Screening Tests for Thyroid Dysfunction

About

High or low thyroid stimulating hormone (TSH) level is indicative of a dysfunctional thyroid.

The thyroid is a gland situated in the front of the neck that secretes two hormones, thyroxine or T4 and thyronine or T3. The thyroid hormones influence important metabolic processes in the body. Increase in thyroid hormone production by the thyroid gland results in a condition called hyperthyroidism, whereas a decrease in thyroid hormones production results in hypothyroidism. Untreated hyperthyroidism can lead to atrial fibrillation or abnormal heart rhythm, heart failure, osteoporosis or bone thinning, and neuropsychiatric problems. Hypothyroidism, if untreated, can lead to fatigue, weight gain, mental slowing, heart failure, and increase in cholesterol levels.

Secretion of thyroid hormones is controlled by a hormone secreted by the pituitary gland (a small gland at the base of the skull) called thyroid stimulating hormone or TSH. If thyroid hormone levels fall (as in hypothyroidism), it is followed by an increased secretion of TSH, which tries to maintain the level of thyroid hormone. On the other hand, if the level of thyroid hormone increases (as in hyperthyroidism), the level of TSH falls. Measurement of the thyroid hormone levels T3 and T4 as well as TSH help to estimate thyroid function.

Subclinical thyroid dysfunction occurs in about 3 to 8 percent individuals of the general population. It is found more commonly in women and older individuals. People with subclinical thyroid dysfunction do not suffer from any symptoms. Their T3 and T4 levels are normal; however, their TSH may be either high or low. These patients could progress to thyroid dysfunction. Subclinical hyperthyroidism (with low TSH levels) may be associated with atrial fibrillation, dementia or forgetfulness, and osteoporosis. Subclinical hypothyroidism may be associated with poor obstetric outcomes and can affect development of brain function in children. Screening for thyroid function could help to detect thyroid dysfunction in these individuals, and enable early treatment.

Effective treatments for thyroid dysfunction are available, thus early diagnosis and timely treatment is definitely useful in preventing complications.

Screening Test for Thyroid Dysfunction

Routine screening for thyroid dysfunction by measuring TSH levels is currently not recommended by the US Preventative Task Force.

What are the current recommendations for screening for thyroid disease?

The US Preventative Task Force, which recommends various screening tests in the United States, does not recommend routine screening for thyroid function. It no doubt agrees that screening does detect cases that do not show any symptoms i.e. cases of subclinical thyroid dysfunction. However, treating these cases has not resulted in much benefit. Mild changes in TSH alone sometimes recover on their own without treatment. False positive results during screening could also result in overtreatment. Taking into consideration the side effects of thyroid and anti-thyroid medications, this could prove more harmful than beneficial.

According to the recommendations from the American College of Gynecology, routine thyroid testing is not recommended in all pregnant women and should be done only in women with symptoms of thyroid disease, history of thyroid disease in the past, or the presence of any other medical conditions associated with it, such as diabetes.

On the other hand, screening is recommended for all newborn infants between two to four days of birth to rule out congenital hypothyroidism or hypothyroidism present from birth. Testing may be done before two days if the baby is discharged early, though false positive results may be obtained in these cases. Congenital hypothyroidism can result in mental retardation unless the condition is diagnosed and treated within the first two weeks of life. Thus, early detection through screening enables early treatment.

What are the tests recommended for screening for thyroid disease?

Screening for thyroid disease is done through a simple and accurate blood test i.e. measurement of TSH levels in the blood. Normal TSH values vary with each laboratory, though the acceptable range in a normal adult is usually between 0.5 and 4.5/5.0. Higher levels indicate hypothyroidism whereas lower levels indicate hyperthyroidism.

Taking a medical history and examining the patient can also help to diagnose thyroid disease, though changes in TSH usually occur before the development of physical changes.

For screening congenital hypothyroidism, two methods are used:

• Measurement of TSH with T4 estimation if TSH levels are high
• T4 measurement with TSH estimation if T4 levels are low

Simultaneous measurement of T4 and TSH is an ideal method for detection of thyroid abnormalities, though it is not practical to be implemented on a large scale as a screening test.

Babies with low T4 levels and TSH levels of more than 40 mU per L are diagnosed as suffering from congenital hypothyroidism. If TSH levels are high but less than 40 mU per L, repeat testing is recommended.

What are the other tests used detect thyroid dysfunction?

Measurement of thyroid hormones T3 and T4 are also used to diagnose thyroid dysfunction. A high level of these hormones above the normal range indicates hyperthyroidism, whereas a low level indicates hypothyroidism.

Tests to detect certain antibodies in the blood can help to diagnose the particular condition. For example, presence of thyroid-stimulating antibodies is observed in patients with Graves' disease.

Radiological tests like ultrasound and CT scan help to diagnose the specific condition and rule out cancer of the thyroid gland. A fine needle aspiration cytology test or FNAC also helps to rule out cancer.

Frequently Asked Questions

1. Which doctor should I visit to get a thyroid test done?

Thyroid tests are done in a pathology laboratory. You could ask your family doctor to recommend a laboratory to you. If the test is abnormal, you may have to visit an endocrinologist for treatment.

2. What treatment is recommended for thyroid disease?

Hypothyroidism and hyperthyroidism are treated with the help of drugs. In hypothyroidism, thyroid hormone is replaced through medications. In hyperthyroidism, medications are given to reduce the production of thyroid hormone.

Radioactive iodine is recommended in some cases. Radioactive iodine gets concentrated in the thyroid gland and destroys the gland. Surgery is recommended in some cases, especially if the gland is large or if cancer is suspected.

3. Which individuals are at high risk for developing thyroid dysfunction?

Elderly individuals, women post delivery, those exposed to high levels of radiation and patients with Down syndrome are at a high risk for developing thyroid dysfunction. Patients with conditions like diabetes, vitiligo, pernicious anemia and premature graying of hair or having family members suffering from these conditions are also at a risk. Drugs like amiodarone used for abnormal heart rhythms and lithium used for manic depressive psychosis could also result in thyroid dysfunction.

Reference:

1. Screening for Thyroid Disease: Recommendation Statement. U.S. Preventative Task Force: Recommendation Statement. Am Fam Physician. 2004 May 15;69(10):2415-2418.
2. Ladenson PW, Singer PA, Ain KB, Bagchi N, Bigos ST, Levy EG, et al. American Thyroid Association Guidelines for Detection of Thyroid Dysfunction. Arch Intern Med. 2000;160:1573-1575
3. Smith L. Updated AAP Guidelines on Newborn Screening and Therapy for Congenital Hypothyroidism. Am Fam Physician 2007;76(3):439-444.