Hypothyroidism - Diagnosis

Diagnosis

The only validated test to diagnose primary hypothyroidism, is to measure thyroid-stimulating hormone (TSH) and free thyroxine (T4). However, these levels can be affected by non-thyroidal illnesses.

High levels of TSH indicate that the thyroid is not producing sufficient levels of thyroid hormone (mainly as thyroxine (T₄) and smaller amounts of triiodothyronine (T₃)). However, measuring just TSH fails to diagnose secondary and tertiary hypothyroidism, thus leading to the following suggested blood testing if the TSH is normal and hypothyroidism is still suspected:

• Free triiodothyronine (fT₃)
• Free thyroxine (fT₄)
• Total T₃
• Total T₄

Additionally, the following measurements may be needed:

• Free T₃ from 24-hour urine catch
• Antithyroid antibodies — for evidence of autoimmune diseases that may be damaging the thyroid gland
• Serum cholesterol — which may be elevated in hypothyroidism
• Prolactin — as a widely available test of pituitary function
• Testing for anemia, including ferritin
• Basal body temperature