carle’ 2006).
The risk for women is even higher. At almost 15% it's almost five-times higher than men.
For US, Europe, and other developed countries, where iodine deficiency is rare, the risk is comparable.
Regularly testing thyroid levels is one way to reduce this risk.
Epidemiology of sub-types of hypothyroidism in Denmark by Allan Carlé and others in European Journal of Endocrinology, vol. 154 (1), 2006.
MORE FROM OUR BLOGS:
Thyroid and Your Health – how thyroid impacts almost everyone.
Pregnancy and Role of Thyroid – why thyroid health is critical during pregnancy.
Thyroid and Iodine Deficiency – why iodine is the most important modulator of thyroid health.
Learn All About Thyroid – summary of thyroid disorders and their symptoms.
The chart below shows ranges for optimal TSH levels. It also includes the lab ranges for other key thyroid hormones and TPO antibodies-a marker of immune response against the thyroid gland.
Normal range for TSH, free T3, free T4 and TPO antibodies for one of our at-home thyroid test.
The distribution of Thyroid Stimulating Hormone in healthy population is close to a bell curve (but it is asymmetric, i.e., non-Gaussian distribution). The curve skews to the right—high TSH levels seem to be more common than low levels.
Healthy values lie between 0.1 mIU/L and 4.5 mIU/L.
Statistically, that's between 2.5th and 97.5th percentile (±3 standard deviation). Which means 95% of the people have thyroid levels in this range.
Age, gender, race, and iodine consumption can cause variations.
The plot below shows data from a study of 1,671 healthy Danish women.
Lower prevalence of mild hyperthyroidism related to a higher iodine intake in the population: prospective study of a mandatory iodization programme by Vejbjerg & others, in Clinical Endocrinology, Vol 71 (3) 2009.
Problems with the hypothalamus, pituitary, or thyroid gland can cause abnormal TSH levels.
Here are few other common factors that play a role:
Circadian and 30 Minutes Variations in Serum TSH and Thyroid Hormones in Normal Subjects by Weeke and Gundersen in Acta Endocrinol (Copenh), 1978, Vol. 89(4), pages 659‐672.
The plot below shows mother’s hormone levels by week of pregnancy:
Thyroid disease in pregnancy: new insights in diagnosis and clinical management by Korevaar and others in Nature Reviews Endocrinology, 2017, Vol. 13, pages 610–622.
Thyroid Stimulating Hormone levels continuously increase with age.
National Health and Nutrition Examination Survey (NHANES III): Serum TSH, T4, and Thyroid Antibodies in the United States Population (1988 to 1994) by Hollowell and others in The Journal of Clinical Endocrinology & Metabolism, Vol. 87(2) 1 February 2002, Pages 489–499.
Well known symptoms of high values include:
Common symptoms of low levels include:
Samples from Framingham Heart Study show a linear correlation between gain in weight and high TSH values. The data suggest women show larger weight gain.
Correlation of Thyroid Function to Body Weight – Cross-sectional and Longitudinal Observations in a Community-Based Sample by Caroline Fox and others in Archives of Internal Medicine, Vol 168 (6), MAR 24, 2008.
Another chart below shows an average weight gain after 6-months of testing. Weight increases almost linearly with higher values. Similar gains have been reported after 5 years, except for those with values above 3.6 mU/L. They gained over 10 lbs!
Small Differences in Thyroid Function May Be Important for Body Mass Index and the Occurrence of Obesity in the Population by Nils Knudsen and others in The Journal of Clinical Endocrinology & Metabolism, Volume 90 (7), July 2005, Pages 4019–4024.
Thyroid health is determined by the hypothalamus-pituitary-thyroid (HPT) axis. One gland often tends to compensate for abnormal behavior of another to maintain a balanced cycle.
Whenever thyroid cannot release sufficient T4, the pituitary compensates additional TSH. Therefore, knowing TSH alone might not be sufficient for a full diagnosis.
There is a well known linear correlation between age and Thyroid Stimulating Hormone. However, T4 values do not show any correlation with age. Therefore, with advancing age, there is a lower pituitary response to thyroid function which a TSH test alone might not capture.
If TSH values 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). Those aged 55+ years with TSH above 4.5 mIU/L have a rate of 36-76% (Boelaert 2013).
In some cases, e.g., during pregnancy, a combination of thyroid dysfunction and high TPO antibodies may predict complications in pregnancy and mother's risk of dying later in life (Männistö 2010).