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 diagnostic test being measuring 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 are typically elevated in hypothyroidism
• Checking free T4 levels, which are usually low or at the lower end of normal range in hypothyroid patients
• Measuring free T3 levels for a more comprehensive assessment
• Additional testing such as thyroid antibodies (particularly thyroid peroxidase antibodies) to identify autoimmune thyroiditis (Hashimoto's disease)

Clinical Symptoms and Physical Examination

• Common symptoms that suggest hypothyroidism include fatigue, cold intolerance, weight gain, dry skin, constipation, and menstrual irregularities
• Physical examination findings might reveal bradycardia, delayed relaxation of reflexes, and possibly goiter

Diagnosis and Treatment

• The diagnosis of hypothyroidism is based on biochemical changes, including an elevated TSH level and a low T4 level, as well as clinical symptoms and physical examination findings 1.
• Treatment typically involves thyroid hormone replacement with levothyroxine, starting at doses of 1.6 mcg/kg/day for most adults, with lower initial doses for elderly patients or those with cardiac disease, as recommended by the USPSTF 1.
• Regular monitoring of TSH levels is necessary to adjust medication dosage appropriately, usually aiming for TSH levels within the reference range.

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 the following key points:

• Serum TSH levels are used to monitor the adequacy of therapy and diagnose hypothyroidism.
• Trimester-specific reference range for serum TSH is used in pregnant patients.
• New onset hypothyroidism is diagnosed based on TSH levels (≥10 IU per liter or < 10 IU per liter). It is essential to note that the label does not provide a clear definition of the diagnostic criteria for hypothyroidism, and the information provided is related to the treatment and monitoring of the condition rather than its diagnosis 2.

From the Research

Diagnostic Criteria for Hypothyroidism

The diagnostic criteria for hypothyroidism are based on blood levels of thyroid-stimulating hormone (TSH) and free thyroxine (FT4) 3, 4. The signs and symptoms of hypothyroidism, such as cold intolerance, fatigue, weight gain, dry skin, constipation, and voice changes, are nonspecific and nondiagnostic, especially early in disease presentation 3, 5.

Laboratory Testing

• Serum TSH measurement is the best diagnostic test for hypothyroidism, with an elevated TSH level almost always signaling primary hypothyroidism 4.
• Serum free thyroxine levels may be below the reference range (overt hypothyroidism) or within the reference range (subclinical hypothyroidism) 4.
• Thyroid hormone assays, including TSH and FT4, are necessary to confirm or rule out hypothyroidism, as physical examination alone is not sufficient 5.

Diagnosis and Classification

• Overt hypothyroidism is diagnosed when serum TSH levels are elevated and serum free thyroxine levels are below the reference range 4.
• Subclinical hypothyroidism is diagnosed when serum TSH levels are elevated, but serum free thyroxine levels are within the reference range 4.
• Myxedema coma is a life-threatening form of decompensated hypothyroidism that requires aggressive treatment with L-thyroxine replacement and other supportive measures 3, 4.