From the Guidelines
Subclinical hypothyroidism is defined as a serum TSH concentration above the statistically defined upper limit of the reference range when serum free T4 (FT4) concentration is within its reference range. This condition is characterized by mildly elevated TSH levels and normal levels of thyroid hormones (T3 and T4) without any specific clinical signs or symptoms of thyroid disease [ 1 ]. The diagnosis of subclinical hypothyroidism is made by excluding other causes of an elevated serum TSH, such as recent adjustments in levothyroxine dosage, transient increase in serum TSH in hospitalized patients, or the presence of heterophilic antibodies against mouse proteins [ 1 ].
Key Characteristics
- Slightly elevated TSH levels
- Normal levels of thyroid hormones (T3 and T4)
- No specific clinical signs or symptoms of thyroid disease
- Exclusion of other causes of elevated serum TSH
Treatment Considerations
Treatment decisions for subclinical hypothyroidism depend on several factors, including TSH level, age, and presence of symptoms. For TSH levels between 4.5-10 mIU/L without symptoms, doctors often recommend monitoring rather than immediate medication [ 1 ]. However, if TSH exceeds 10 mIU/L or if symptoms are present, levothyroxine treatment may be prescribed, typically starting at 25-50 mcg daily for older adults or those with heart disease, and 50-75 mcg daily for younger patients [ 1 ].
Monitoring and Progression
Regular monitoring is essential as subclinical hypothyroidism may progress to overt hypothyroidism in some cases, particularly in those with thyroid antibodies or higher initial TSH levels [ 1 ]. The condition occurs because the thyroid gland begins to fail, requiring the pituitary gland to produce more TSH to stimulate adequate thyroid hormone production. Approximately 2% to 5% of patients with subclinical hypothyroidism will progress to overt hypothyroidism per year [ 1 ].
From the Research
Definition of Subclinical Hypothyroidism
- Subclinical hypothyroidism is defined as an elevated thyroid-stimulating hormone (TSH) concentration in the presence of normal serum free thyroxine (T4) and triiodothyronine (T3) concentrations 2.
- It is a grade of primary hypothyroidism where the thyroid gland does not produce enough thyroid hormone, but the levels of T4 and T3 are still within the normal range 2.
- Subclinical hypothyroidism can be caused by autoimmune thyroiditis, such as Hashimoto's thyroiditis, and can also occur in elderly patients without thyroid disease 3.
Diagnosis and Treatment
- The diagnosis of subclinical hypothyroidism should be confirmed by repeat thyroid function tests, ideally obtained at least 2 months later, as 62% of elevated TSH levels may revert to normal spontaneously 4.
- Treatment is not necessary unless the TSH exceeds 7.0-10 mIU/L, and treatment does not improve symptoms or cognitive function if the TSH is less than 10 mIU/L 4.
- Levothyroxine treatment is indicated in patients with subclinical hypothyroidism and serum TSH levels of 10 mIU/L or higher, or for young and middle-aged individuals with subclinical hypothyroidism and symptoms consistent with mild hypothyroidism 5, 3.
- Treatment should be considered in symptomatic patients, patients with infertility, and patients with goitre or positive anti-thyroid peroxidase (TPO) antibodies 2.
Risks and Benefits of Treatment
- Levothyroxine treatment may be associated with iatrogenic thyrotoxicosis, especially in elderly patients, and there is no evidence that it is beneficial in persons aged 65 years or older 3.
- Treatment may reduce the risk of adverse cardiovascular events, such as heart failure and coronary artery disease, in patients with subclinical hypothyroidism 6, 3.
- The risks and benefits of treatment should be weighed on an individual basis, taking into account the patient's age, symptoms, and comorbidities 2, 5.