Thyroid Replacement Therapy

The thyroid gland is part of the endocrine system. It is a hormone that produces, stores, and releases chemicals into the bloodstream. The hormones are then carried in the bloodstream to their target cells or organs in various parts of the body. The thyroid gland uses iodine from the foods we eat as a substrate to make two main hormones through which it exerts its effects on the body:

  1. Triiodothyronine (T3)
  2. Thyroxine (T4)

Among the two, T3 is considered the more active and stronger hormone. Thyroid hormones regulate the body’s metabolism, heart rate, and how fast the intestines process food. They also regulate the internal body temperature and cholesterol levels. The thyroid gland produces these hormones under the influence of other hormones secreted from the pituitary gland known as T3 and T4.

Under activity of the thyroid gland (T3 and T4 hormones are lower than normal) is hypothyroidism. Overactivity (the T3 and T4 hormones are higher than normal) is hyperthyroidism. The levels of TSH are generally inversely related, i.e they are high in hypothyroidism and low in hyperthyroidism.


Hypothyroidism is a serious disorder and if unchecked, may even be fatal. Since it takes a long time for the more serious complications to manifest, people with suspicious symptoms such as cold intolerance, unexplained weight gain, dry skin, depression etc should undergo testing and treatment initiated if found to have how hormone levels. Some of the most common symptoms of hypothyroidism include the following:

  1. Dry skin and hair
  2. Depression
  3. Joint and muscle pain
  4. Tiredness and fatigue
  5. Difficulty concentrating
  6. Disturbed sleep

There are various causes for under activity of thyroid gland including iodine deficiency, viral infection and various autoimmune disorders. Few of these are reversible, but majority result in a permanent reduction in thyroid hormone production. In all these cases, treatment of hypothyroidism consists of replacing the thyroid hormone externally, a treatment referred to as thyroid replacement therapy.

The standard treatment for hypothyroidism, even when T3 is more active, is a synthetic T4 hormone; the reason being that most of the T3 in the body was once T4. When the T4 hormone comes into contact with cells, it gives up an iodine atom to interact with those cells. When T4 loses an iodine atom, it becomes T3.  T4 stays for a longer period in the body after ingestion when compared to T3.

Thyroid replacement therapy usually requires the patient to take a daily dose of T4 in a pill orally to compensate for the under secretion. However, each persons’ requirement and their reaction to how the hormone is absorbed by the body are different. So there is no hard and fast rule as to the amount or dosage for every person in common. The adequacy of thyroid hormone replacement therapy is monitored by monitoring the levels of thyroid-stimulating hormone (TSH).

Written by MedPlus