What are the diagnostic criteria for hypothyroidism (underactive Thyroid (thyroid gland))?

From the Guidelines

Hypothyroidism is diagnosed through a combination of blood tests, clinical symptoms, and physical examination, with the primary test being the measurement of thyroid stimulating hormone (TSH) levels, which are typically elevated in hypothyroidism, as defined by a TSH level exceeding the upper threshold of a specified laboratory reference interval, commonly 4.5 mIU/L, and a normal thyroxine (T4) level 1. The diagnostic criteria for hypothyroidism involve biochemical changes, including an elevated TSH level and a low T4 level, which may or may not be associated with clinical symptoms such as fatigue, feeling cold, weight gain, hair loss, and constipation.

Key Diagnostic Tests

• Measuring TSH levels, which is the primary screening test for thyroid dysfunction
• Free T4 (thyroxine) levels, which are usually low or at the lower end of normal range in hypothyroidism
• Additional tests, such as free T3 (triiodothyronine) and thyroid antibodies like thyroid peroxidase antibodies (TPOAb), to identify autoimmune causes like Hashimoto's thyroiditis

Clinical Symptoms and Physical Examination

• Common symptoms that prompt testing include fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, and menstrual irregularities
• Physical examination may reveal slow heart rate, delayed reflexes, puffy face, or enlarged thyroid gland (goiter)

Diagnosis and Classification

• Subclinical hypothyroidism is defined as an asymptomatic condition with a serum TSH level exceeding the upper threshold of a specified laboratory reference interval, but a normal T4 level
• Overt hypothyroidism is defined biochemically by an elevated TSH level and a low T4 level, and may or may not be associated with clinical symptoms
• The USPSTF recommends using the serum TSH test as the primary screening test for thyroid dysfunction, with multiple tests done over a 3- to 6-month interval to confirm or rule out abnormal findings 1.

From the FDA Drug Label

The general aim of therapy is to normalize the serum TSH level TSH may not normalize in some patients due to in utero hypothyroidism causing a resetting of pituitary-thyroid feedback. Failure of the serum T4 to increase into the upper half of the normal range within 2 weeks of initiation of levothyroxine sodium therapy and/or of the serum TSH to decrease below 20 IU per litre within 4 weeks may indicate the patient is not receiving adequate therapy Monitor serum TSH levels after an interval of 6 to 8 weeks after any change in dosage. In patients on a stable and appropriate replacement dosage, evaluate clinical and biochemical response every 6 to 12 months and whenever there is a change in the patient’s clinical status In pregnant patients with primary hypothyroidism, maintain serum TSH in the trimester-specific reference range New onset hypothyroidism (TSH ≥10 IU per liter) New onset hypothyroidism (TSH < 10 IU per liter)

The diagnostic criteria for hypothyroidism are not explicitly stated in the provided drug label. However, the label mentions that hypothyroidism is diagnosed based on serum TSH levels and free-T4 levels.

• Serum TSH levels are used to assess the adequacy of therapy and to diagnose hypothyroidism.
• Free-T4 levels are used to assess the adequacy of therapy in patients with secondary and tertiary hypothyroidism.
• Trimester-specific reference range is used to diagnose and manage hypothyroidism in pregnant patients.
• TSH ≥10 IU per liter and TSH < 10 IU per liter are mentioned as criteria for new onset hypothyroidism. However, the label does not provide a clear and direct answer to the question, and the information provided is more related to the management and treatment of hypothyroidism rather than its diagnosis 2.

From the Research

Diagnostic Criteria for Hypothyroidism

The diagnosis of hypothyroidism is based on laboratory tests that measure the levels of thyroid hormones (T3 and T4) in the blood 3. The following are some key points to consider:

• Serum thyroid-stimulating hormone (TSH) measurement is the best diagnostic test; an elevated TSH level almost always signals primary hypothyroidism 4.
• Serum free thyroxine levels may be below the reference range (overt hypothyroidism) or within the reference range (subclinical hypothyroidism) 4.
• Common symptoms of hypothyroidism include fatigue, weight gain, increased sensitivity to cold, irregular bowel movements, and dry skin 3.
• Physical examination alone is not sufficient to confirm or rule out hypothyroidism, as no single sign can easily discriminate a euthyroid from a hypothyroid patient 5.

Laboratory Tests

The following laboratory tests are used to diagnose hypothyroidism:

• Thyroid-stimulating hormone (TSH) measurement
• Free thyroxine (FT4) measurement
• Free triiodothyronine (FT3) measurement These tests help to determine the levels of thyroid hormones in the blood and diagnose hypothyroidism 3, 4.

Diagnostic Challenges

The diagnosis of hypothyroidism can be challenging due to the non-specific nature of its symptoms, which can overlap with other conditions 3, 5. Additionally, some patients with hypothyroidism may not exhibit typical signs and symptoms, making diagnosis more difficult 5. Therefore, laboratory tests are essential for an accurate diagnosis.