From the Guidelines
Primary hypothyroidism is characterized by elevated thyroid-stimulating hormone (TSH) levels above 4.5-5.0 mIU/L, often reaching values of 10-100 mIU/L in severe cases, accompanied by decreased free thyroxine (T4) levels below 0.8-1.0 ng/dL. The diagnosis of primary hypothyroidism is based on laboratory findings, including TSH and free T4 levels.
Key Laboratory Findings:
- Elevated TSH levels: above 4.5-5.0 mIU/L, often reaching values of 10-100 mIU/L in severe cases
- Decreased free T4 levels: below 0.8-1.0 ng/dL
- Free triiodothyronine (T3) may also be low but is less reliable for diagnosis
- Anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin antibodies are frequently elevated in autoimmune hypothyroidism (Hashimoto's thyroiditis), the most common cause
- Other laboratory findings may include:
- Elevated total cholesterol, LDL cholesterol, and triglycerides
- Mild anemia (normocytic or macrocytic)
- Hyponatremia
- Elevated creatine kinase
- Elevated prolactin These abnormal values reflect the widespread effects of thyroid hormone deficiency on metabolism, protein synthesis, and multiple organ systems, as noted in studies such as 1 and 1.
Treatment and Monitoring:
Treatment with levothyroxine replacement therapy should normalize TSH and free T4 levels over time, with TSH being the primary marker used to monitor treatment adequacy, as discussed in 1 and 1. It is essential to note that the decision to start therapy in patients with TSH levels between 4.5 and 10.0 mIU/L is more controversial, and a large magnitude of overdiagnosis and overtreatment is a likely consequence of screening for thyroid dysfunction, as highlighted in 1 and 1. Therefore, it is crucial to carefully evaluate the laboratory findings and clinical symptoms before initiating treatment, considering the potential harms associated with antithyroid medications, ablation therapy, and long-term thyroid hormone therapy, as mentioned in 1 and 1.
From the Research
Typical Lab Values in Primary Hypothyroidism
The typical lab values in primary hypothyroidism are characterized by:
- Elevated thyroid-stimulating hormone (TSH) concentration 2, 3
- Reduced serum free thyroxine (fT4) and triiodothyronine (fT3) levels in manifest hypothyroidism 2, 3
- Normal serum fT4 and fT3 levels in subclinical hypothyroidism, with elevated TSH 2, 3
- The biochemical treatment goal for T4 replacement in primary hypothyroidism is a TSH level within the reference range (0.4-4.0 mU/l) 2
Classification of Hypothyroidism
Hypothyroidism can be classified as:
- Primary hypothyroidism: defined as elevated TSH concentration in combination with free thyroxine (fT4) concentration below the reference range 3
- Subclinical hypothyroidism: defined by elevated TSH concentrations but fT4 concentrations within the reference range 3
- Central or peripheral hypothyroidism: based on pathology in the pituitary or hypothalamus, or peripheral tissue, respectively 3
Treatment of Primary Hypothyroidism
The treatment of primary hypothyroidism typically involves:
- Levothyroxine (T4) monotherapy as the standard of care 2
- Initiation of levothyroxine therapy based on the level of TSH elevation, as well as other factors such as patient age, presence of pregnancy or comorbidities 2
- Adjustment of thyroid hormone dosage based on serum TSH levels in primary hypothyroidism, and serum fT4 levels in secondary hypothyroidism 2