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
- 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 Findings
- Common symptoms that prompt testing include fatigue, cold intolerance, weight gain, dry skin, constipation, and menstrual irregularities
- Physical findings might include bradycardia, delayed reflexes, and thyroid enlargement
Treatment
- 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 levels every 6-8 weeks until stabilized, as recommended by the USPSTF 1
- The goal is to normalize TSH levels, usually to 0.5-2.5 mIU/L, which restores normal thyroid hormone function throughout the body and alleviates symptoms.
From the Research
Diagnostic Criteria for Hypothyroidism
The diagnosis of hypothyroidism is often missed due to its non-specific signs and symptoms 2. The following are the diagnostic criteria for hypothyroidism:
- Serum thyroid-stimulating hormone (TSH) measurement 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
- Physical examination signs such as coarse skin, puffy face, slow movements, bradycardia, pretibial oedema, and ankle reflex may be present, but no single sign can easily discriminate a euthyroid from a hypothyroid patient 2
- The combination of signs that had the highest likelihood ratios (coarse skin, bradycardia, and delayed ankle reflex) was associated with modest accuracy, but clinicians cannot rely exclusively on physical examination to confirm or rule out hypothyroidism 2
Laboratory Testing
Laboratory testing is necessary to confirm the diagnosis of hypothyroidism. The following tests are used:
- Thyroid-stimulating hormone (TSH) measurement 3
- Free thyroxine (FT4) levels 3, 4
- Thyrotropine concentrations [TSH] 4
Treatment and Management
The treatment and management of hypothyroidism involve:
- Oral L-thyroxine as the treatment of choice 3
- Therapy goals are symptom relief and maintenance of serum TSH levels within the reference range 3
- Levothyroxine doses should be optimized aiming for a TSH in the 0.3-2.0 mU/L range for 3 to 6 months before a therapeutic response can be assessed 5
- In some patients, a trial of liothyronine/levothyroxine combined therapy may be warranted 5