Hypothyroidism Lab Values - How to Decipher

Hypothyroidism is the state when the thyroid gland is hypoactive and produces low levels of thyroid hormones. The thyroid is a gland located in the neck, and its role is to produce and secrete the thyroid hormones, triiodothyronine (T3) and thyroxine (T4), and the hormone calcitonin.

It is essential to understand some basic ideas of the thyroid hormone production process, the hormones involved in it, and the regulation of this process by the hypothalamic-pituitary-thyroid axis that keeps the hormone levels within reasonable limits. The first step occurs in the hypothalamus, where the thyrotropin-releasing hormone (TRH) is produced and released to stimulates the anterior pituitary gland to secretes the thyroid-stimulating hormone (TSH). The TSH stimulates the thyroid gland to produce thyroid hormones, incorporating the iodine, existing in the bloodstream, in the molecules of thyroglobulin by iodine-tyrosine attach. The thyroxine (T4) is secreted in a higher amount and is converted into triiodothyronine (T3). Once T4 and T3 are produced, they give negative feedback to the pituitary and the hypothalamus, decreasing the production of TSH and TRH. Reduced TSH lead to reduced TSH and those levels reduce the thyroid hormone production.

Anomalies in the physiology or anatomy or the thyroid gland, thyroid surgery or some medications will result in thyroid disease; as a result, the production of thyroid hormones will be affected in several ways. Depending on the thyroid hormones, TRH, and TSH produced and circulating in the blood, and the cause of the problem, symptoms will appear correlating the lab values with the pathologic stage. High levels of thyroid hormones can be noticed in the person that shows some of these symptoms: anxiety, nervousness, insomnia, tremors in hands, bulging of the eyes, weight loss, palpitations, and intolerance to the heat; this situation is called hyperthyroidism.

Low thyroid hormone is called hypothyroidism. Symptoms of hypothyroidism are tiredness, fatigue, slow heart rate, weight gain, constipation, hair loss, dry skin, muscle weakness, muscle aches, joint pain, hoarseness, heavy menstrual bleeding, cold intolerance, high cholesterol level, infertility, depression, and impaired memory.

There are different clinical types of hypothyroidism, depending on the causes, the physiology and the hormones affected. Primary hypothyroidism is a state when the gland does not produce the right amount of thyroid hormones. The pituitary gland produces more TSH trying to stimulate the production of T4 and T3. In secondary hypothyroidism or central hypothyroidism, the hypothalamus does not generate enough TRH or the pituitary gland does not produce enough TSH, there is not adequate thyroid gland stimulation and thyroid hormones will be not created or will be produced deficiently. The subclinical hypothyroidism is a stage that not always shows signs and symptoms. It may be caused by Hashimoto’s thyroiditis.

Diagnosis

TSH and thyroid hormones are levels obtained, at least, annually in older women, because of the rate that shows that hypothyroidism or thyroid disease is more frequent in this group of the population. Expecting women or women planning to get pregnant should be screened too. When patients present symptoms related to the gland-related illness such as tiredness, dry skin, constipation, weight gain, increased heart rate, the tests should also be performed. The other group of people that must be tested yearly is those with previous thyroid problems and the people with thyroid hormone replacement as it is explained later.

The laboratory tests used to check on the thyroid gland function and its abnormalities are the TSH, T3 and the Thyroxine hormone (t4). TSH level screen is the best test because of its sensitivity. TSH can be affected and show variations even when symptoms of thyroid disease are not present yet. Determining T3 levels is useful for hyperthyroidism diagnosis and management. Combining TSH test with T4 (free) is the way to study the thyroid gland function. A normal function of the thyroid gland will have normal levels of TSH and T4.

In primary hypothyroidism T4 levels are low, and the TSH is elevated, trying to stimulate the thyroid hormones production. Central hypothyroidism will be diagnosed with low TSH and low T4. In Subclinical hypothyroidism, the lab test will show elevated TSH and T4 will be within the range.

Other tests performed in the presence of thyroid disease, are thyroid peroxidase and thyroglobulin; these are thyroid antibodies that can help diagnose the cause of the thyroid problem. In a hypothyroid person, with anti-thyroid peroxidase or anti-thyroglobulin antibodies positive, Hashimoto’s thyroiditis can be diagnosed, but, if the antibodies are positive in somebody with hyperthyroidism, the diagnosis will be autoimmune thyroid disease.

Radioactive iodine intake by the thyroid gland can help to determine if the thyroid is functioning correctly. The person is asked to swallow a small amount of iodine, and pictures of the thyroid will be taken. A very high uptake of the radioactive iodine shows that we are in the presence of hyperthyroidism, and low iodine uptake means hypothyroidism.

Lab tests are the way to determine the thyroid gland functioning, and depending on the results combining, we can get information about the thyroid and the pituitary glands. Once the results show if we are in the presence of primary, secondary or subclinical hypothyroidism, more test should be performed to determine the cause of that pathology. Even when measuring T4 is a useful way to get information about hypothyroidism, the first choice test to diagnose and control this disease is measuring the blood levels of thyroid-stimulating hormone (TSH). TSH will not only show the thyroid or pituitary gland pathology but is also, the way to determine the right medication to be given to a patient, to check the response of the body to the treatment.

The Levothyroxine (synthetic replacement of the T4) will be given daily. After three months of use, TSH should be determined, and if it still shows anomalies, a dosage adjustment must be considered. After two or three periodic TSH normal level obtained, the test will be performed twice or once a year over time as a routine. If the patient starts showing thyroid disease symptoms, the lab work must be done again.

References:

1. Becker, K. L. (Ed.). (2001). Principles and practice of endocrinology and metabolism. Lippincott Williams & Wilkins.