What tests are used to diagnose the causes of hypothyroidism?

From the Guidelines

The primary test used to diagnose the causes of hypothyroidism is the thyroid stimulating hormone (TSH) test, which is typically elevated in hypothyroidism, as stated in the USPSTF recommendation statement 1. To determine the specific cause of hypothyroidism, several tests are utilized, including:

• Free T4 (thyroxine) levels, which are usually low in hypothyroid patients
• Thyroid antibody tests, particularly thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb), to identify autoimmune thyroiditis (Hashimoto's disease), the most common cause of hypothyroidism
• Thyroid imaging studies like ultrasound to evaluate thyroid structure and identify nodules or abnormal tissue
• Radioactive iodine uptake tests to assess thyroid function and help distinguish between different causes A complete medical history and physical examination are also crucial parts of the diagnostic process, as they can reveal symptoms, risk factors, and physical signs of thyroid dysfunction, as noted in the USPSTF recommendation statement 1. The USPSTF found that the serum TSH test is the primary screening test for thyroid dysfunction, and multiple tests should be done over a 3- to 6-month interval to confirm or rule out abnormal findings, as stated in the recommendation statement 1. Additionally, follow-up testing of serum T4 levels in persons with persistently abnormal TSH levels can differentiate between subclinical (normal T4 levels) and “overt” (abnormal T4 levels) thyroid dysfunction, as noted in the USPSTF recommendation statement 1. It is essential to use these comprehensive tests to accurately diagnose hypothyroidism and determine its underlying cause, enabling appropriate treatment, as recommended by the USPSTF 1.

From the Research

Diagnosis of Hypothyroidism

To diagnose the causes of hypothyroidism, several tests can be used, including:

• Analysis of the concentrations of free thyroxine (T4) and thyroid-stimulating hormone (TSH) in serum 2, 3, 4
• Thyroid hormone assays (TSH and FT4) to confirm the diagnosis of hypothyroidism 5
• First-line combined TSH and FT4 testing to detect central hypothyroidism 6

Types of Hypothyroidism

There are different types of hypothyroidism, including:

• Primary hypothyroidism, caused by disease or treatment that destroys the thyroid gland or interferes with thyroid hormone biosynthesis 2
• Secondary or central hypothyroidism, caused by a lack of TSH or TSH activity due to a pituitary or hypothalamic cause 2
• Subclinical hypothyroidism, characterized by a normal serum level of T4, an increased level of TSH, and the absence of clinical symptoms 2, 4

Laboratory Tests

Laboratory tests used to diagnose hypothyroidism include:

• TSH test to measure the level of TSH in the blood 2, 3, 4
• Free T4 (FT4) test to measure the level of FT4 in the blood 2, 3, 4
• Thyroid hormone assays (TSH and FT4) to confirm the diagnosis of hypothyroidism 5

Clinical Evaluation

Clinical evaluation is also important in the diagnosis of hypothyroidism, including:

• Physical examination to look for signs and symptoms of hypothyroidism, such as coarse skin, puffy face, slow movements, bradycardia, and pretibial edema 5
• Medical history to determine the presence of any underlying conditions that may be causing hypothyroidism 3, 4