Diagnosis of Underactive Thyroid (Hypothyroidism)

Diagnosis of hypothyroidism can be established based on the results of tests done to measure the levels of these hormones in the blood. Commonly T4 and TSH in blood are measured. Since hypothyroidism is about deficiency or low levels of thyroid hormones, T4 (and T3) levels in the blood are below normal. TSH levels are high in case of Primary hypothyroidism and this can be explained as follows. Since the thyroid is not able to produce adequate amounts of T4 and T3, the pituitary senses this and increases TSH (Thyroid stimulating hormone) production which stimulates the thyroid to increase T4and T3 production.

In case of secondary hypothyroidism, the pituitary gland itself fails to produce enough TSH and hence the levels of TSH as well asT4 and T3 will be below normal in the blood.

Some important points to remember are:

• Symptoms may not always correspond to the severity of the lab test abnormalities i.e. some patients may have severe hypothyroidism as revealed by their laboratory values but they may have very mild symptoms.
• On the other hand, some patients may not have significantly abnormal values but they may be having significant symptoms.
• It is important to remember that the treatment must not only correct the laboratory values but also make the patient feel better.
• During the initial stages of Hypothyroidism, increased conversion of T4 to T3 occurs and hence T3 levels are maintained but T4 levels may be low to normal.

In order to diagnose Hashimoto's Thyroiditis (as an etiology of primary thyroid failure), Anti-TPO antibody testing may be done. The Anti-TPO Ab which was previously referred to as the Antimicrosomal antibody (AMA) is a useful marker for the diagnosis and management of autoimmune thyroid disease.

Ultrasonography (USG) of the neck and thyroid can be used to detect nodules and infiltrative disease.