Thyroid Blood Tests

Medical Revising Author: Ruchi Mathur, M.D.
Medical Revising Editor: William C. Shiel Jr, MD, FACP, FACR

• Introduction
• What are thyroid hormones?
• Thyroid hormone regulation -- the chain of command
• How is hypothyroidism diagnosed?
• How is hyperthyroidism diagnosed?
• Are there other tests of the thyroid gland?

Introduction

The thyroid gland produces hormones that are essential for normal body metabolism. Blood testing is now commonly available to determine the adequacy of the levels of thyroid hormones. These blood tests can define whether the thyroid gland's hormone production is normal, overactive, or underactive.

What are thyroid hormones?

Thyroid hormones are produced by the thyroid gland. This gland is located in the lower part of the neck, below the Adam's apple. The gland wraps around the windpipe (trachea) and has a shape that is similar to a butterfly - formed by two wings (lobes) and attached by a middle part (isthmus).

The thyroid gland uses iodine (mostly available from the diet in foods such as seafood, bread, and salt) to produce thyroid hormones. The two most important thyroid hormones are thyroxine (T4) and triiodothyronine (T3), which account for 99.9% and 0.1% of thyroid hormones present in the blood respectively. However, the hormone with the most biological activity is T3. Once released from the thyroid gland into the blood, a large amount of T4 is converted into T3 - the active hormone that affects the metabolism of cells.

Next: Thyroid hormone regulation -- the chain of command »

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Last Editorial Review: 10/18/2007