Hypothyroidism is a condition of the thyroid gland that affects the production of thyroid hormones, important hormones in the regulation of metabolism. The symptoms of this condition are non-specific and develop slowly, meaning they are often missed or confused with symptoms of other conditions. Hypothyroidism therefore sometimes fails to be diagnosed for years.

Some of the signs and symptoms of hypothyroidism include the folowing:

• Weakness
• Lethargy
• Sensitivity to cold
• Dry skin and hair
• Weight gain
• Muscle ache
• Constipation
• Loss of libido
• Depression

The only reliable way to determine whether a patient has hypothyroidism is to perform thyroid function tests that assess the levels of hormones produced by the thyroid gland. These tests are describe below.

• A thyroid function test involves assessing the level of thyroid stimulating hormone (TSH) and thyroxine (T4) in a patient’s blood.

TSH is a hormone released from the pituitary gland in the brain. This hormone stimulates the thyroid gland into secreting thyroxine and thyroid hormone. In hypothyroidism, the pituitary gland senses a deficiency in the hormones and secretes more TSH than usual. The levels of TSH are therefore high. The ultrasensitive TSH test is usually the first test performed to detect hypothyroidism and is the most accurate measure of thyroid activity currently available. If the levels of TSH are below normal, a hyperactive thyroid gland is indicated.

• Measuring the level of thyroxine (the prohormone of thyroid hormone) indicates the level of thyroid hormone circulating in the blood.
• If the TSH level is raised but the T4 level is low, the patient has an underactive thyroid.
• If the TSH is raised but the T4 level is normal, the patient is at risk of developing hypothyroidism and maybe asked to attend repeat blood tests, in case they eventually develop the condition.  
• A physician may also want to check for Hashimoto’s disease, an autoimmune condition where the immune system attacks the thyroid gland. A blood test is arranged to check for the presence of antibodies against an enzyme called thyroid peroxidase, which plays a key role in the production of thyroid hormones.