Tests to diagnose Hypothyroidism

Blood tests. The most sensitive test is the measurement of thyrotropin or thyroid-stimulating hormone (TSH). High TSH levels generally indicate hypothyroidism. The next test is to determine the active circulating thyroxine, free thyroxine (T4L), which is low in hypothyroidism, although in some cases it may be normal and indicate non-symptomatic hypothyroidism. If the levels of TSH and T4L are low it means that the thyroid is functioning correctly and the hypothyroidism is caused by a lack of TSH. This is because when the pituitary gland detects low levels of T4L, it is not able to boost the release of TSH, so it does not stimulate the thyroid to produce more hormones, causing hypothyroidism in the patient.  

General biochemical parameters such as cholesterol, liver transaminases or blood sugar (glycaemia) may present transient variations in the course of the disease because the thyroid gland is the main controller of the metabolism.  

To determine whether the disease is an autoimmune disease, caused by the immune system overreacting against the thyroid, the presence of "anti-thyroid" antibodies is tested: anti-peroxidase (anti-TPO) and anti-thyroglobulin (anti-TG) antibodies. 

Imaging tests. These are usually used to study the shape of any nodules or goitre associated with hypothyroidism, but not for hypothyroidism itself.