Thyroid Function Assumed to be Normal When Only TSH is Tested

DISCLAIMER (August 14, 2022): The information in this post should in no way be taken as a recommendation to self-diagnose, self-interpret diagnostic tests, or self-treat any suspected disorder, including thyroid function. It is essential that people who suspect they may have symptoms of any condition consult with their doctor, as only a medical doctor can diagnose and treat.

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In Canada and many places in the US, the standard screening test for abnormal thyroid function is thyroid stimulating hormone (TSH). As outlined below, TSH is a hormone that is released from the pituitary gland, not the thyroid. If TSH results falls within normal range, no testing of thyroid hormones occurs. Thyroid response to TSH is presumed to be normal.

Thyroid Hormones and Lab Tests Used to Assess Thyroid Function

Thyroid Stimulating Hormone (TSH) is a hormone that is produced by the pituitary gland in response to a hormone called Thyrotropin-Releasing Factor (TRF) that is released by the hypothalamus of the brain.

Thyroid Stimulating Hormone (TSH) released from the pituitary gland acts on the thyroid gland, a butterfly shaped gland in the front of the neck. The action of pituitary TSH on the thyroid results in the release of thyroxine, also called Free T4 (fT4).  Thyroxine (fT4) is reduced to Triiodothyronine also called Free T3 (fT3), which is the active form of thyroid hormone.

Central Hypothyroidism is where a problem exists in either the hypothalamus or the pituitary gland that results in a decreased in TSH release from the pituitary gland. On lab tests, a low TSH and low T4 indicates central hypothyroidism. This is often treated by administration of growth hormone, or using T3 containing medications. 

Primary hypothyroidism is where there is no abnormality in the hypothalamus or the pituitary gland. Primary hypothyroidism is diagnosed when there is high TSH and normal or low free thyroxine (free T4 / fT4). In many places in Canada and the USA, if TSH is normal, no further testing is done. It is assumed that the action of pituitary TSH on the thyroid gland results in sufficient release of T4.

Thyroid Function – different causes of primary hypothyroidism

Hashimoto’s Disease

The most common form of primary hypothyroidism in the western world is Hashimoto’s disease which is an autoimmune disorder where the body attacks the thyroid. Thyroid Peroxidase Antibody (TPO antibody) is the marker for Hashimoto’s hypothyroidism [6]. 

Prior Thyroid Surgery or Radiation of the Neck

According to Endocrinologist, Dr. Theodore C. Friedman MD, PhD,  Professor of Medicine at UCLA, primary hypothyroidism can also result from prior thyroid surgery to remove a tumor or nodule, or due to radiation of the neck [6].

Dietary Deficiency

Iodine is essential for thyroid function and in the developing world, the most common type of primary hypothyroidism is related to iodine deficiency. Iodine deficiency is assumed to be rare in the West since iodine is added to salt (iodized salt), in the same way that vitamin D is routinely added to milk.  I have noticed a significant increase in the use of Himalayan pink salt and sea salt for home use in the last decade or so, and wondered how much of the salt being used currently is iodized. Data from 2015 indicates that only 53% of salt sales in the US were iodized [7], so I have to wonder what effect this decreased intake of iodized salt may be having on the prevalence of hypothyroidism. 

Selenium is another mineral that is essential for thyroid function as it functions in the conversion of (inactive) T4 to (active)T3. Like iodine deficiency, selenium deficiency is a significant problem in the developing world, but thought to be rare in the West. Research from 2012 indicates that the selenium content of the soil in the US was already lowest in the major agricultural areas of the Northwest, Northeast, Southeast, and areas of the Midwest near the Great Lakes[8] and at the time, only the Great Plains and the Southwest were reported to have adequate selenium content in the soil [8].

Given the decreased use of iodized salt and decreased presence of selenium in the soil where much of domestic food is grown, I wonder what effect this may be having on formerly rare incidence of nutrient-related hypothyroidism in the US and Canada.

Assessing Thyroid Function – testing for hypothyroidism

Each province in Canada sets its own policy for which laboratory tests are covered by provincial medical plans, and in the US which testing is covered is determined by whether they are performed by in-network or out-of-network labs. 

In British Columbia, thyroid testing covered by the provincial plan is determined by a 2018 document titled Thyroid Function Testing in the Diagnosis and Monitoring of Thyroid Function Disorder [9]. These guidelines outline testing for thyroid stimulating hormone (TSH), free thyroxine (fT4), free triiodothyronine (fT3) and anti-thyroid peroxidase (TPO).

According to the guidelines, risk factors for thyroid disease include [9]:

• men: age ≥ 60 years
• women: age ≥ 50 years
• personal history or strong family history of thyroid disease
• diagnosis of other autoimmune diseases
• past history of neck irradiation
• previous thyroidectomy or radioactive iodine ablation
• drug therapies such as lithium and amiodarone
• dietary factors (iodine excess and iodine deficiency in patients from developing countries)
• certain chromosomal or genetic disorders

Note: Iodine nutrient deficiency for those who are not from developing countries is not included. 

Indications for Testing

1. 1. Routine thyroid function testing is not recommended in asymptomatic patients Testing may be indicated when non-specific symptoms or signs are present in patients who have specific risk factors for thyroid disease.
   2. Testing is indicated for patients with a clinical presentation consistent with thyroid disease as delineated in Table 1: Symptoms and Signs of Thyroid Disease, below.
   3. Where thyroid testing in an asymptomatic patient has occurred and the patient has been diagnosed with subclinical thyroid disease (subclinical hypothyroidism: TSH is elevated in the presence of normal levels of fT4)
   4. If initial testing (i.e. TSH) is normal, repeat testing is unnecessary unless there is a change in clinical condition*.  

The Guidelines (page 3) states, “A TSH value within the laboratory reference interval excludes the majority of cases of primary thyroid dysfunction.

[The reference provided for this is: Jameson, JL., Weetman, A.P.,  Disorders of the Thyroid Gland, Harrison’s Principles of Internal Medicine. 17th ed. New York: McGraw-Hill; 2008. p. 2224–47]

The guide also indicates that “the TSH reference interval will vary depending on the testing laboratory. ” What that means is the cutoff points for an abnormal level of TSH vary between labs in BC.

*What this means is, if TSH lab test results come back in the normal range, no further testing is performed unless the person begins to show some of the accepted hypothyroid signs and symptoms in Table 1, below.

Note: As outlined above, if TSH is found to be normal we know that TSH release from the pituitary gland is normal. There is no testing for the thyroid gland’s response to TSH released from the pituitary gland, it is assumed to be functioning.

Below are the accepted hypothyroid signs and symptoms that warrant testing (from Table 1 [9]).

Only the above list of clinical presentation symptoms is recognized as consistent with hypothyroidism, warranting lab testing.

Looking at the list in Table 1, how many other conditions, including having Covid-19 result in people feeling depressed, having decreased mental function (“brain fog”), feeling physically tired, feeling cold, having reduced degree of movement and muscle weakness, dry flaking skin, and a hoarse voice?  Unless a person has burning or prickling sensation in their hands or feet, or swelling under their eyes, or are sweating less than usual, it is unlikely they would notice anything unusual outside of common flu or Covid symptoms that would have them mention the above to their doctor.

Below is a fuller list of clinical presentation symptoms associated with hypothyroidism, with the ones NOT recognized for testing in italics.

The Guidelines Summarized

Unless you are either a man ≥ 60 years, or a woman ≥ 50 years with a personal history or strong family history of thyroid disease, a diagnosis of other autoimmune diseases, a past history of neck irradiation or previous removal of your thyroid or destruction of your thyroid for medical reason using radioactive iodine, are not on medications such as lithium or amiodarone, and aren’t from a developing country with either iodine excess or iodine deficiency, or have a specific chromosomal or genetic disorder listed, you do not qualify for TSH testing unless you display the specific symptoms listed in Table 1, above).

What if the other common presentations that are NOT also common in colds, flu or Covid-19 were included in the checklist such as;

• • non-pitting edema (swelling) in the lower legs and ankles
  • a puffy swollen face
  • an enlarged tongue (with or without scalloped edges)
  • enlarged saliva glands including under the tongue
  • hair thinning
  • loss of the outer third of eyebrows
  • pale or bluish lips

…would it be more likely that people experiencing these symptoms would go to their doctors, and be tested?

Summary of Assessing Thyroid Function

1. To be diagnosed with hypothyroidism, requires a high TSH and normal or low free free T4 but in many places in Canada and the USA, if TSH is normal, no further testing is done.

2. How one defines “high TSH” is important. In British Columbia, the cutoff points vary between labs, but lab normal values at the labs near me are the normal range is 0.27-0.42 mU/L, but is a result of 3.9 mU/L or 4.0 mU/L really “normal”?

Thyroid Function

TSH                    4.0  (0.27-4.2) mU/L

It is “normal enough” that no further testing is done.

3. Someone can have common clinical manifestations of hypothyroidism (non-pitting edema in the lower legs and ankles, a puffy swollen face, enlarged tongue (with or without scalloped edges), enlarged saliva glands, hair thinning, loss of the outer third of eyebrows or pale or bluish lips but if their symptoms are not on the list in Table 1, they are not eligible for testing of thyroid hormones, fT4 / fT3.

4. Unless a person is from a family with specific risk factors (older age with other autoimmune conditions, or had neck irradiation, a thyroidectomy or radioactive iodine ablation,  or take lithium or amiodarone, or are from a developing country that had iodine excess and iodine deficiency) they are not eligible for testing of thyroid hormones, fT4 / fT3.

Final Thoughts…

If some one has their TSH tested and the results come back in the normal range for that particular lab, no further testing is done — even if they have symptoms documented in the literature to be associated with hypothyroidism. 

My concern is if the definition of who qualifies for fT4 testing may be too narrow,. People with high-normal TSH and symptoms that are associated with hypothyroidism, but not on the “list” in Table 1, will not be tested. Could it be that some people could have greatly improved quality of life if thyroid hormones were evaluated, found to be low, and treatment initiated? [Please see note added July 17, 2022 about a diagnosis of subclinical hypothyroidism where TSH > 4mIU/ with normal T4.]

NOTE, July 16, 2022: It is not the normal TSH test result in the absence of symptoms that I am addressing in this article, but the absence of T4 testing in someone with presenting symptoms of hypothyroidism beyond the official “list”. It is my hope that if someone has those symptoms and a TSH value that is normal, that physicians would remain curious and ask for additional testing.

NOTE: July 17, 2022: I came across an academic paper from 2016 that indicates that diagnosis of subclinical hypothyroidism (SCH) exists in western countries for TSH >4 mIU/L, with normal T4 [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4740939/], whereas the cutoff here for a diagnosis of SCH is >10 mIU/L. (which is based on 2008 reference, as outlined above). Also of interest, another paper I came across from 2016 reported that several previous studies found no significant difference in symptoms between people with  subclinical hypothyroidism  and those with overt hypothyroidism [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4740939/] . The most common symptoms reported of both SCH and overt hypothyroidism were poor memory, slow thinking, muscle cramps, muscle weakness, tiredness, dry skin, feeling colder, hoarse voice, puffy eyes, more constipation.

More Info?

If you have been diagnosed with hypothyroidism and would like to better understand the condition, or would like make sure that you have adequate intake of nutrients known to be important in thyroid health, please send me a note through the Contact Me form and I will reply when I can.

To your good health!

Joy

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References

1. Crofts, C., et al., Identifying hyperinsulinaemia in the absence of impaired glucose tolerance: An examination of the Kraft database. Diabetes Res Clin Pract, 2016. 118: p. 50-7.
2. Pareek, M., Bhatt, D.L., Nielsen, M.L., et al,  Enhanced Predictive Capability of a 1-Hour Oral Glucose Tolerance Test: A Prospective Population-Based Cohort Study. Diabetes Care 1 January 2018; 41 (1): 171–177. https://doi.org/10.2337/dc17-1351
3. Bergman M, Chetrit A, Roth J, Dankner R (2016) One-hour post-load plasma glucose level during the OGTT predicts mortality: observations from the Israel Study of Glucose Intolerance, Obesity and Hypertension. Diabet Med 33:1060–1066
4. Hulman, A., Vistisen, D., Glümer, C. et al. Glucose patterns during an oral glucose tolerance test and associations with future diabetes, cardiovascular disease and all-cause mortality rate. Diabetologia 61, 101–107 (2018). https://doi.org/10.1007/s00125-017-4468-z
5. Berkow, R., Beers, M. H., & Fletcher, A. J. (1997). The Merck Manual of Medical Information. Whitehouse Station, N.J.: Merck Research Laboratories.
6. Freidman, Theodore C., Nuances of Hypothyroidism, The MAGIC Foundation’s Annual Conference for Adults With Endocrine Disorders (Phoenix, AZ),  March 3, 2019
7. Maalouf J, Barron J, Gunn JP, Yuan K, Perrine CG, Cogswell ME. Iodized salt sales in the United States. Nutrients. 2015 Mar 10;7(3):1691-5. doi: 10.3390/nu7031691. PMID: 25763528; PMCID: PMC4377875.
8. Mistry HD, Broughton Pipkin F, Redman CW, Poston L. Selenium in reproductive health. Am J Obstet Gynecol. 2012 Jan;206(1):21-30
9. BC Guidelines & Protocols Advisory Committee, Thyroid Function Testing in the Diagnosis and Monitoring of Thyroid Function Disorder, October 24, 2018

 

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