If you’re like most primary care clinicians, your email inbox is flooded with messages from patients with questions about lab results. A common query: Should I be worried about an abnormal value on a test of thyroid-stimulating hormone (TSH)?
For guidance, this news organization spoke with Angela Leung, MD, associate professor of medicine in the Division of Endocrinology, Diabetes & Metabolism at the UCLA David Geffen School of Medicine and an endocrinologist at UCLA and the VA Greater Los Angeles Healthcare System, and Karen Tsai, MD, assistant clinical professor of endocrinology at City of Hope Comprehensive Cancer Center in Duarte, California. The following interview has been edited for length and clarity.
Question: Why do you usually start by measuring TSH levels?
Dr. Leung: We need to measure the thyroid status in a way that integrates more information about the long-term thyroid status and not small changes in thyroid hormone levels. TSH is made by the pituitary gland in the brain, which integrates information about the signals of high and low levels from each of the different thyroid hormones.
Now we can measure the actual thyroid hormones — primarily we’re talking about T3 and T4 — but if we do that, we are relying on a single snapshot in the bloodstream at that moment. The levels might change throughout the day in response to ongoing metabolism and outside stresses. So we usually start by measuring the TSH level, which is a good representation of the compilation of all those things over the past 30 days or so.
Question: How do you describe a low TSH result to patients?
Dr. Leung: Whenever we encounter a low TSH level, we want to repeat the test because it is a dynamic test, and it can change in response to several factors. If it is indeed low, we’re thinking that perhaps there’s a little bit of extra thyroid hormone in the body. It can be either temporary or more chronic, but that higher amount of thyroid hormone is telling the pituitary gland in the brain to start making less. So TSH levels go low when we need less thyroid hormone.
Question: What are some of the reasons for a low TSH level?
Dr. Leung: One of the most common situations for a temporarily low TSH level I see is what we call nonthyroidal illness, like a common cold or just being under the weather. Other things that can artifactually lower the TSH level could be the use of steroids, such as prednisone for asthma or some sort of a rheumatologic condition. Also, the TSH level could be low if a person has been recently exposed to very high amounts of iodine, such as iodinated contrast needed for a CT scan.
If the TSH level remains persistently low, usually in the presence of high thyroid hormone (T3 and/or T4) levels, the most common reason for hyperthyroidism is Graves disease, in which there are autoantibodies — measurable in the blood — that can stimulate the thyroid gland in the neck to make extra thyroid hormone.
Question: And what does an elevated TSH level mean?
Dr. Leung: Again, we want to confirm that it is elevated. We need at least two tests to confirm a high TSH level. A persistently elevated TSH level is a signal there might be low thyroid hormone levels in the body, which could be transient or more longer lasting.
Question: What are some of the most common causes of an elevated TSH level?
Dr. Leung: If the TSH level is confirmed high and the thyroid hormone levels are low, the most common cause of hypothyroidism here in the United States is Hashimoto thyroiditis.
Globally, iodine deficiency is the most common reason for hypothyroidism and may be a problem in parts of the globe where there are endemically low iodine levels in soil, crops, and the food supply like not having enough iodized salt. The thyroid is reliant on having enough iodine as a micronutrient to make thyroid hormone. If it doesn’t, the thyroid really can’t make thyroid hormone. It’s important to also remember, though, that having too much iodine can result in hypo- or hyperthyroidism.
Dr. Tsai: I take a glance at their medication list. Some of the patients are on methimazole or levothyroxine, and those medications should be adjusted first to normalize the TSH level. Other medications like lithium and amiodarone can also cause elevated TSH levels. We are also seeing a lot of patients on cancer therapies, such as tyrosine kinase inhibitors or immunotherapy, that can cause an elevated TSH level.
Question: If the repeat TSH test shows that TSH levels are still elevated, what comes next in your workup?
Dr. Tsai: If there’s not a real clear-cut diagnosis, I’ll order the thyroid peroxidase antibody and the thyroglobulin antibody, although thyroid peroxidase antibody, which is indicative of autoimmune thyroid disease, alone is usually sufficient to make that diagnosis.
Question: Should clinicians follow thyroid antibodies over time?
Dr. Tsai: I usually don’t repeat the antibody tests. In those circumstances where patients who were diagnosed 50-60 years ago and perhaps it is unknown if they had the thyroid antibodies measured at the time and now they’re saying, “Do I actually have Hashimoto’s?” or “Do I really need to continue this for the rest of my life?” I do repeat antibody tests to help gauge if the patient’s levothyroxine can be stopped.
Question: How important is it to follow T4 or T3 levels?
Dr. Tsai: T4 and T3 levels can help differentiate overt thyroid dysfunction — where T3 and/or T4 levels will be abnormal — from subclinical thyroid dysfunction — where T3 and T4 levels would be normal. In general, although we do not fully appreciate the best metric to monitor hypo- or hyperthyroidism, because some patients with a normal TSH level still may have symptoms of thyroid dysfunction, these peripheral thyroid hormone levels are usually the most helpful at the time of initial diagnosis.
Question: What are your criteria for initiating treatment for hypothyroidism?
Dr. Tsai: If the TSH level > 10 mIU/L, I recommend levothyroxine hormone replacement. A lot of published data support clinical benefit in this group.
There is a gray area in those patients who have a TSH level higher than the upper limit of the reference range but less than 10. If the patient doesn’t have overt hypothyroid symptoms, I discuss the findings with the patient but don’t really feel eager to treat. I recommend checking the levels again in 6 months to see where that TSH goes, and if it worsens or becomes greater than 10 mIU/L, I then recommend levothyroxine hormone replacement.
It is also important to note that a TSH level of 5-7 may be an acceptable range for older patients, and they do not require levothyroxine.
The other category is patients whose TSH level is greater than the upper limit of the normal reference range but less than 10 and with overt hypothyroid symptoms such as fatigue, unintentional weight gain, constipation, or cold intolerance. In these patients, it is worthwhile to try a low dose of levothyroxine (25-50 mcg/d) and repeat TSH and free T4 tests in 6-8 weeks and see if the TSH level normalizes.
Dr. Leung: When you look at subclinical hypothyroidism, the situation of an isolated high TSH level in the setting of normal T4 levels, if the TSH level is mildly elevated in the 5-7 mIU/L range, there’s a 60% chance that it will normalize within 6 months.
Going back to Karen’s point, a lot of people are started and maintained on low doses of thyroid hormone forever and ever. A recent study on levothyroxine use found half of the prescriptions were unnecessary.
Question: In an era where many patients obtain much of their health information from TikTok, what’s your approach with patients with a normal TSH level who feel that they should have more testing or start treatment?
Dr. Tsai: Fatigue is one of the common referrals we get into our endocrinology practice, and everyone is convinced that their thyroid is the culprit. It is important to note, however, that fatigue can be due to different diseases such as anemia, depression, sleep disorders, or a recent viral illness.
TSH tests are readily available and cheap. I don’t mind ordering the lab test again if it helps give the patient some reassurance. I also find that patients are relieved once they hear from their endocrinologist that their thyroid is unlikely to be the cause of their fatigue.
Some other endocrine causes we may consider additionally working up include adrenal insufficiency, vitamin D deficiency, and diabetes. A comprehensive metabolic panel and complete blood count is part of my workup to rule out any gross electrolyte abnormalities or any new diagnosis of anemia, liver disease, or chronic kidney disease.
Question: What are your criteria for recommending that someone see an endocrinologist?
Dr. Tsai: Our primary care colleagues can do a workup and interpretation of thyroid function tests in most cases. In the situations where the thyroid function test results are discordant (ie, elevated TSH and elevated free T4 levels or low TSH and low free T4 levels) or difficult to interpret, it would be appropriate to refer the patient to an endocrinologist.
One of the common referrals that we do get from the community is a patient’s thyroid function tests going from hyperthyroid to hypothyroid without a clear explanation or the patient is suboptimally controlled with levothyroxine or methimazole. In those circumstances, it would be worthwhile to send to an endocrinologist try to discern an underlying cause or for optimization of medication.
Dr. Leung and Dr. Tsai had no financial disclosures.
A version of this article appeared on Medscape.com.