May 25 is World Thyroid Day. This is dedicated to millions of people suffering from a spectrum of diseases affecting the butterfly-shaped gland. Small as it may seem, the thyroid gland plays a huge role in regulating metabolism through hormones. Unfortunately, some individuals suffer from thyroid function disorders ─a subset of disease from the spectrum. These disorders affect hormone production.
Thyroid function disorders are commonly encountered in primary care. In iodine-sufficient countries, hyperthyroidism has a prevalence of 0.2 to 1.3% while hypothyroidism has 1 to 2% prevalence. Variation in the global prevalence exists mainly due to the iodine nutrition of a locale and population dynamics with more cases of hypothyroidism among women and in the elderly.
The Philippine Thyroid Diseases Study (PhilTiDeS) found the prevalence rate of thyroid function disorders to be at 8.53% in 2008. Subclinical disease (those with abnormal thyroid function test but are asymptomatic or with mild thyroid-related symptoms) ranked high with subclinical hyperthyroidism at 5.33% and subclinical hypothyroidism at 2.18%. Overt hyperthyroidism had a prevalence rate of 0.61% while overt hypothyroidism had 0.41%.
The prevalence of overt hypothyroidism in the Philippines is comparable to that of iodine-deficient countries but it is lower than the prevalence in iodine-sufficient countries. This may reflect an improvement in iodine supplementation and intake. The prevalence of overt hyperthyroidism is compatible with the global prevalence in iodine-replete countries.
The Highs and Lows of Thyroid Function Disorder
Hyperthyroidism is associated with irritability, heat intolerance, palpitations, fatigue, weight loss, diarrhea, tachycardia, and tremors. Hypothyroidism, on the other hand, may present with tiredness, dry skin, feeling cold, difficulty concentrating, poor memory, constipation, weight gain, puffy face, hands, and feet (myxedema), and decreased deep tendon reflexes.
The abovementioned signs and symptoms are true for overt disease. Although subclinical diseases have little to no symptoms, they still pose harm on patients. Subclinical hyperthyroidism predisposes patients to disturbances in bone metabolism and cardiovascular function while those with subclinical hypothyroidism are at risk for dyslipidemia.
The quality of life, activities of daily living, and functional capacities are affected by thyroid function disorders. If hyperthyroidism is left untreated or partially treated, complications which are mostly cardiovascular in nature may arise. This is particularly true among the elderly. Those with hyperthyroidism may develop atrial fibrillation and embolic events. Hypothyroid elderly patients are also predisposed to cardiovascular conditions like heart failure.
Is Thyroid Function Over-tested?
Diagnostic tests in primary care help general practitioners screen and confirm diseases. One issue though is over-testing. This happens when diagnostic tests are ordered injudiciously. With the cost and availability of thyroid function tests, primary care physicians should know when and which tests to request.
A 2013 study in the United Kingdom showed that thyroid function has been over-tested. A study in Spain published in 2016 showed variability among health institutions in terms of ordering thyroid function test. In both studies, Thyroid Stimulating Hormone (TSH) was over-tested or inappropriately requested.
Werhun and Hamilton in 2015 examined the number of thyroid function testing done in a primary care setting in the UK. Of the 16,407 patients seen in a single practice clinic at Devon, 2,035 (12%) underwent thyroid function testing. Only 42 patients or about 2.1% of those tested had abnormal TSH levels.
The 2.1% prevalence of thyroid function disorder detected in Werhun and Hamilton’s study is comparable to the overall prevalence of thyroid function disorder in the UK which is at 2.4%. Their level of testing seems appropriate but they are still advocating for more discriminate testing.
A systematic review by Zhelev, Z. et al. in 2016 found that behavioral change interventions were effective in terms of lowering the number of thyroid function test ordered10. The patterns in ordering the tests as well as compliance to guidelines were improved. In addition, the costs were reduced as inappropriate tests were avoided.
Detection of Thyroid Function Disorder in Primary Care
Before ordering tests, primary care physicians should know whether the patient’s disorder is iatrogenic. Use of thyroid medications and other drugs such as amiodarone or lithium must be ruled-out.
Pregnant patients are also predisposed to thyroid function abnormalities. Estrogen increases thyroid binding globulin which in turn decreases free thyroid hormones leading to a hypothyroid state. Some are also predisposed to hyperthyroidism especially during the first trimester due to human chorionic gonadotropin (hCG) which has thyrotropic actions.
Aside from knowing the thyroid function disorder, the primary care physician should investigate the etiology of the disorder. To confirm other etiologies, thyroid ultrasound, scan, and uptake, as well as antibodies may be needed.
When to Request Thyroid Function Test?
Thyroid function test is clearly indicated among patients presenting with signs and symptoms consistent with hyper- or hypothyroidism, more so if they have a positive neck examination for diffuse or nodular goiter. But it is important to know that some patients with goiter have normal thyroid hormone levels.
The number of subclinical diseases may be significant in certain countries such as the Philippines but that does not warrant the need for screening the general population. It is prudent to exercise caution when ordering thyroid function tests to prevent overdiagnosis. Patients already tested who have subclinical disease may be retested after 3-6 months.
TSH is the most important initial test that can be ordered for screening thyroid function disorders. Serum thyroxine (T4) levels can be used for confirmation of a hyper- or hypothyroid state. Triiodothyronine (T3) is rarely ordered unless T3 thyrotoxicosis is suspected.
Pregnant women, women over 40, and elderly patients may benefit from thyroid function testing. However, clear local guidelines must first be developed before tests are recommended and done.
The thyroid gland is a vital organ for growth and metabolism. If left unchecked, it may cause serious consequences. With the data presented in this article, everyone must know when it’s time to test their thyroid.
- American Thyroid Association. (2010). American Thyroid Association Supports World Thyroid Day [Press release]. Retrieved from: https://www.newswise.com/artic...
- Taylor, P. et al. Global epidemiology of hyperthyroidism and hypothyroidism. Nature reviews. May 2018.
- Carlos-Raboca, J. et al. The Philippine Thyroid Diseases Study (PhilTiDeS 1): Prevalence ofThyroid Disorders Among Adults in the Philippines. 2012.
- Jameson, J. et al. Harrison’s Principles of Internal Medicine 20th Edition. McGraw Hill.2018.
- Ross, D. et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016.
- Moynihan R., Doust J., ND Henry D. Preventing overdiagnosis: how to stop harming the healthy. British Medical Journal. 2012.
- Vaidya, B. et al. Variability in thyroid function test requests across general practices in south-west England. Quality in Primary Care. 2013.
- Salinas, M. et al. Request of thyroid function tests from Primary Care in Spain. Endocrinol Nutr. 2016.
- Werhun, A. and Hamilton, W. Thyroid function testing in primary care: overused and under-evidenced? A study examining which clinical features correspond to an abnormal thyroid function result. Oxford University Press. 2015.
- Zhelev, Z. et al. Effectiveness of interventions to reduce ordering of thyroid function tests: a systematic review. British Medical Journal. 2016.