Thyroid hormones are key to maintaining everyday health.
In fact, abnormal thyroid levels can have serious consequences for healthy metabolism, growth, and development.
In general, overt hypothyroidism and hyperthyroidism are common thyroid disorders that occur due to abnormal thyroid-stimulating hormone or thyrotropin.
The risk of thyroid disorders in our lifetime is quite high. It increases with age and higher occurrence found in senior population.
In one study from Denmark, the overall risk of overt hypothyroidism or hyperthyroidism diagnosis–over the lifetime–was 9.2% (Carle’ 2006).
The risk in women was found to be much higher, at 14.6% compared to 3.7% in men. For US, Europe, and countries with no iodine deficiency the risk appears similar.
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 health 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.
At Home Thyroid Test – a comprehensive test that measures TSH, free T4, free T3, and TPO antibodies.
Chart below shows the ranges for optimal TSH levels and other key thyroid hormones as well as TPO antibodies.
Normal range for TSH, free T3, free T4 and TPO antibodies based on our at-home thyroid test.
The distribution of Thyroid Stimulating Hormone in healthy population is close to a bell curve (but it is non-Gaussian distribution, i.e., asymmetric). The curve skews towards high TSH levels. Data for 1,671 healthy Danish women are plotted in the figure below.
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 population has thyroid levels in this range. However, variations occur due to age, gender, race, and iodine consumption.
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 trend of mother’s hormone levels during pregnancy (plotted by week)
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).