From the Guidelines
The gold standard for diagnosing hypothyroidism is a blood test measuring thyroid hormone levels, specifically thyroid-stimulating hormone (TSH) and free thyroxine (T4), as it is highly reliable and allows for precise monitoring of thyroid function over time. This diagnostic approach is preferred because it's minimally invasive and highly accurate. An elevated TSH level combined with a low free T4 level confirms primary hypothyroidism 1. The test is typically performed after fasting overnight, though this isn't strictly necessary. Normal TSH ranges from approximately 0.4 to 4.0 mIU/L, with levels above this range suggesting hypothyroidism.
Key Points to Consider
- 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 1.
- The optimal screening interval for thyroid dysfunction is unknown, but the USPSTF recommends measuring thyroid function in all adults beginning at age 35 years and every 5 years thereafter, noting that more frequent screening may be appropriate in high-risk or symptomatic individuals 1.
- Once diagnosed, treatment typically involves thyroid hormone replacement with levothyroxine, starting at doses of 25-50 mcg daily for mild cases or elderly patients, and 75-100 mcg daily for most adults, with adjustments based on follow-up TSH testing every 6-8 weeks until levels normalize.
Important Considerations
- The most common cause of hypothyroidism in the United States is chronic autoimmune (Hashimoto) thyroiditis, and risk factors for an elevated TSH level include female sex, advancing age, white race, type 1 diabetes, Down syndrome, family history of thyroid disease, goiter, previous hyperthyroidism, and external-beam radiation in the head and neck area 1.
- The USPSTF found no direct evidence that treatment of thyroid dysfunction based on risk level alters final health outcomes, but treatment is generally recommended for patients with a TSH level that is clearly abnormal.
From the Research
Diagnosis of Hypothyroidism
The diagnosis of hypothyroidism is primarily based on clinical signs and symptoms as well as measurement of thyroid-stimulating hormone (TSH) concentration 2.
- Clinical signs and symptoms include fatigue, bradycardia, constipation, and cold intolerance.
- Measurement of TSH concentration is the best diagnostic test, with an elevated TSH level almost always signaling primary hypothyroidism 3.
- Serum free thyroxine levels may be below the reference range (overt hypothyroidism) or within the reference range (subclinical hypothyroidism) 3.
Gold Standard for Diagnosing Hypothyroidism
The gold standard for diagnosing hypothyroidism is based on the measurement of TSH and free thyroxine (FT4) levels in serum 4.
- In primary hypothyroidism, the serum content of FT4 is low and that of TSH is high.
- In central hypothyroidism, the serum content of FT4 is low and that of TSH is generally low or normal.
- Subclinical hypothyroidism is characterized by a normal serum level of FT4, an increased level of TSH, and the absence of clinical symptoms.
Laboratory Testing
Laboratory testing is essential for the diagnosis of hypothyroidism, as the clinical features are often nonspecific 5, 3.
- Serum TSH measurement is the best diagnostic test.
- Serum free thyroxine levels may be measured to confirm the diagnosis.
- Thyroid peroxidase antibody may be elevated in some cases of hypothyroidism 5.