From the Guidelines
Subclinical hypothyroidism is defined as an asymptomatic condition in which a patient has a serum thyroid-stimulating hormone (TSH) level exceeding the upper threshold of a specified laboratory reference interval (commonly but arbitrarily defined as 4.5 mIU/L) but a normal thyroxine (T4) level. This condition represents an early stage of thyroid failure where the pituitary gland increases TSH production to stimulate the thyroid to maintain normal hormone levels, as noted in the study by 1. Patients typically have TSH values between 4.5-10 mIU/L with normal free T4 levels. Most people with subclinical hypothyroidism don't experience symptoms, though some may report fatigue, mild depression, or other subtle symptoms associated with hypothyroidism.
Key Characteristics of Subclinical Hypothyroidism
- Elevated TSH levels above 4.5 mIU/L
- Normal thyroxine (T4) levels
- Often asymptomatic, but may include subtle symptoms like fatigue or mild depression
- More common in women, older adults, and those with autoimmune disorders, as discussed in 1 and 1
Importance of Accurate Diagnosis
Accurate diagnosis of subclinical hypothyroidism is crucial, as it can progress to overt hypothyroidism in some cases, particularly those with thyroid antibodies. Regular monitoring with TSH tests every 6-12 months is important for early detection and treatment, as recommended by 1. Treatment decisions are individualized, with levothyroxine therapy generally recommended for those with TSH levels above 10 mIU/L, those with symptoms, pregnant women, women planning pregnancy, or patients with positive thyroid antibodies.
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, 3, 4, 5.
- It is also referred to as compensated hypothyroidism or mild hypothyroidism, and is characterized by a raised serum concentration of TSH but a normal serum free thyroxine (FT4) 6.
- The condition can be primary (abnormality in thyroid gland itself) or secondary/central (as a result of hypothalamic or pituitary disease) 2.
Thyroid-Stimulating Hormone (TSH) Regulation
- Subclinical hypothyroidism is associated with an elevated TSH level, which can progress to overt hypothyroidism in approximately 2-5% cases annually 2.
- The risk of progression to overt hypothyroidism is higher in patients with thyroid autoantibodies and higher TSH levels 6.
- The TSH level is used to monitor treatment in primary hypothyroidism, with a target of 0.5-2.0 mIU/L 2.
Classification of Subclinical Hypothyroidism
- Subclinical hypothyroidism can be classified into a milder condition with TSH levels between 4.0 and 10.0 milli-international units (mIU)/l (mild-SCH) and a severe form with TSH >10.0 mIU/l (severe-SCH) 4.
- The classification is important for determining the risk of progression to overt hypothyroidism and the need for treatment 4.