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 in hypothyroid patients
• Additional tests, such as free T3 (triiodothyronine) and thyroid antibodies like thyroid peroxidase antibodies (TPOAb), to determine if the cause is autoimmune (Hashimoto's thyroiditis)

Clinical Symptoms and Physical Examination

• Common symptoms that prompt testing include fatigue, weight gain, cold intolerance, constipation, dry skin, and hair loss
• Physical examination may reveal bradycardia, delayed reflexes, or thyroid enlargement

Treatment and Monitoring

• Treatment typically involves thyroid hormone replacement with levothyroxine, with dose adjustments based on TSH levels measured 6-8 weeks after starting therapy
• The goal is to normalize TSH levels, usually to 0.5-4.0 mIU/L, as recommended by the USPSTF 1
• Regular monitoring is essential, with TSH checks every 6-12 months once stable, and medication should be taken on an empty stomach, 30-60 minutes before breakfast, and separated from calcium, iron supplements, and certain medications that can interfere with absorption.

Risk Factors

• The most common cause of hypothyroidism in the United States is chronic autoimmune (Hashimoto) thyroiditis
• 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

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.