Thyroid gland

The thyroid is an endocrine gland in the neck. It secretes thyroid hormones, which regulate the body's rate of metabolism. The production of thyroid hormones is stimulated by thyrotropin or Thyroid Stimulating Hormone (TSH), which is produced by the pituitary.

Types of thyroid hormones

Triiodothyronine and thyroxine are the two main forms of thyroid hormones. Comprised primarily of iodine, they act on nearly every cell in the body to regulate the basal metabolic rate.

Thyroxine (T4)

Thyroxine (abbreviate 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." (cite: https://www.frontiersin.org/articles/10.3389/fendo.2018.00097/full)

enzyme a substance produced by a living organism which acts as a catalyst to bring about a specific biochemical reaction.

metabolomics The analysis of the chemical metabolism within cells, tissues or organisms. The term is often used to refer to the full set of metabolites found in a cell in a given environment.