What is the recommended initial thyroid function test for patients with possible thyroid disorders?

The thyroid-stimulating hormone (TSH) test is considered the recommended initial test for evaluating patients with suspected thyroid disorders because it is a highly sensitive marker of thyroid function. TSH is produced by the pituitary gland and regulates the production of thyroid hormones by the thyroid gland. When the thyroid is underactive (hypothyroidism), TSH levels typically rise due to the pituitary's attempt to stimulate hormone production. Conversely, in hyperthyroidism or in conditions where there is excess thyroid hormone, TSH levels generally decrease.

By measuring TSH first, healthcare providers can effectively assess the overall function of the thyroid gland and differentiate between different types of thyroid dysfunction. If TSH is abnormal, further testing of free T4 and possibly T3 can help to determine the specific nature and cause of the thyroid disorder. This sequential approach, starting with TSH, is endorsed by various clinical guidelines and is more cost-effective than directly measuring thyroid hormones initially.