From the Guidelines
Subclinical hypothyroidism is characterized by elevated thyroid-stimulating hormone (TSH) levels with normal free thyroxine (T4) levels, typically with TSH values ranging between 4.5-10 mIU/L.
Key Laboratory Findings
- TSH levels are elevated, usually between 4.5-10 mIU/L, though lab reference ranges may vary slightly 1.
- Free T4 levels remain within normal limits.
- Free triiodothyronine (T3) levels are also usually normal.
- Anti-thyroid peroxidase (anti-TPO) antibodies may be present in many cases, indicating an autoimmune etiology like Hashimoto's thyroiditis.
- Complete blood count and metabolic panels are generally normal, though some patients might show mildly elevated cholesterol levels.
Monitoring and Treatment
- Monitoring involves checking TSH and free T4 every 6-12 months, as 2-5% of cases progress to overt hypothyroidism annually 1.
- Treatment with levothyroxine is typically considered when TSH exceeds 10 mIU/L, or at lower levels if the patient has symptoms, positive antibodies, or cardiovascular risk factors.
- The goal of treatment, when initiated, is to maintain TSH within the reference range, usually between 0.5-4.0 mIU/L. This condition represents early thyroid dysfunction where the pituitary is compensating for subtle thyroid hormone deficiency by increasing TSH production, while still maintaining adequate circulating thyroid hormone levels 1.
From the Research
Laboratory Findings for 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.
- 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 3.
- Serum TSH levels are used to diagnose and monitor subclinical hypothyroidism, with a target range of 0.5-2.0 mIU/L for primary hypothyroidism 2.
- In patients with central hypothyroidism, treatment is tailored according to free or total T4 levels, which should be maintained in the upper half of the normal range for age 2.
TSH Levels and Treatment
- Treatment is generally not necessary unless the TSH exceeds 7.0-10 mIU/L 3.
- TSH goals are age-dependent, with a 97.5 percentile (upper limit of normal) of 3.6 mIU/L for patients under age 40, and 7.5 mIU/L for patients over age 80 3.
- Patients with subclinical hypothyroidism and serum TSH levels of 10 mU/L or higher, or young and middle-aged individuals with subclinical hypothyroidism and symptoms consistent with mild hypothyroidism, may benefit from treatment 4.
Special Considerations
- Macro-thyroid-stimulating hormone (m-TSH) is a rare cause of subclinical hypothyroidism, and should be considered in patients with elevated TSH levels without hypothyroidism symptoms or who require a higher levothyroxine replacement dose than expected to make them euthyroid 5.
- The presence of circulating thyroid peroxidase antibodies increases the risk of progression from subclinical to overt hypothyroidism 4.
- Subclinical hypothyroidism may be associated with an increased risk of heart failure, coronary artery disease events, and mortality from coronary heart disease, as well as cognitive impairment and nonspecific symptoms such as fatigue and altered mood 4.