Thyroid hormone

Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4). They are primarily responsible for regulation of metabolism, and thus act on nearly every cell in the body.

Thyroid hormones are partially comprised of iodine and a deficiency of iodine in the diet can lead to decreased production of these hormones. If left uncorrected the thyroid tissue can enlarge, resulting in a pronounced swelling in the neck, called a 'simple goitre.'

Over production of thyroid hormones (hyperthyroidism) can accelerate your body's metabolism causing sudden weight loss, a rapid or irregular heartbeat, sweating, nervousness or irritability, and other symptoms. Under production of thyroid hormones (hypothyroidism) can lower your body's metabolism causing obesity, fatigue, muscle weakness, thinning hair, slowed heart rate, depression and other symptoms.

Thyroxine (T4)
Thyroxine (abbreviated as T4).

Triiodothyronine (T3)
Triiodothyronine (abbreviated as T3) is the active form of the hormone, and is converted from T4 within the cell by deiodinases, which are selenium-dependent enzymes. Selenium sufficiency is therefore important for the proper conversion of T4 to T3.

Hormone replacement
Patients with hypothyrodism may need hormone replacement. The standard replacement therapy is Levothyroxine, a synthetic form of T4. However, some patients on Levothyroxine report continuing symptoms of hypothyrodism despite normal laboratory results, owing to poor conversion of T4 to T3. In these cases, patients may add a T3 replacement hormone drug, such as Cytomel or Armour thyroid, which is whole desiccated thyroid glandular from pigs.

Role in Chronic Fatigue Syndrome
A study of 99 patients with a diagnosis of Chronic Fatigue Syndrome found similar levels of TSH to healthy controls but lower free triiodothyronine (FT3), total thyroxine (TT4), total triiodothyronine (TT3), %TT3 (4.7%), deiodinase activity, secretory capacity of the thyroid gland, lower 24-h urinary iodine (27.6%), and higher % reverse T3 (rT3). "Low circulating T3 and the apparent shift from T3 to rT3 may reflect more severely depressed tissue T3 levels. The present findings might be in line with recent metabolomic studies pointing at a hypometabolic state."

Notable studies

 * 2018, Higher Prevalence of “Low T3 Syndrome” in Patients With Chronic Fatigue Syndrome: A Case–Control Study Full Text)

Learn more

 * American Thyroid Association webpage