Ruchi Mathur, MD, FRCP(C) is an Attending Physician with the Division of Endocrinology, Diabetes and Metabolism and Associate Director of Clinical Research, Recruitment and Phenotyping with the Center for Androgen Related Disorders, Department of Obstetrics and Gynecology at Cedars-Sinai Medical Center.
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
is a condition characterized by abnormally low thyroid
hormone production. There are many disorders that result in
hypothyroidism. These disorders may directly or indirectly involve the thyroid
gland. Because thyroid hormone affects growth, development, and many cellular processes, inadequate thyroid hormone has widespread consequences for the body.
This article will focus specifically on hypothyroidism in adults.
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% and 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.
Thyroid hormone regulation- the chain of command
The thyroid itself is regulated by another gland that is located in
the brain, called the pituitary. In turn, the pituitary is regulated in part by the thyroid
(via a "feedback" effect of thyroid hormone on the pituitary gland) and by another gland called
The hypothalamus releases a hormone called thyrotropin releasing hormone (TRH), which sends a
signal to the pituitary to release thyroid stimulating hormone (TSH). In turn, TSH sends a
signal to the thyroid to release thyroid hormones. If a disruption occurs at any of these
levels, a defect in thyroid hormone production may result in a deficiency of thyroid hormone
Hypothalamus - TRH
Thyroid- T4 and T3
The rate of thyroid hormone production is controlled by the pituitary gland. If there is an
insufficient amount of thyroid hormone circulating in the body to allow for normal functioning,
the release of TSH is increased by the pituitary gland in an attempt to stimulate more thyroid
hormone production. In contrast, when there is an excessive amount of circulating thyroid
hormone, TSH levels fall as the pituitary attempts to decrease the production of thyroid hormone.
In persons with hypothyroidism, there is a persistent low level of circulating thyroid
Medical Author: Ruchi Mathur, MD Medical Editor: William C. Shiel, Jr, MD, FACP, FACR
Many of my patients ask questions or make comments about iodine use in thyroid disease. Examples are: " Should I increase the iodine in my diet if I'm hypothyroid?" "My mother had hyperthyroidism, and they told her it was because of a lack of iodine." " My sister has thyroid disease, and to avoid getting it, I'm taking Kelp tablets." "Can I eat sushi if I take Synthroid?"
Although these questions and comments are relevant, this subject is peppered with "old wives tales" and folklore. In the first part of this discussion, I'd like to focus on the role iodine in relation to the thyroid gland and its function.... a bit of physiologyfor the beginner. Later, I will discuss how an excess or deficiency of iodine can contribute to diseases of the thyroid gland.