What is the thyroid gland and what is its function?
What happens if the thyroid gland does not function or is removed?
The thyroid gland is one of the larger endocrine (ductless glands secreting hormone directly into the bloodstream) glands in the body. It is a butterfly or horseshoe shaped gland that lies in front of the trachea (windpipe) just below the thyroid cartilage (Adam’s apple). It consists of two lobes on either side of the trachea connected by an isthmus of glandular tissue crossing over the trachea. The recurrent laryngeal nerves that enter the muscles controlling movement of the vocal cords run behind the gland adjacent to the trachea. There are four, paired, tiny endocrine glands called parathyroid glands that make another hormone called parathormone. This hormone precisely maintains the proper balance of calcium in the bloodstream by regulating calcium absorption from food, mineral content in the bones and elimination by the kidneys. The parathyroid glands are located next to and usually behind the thyroid gland. These adjacent structures are of clinical importance since surgical removal of the thyroid gland places both of the nerves at risk of injury possibly resulting in permanent hoarseness due to a weak or immobile vocal cord. Inadvertent removal of the parathyroid glands in the course of thyroidectomy may result in temporary or rarely, permanent hypocalcaemia (low blood calcium levels).
As noted earlier, the thyroid, as with all other endocrine glands, secretes hormones into the blood directly, rather than through a duct. The thyroid gland secretes three hormones. Two of these hormones; thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3), control how quickly the body uses energy (metabolism) along with the growth and function of many body systems. Most of the circulating thyroid hormone is in the form of T4 and gets converted to T3 after entering individual cells as the more active form. The third hormone is calcitonin which is secreted by a special type of cell in the gland known as the parafollicular C-cell. Calcitonin has the opposite effect of parathyroid hormone resulting in the lowering of serum calcium levels. The amount and rate of secretion of the two thyroid hormones, T3 and T4 is controlled by the pituitary, another endocrine gland, located at the base of the brain, which secretes thyroid stimulating hormone (TSH). The control of thyroid gland secretion by the pituitary gland is via a “negative feedback” loop functioning much like a thermostat. As the levels of T3 and T4 rise in the blood, the amount of TSH secreted by the pituitary is diminished. Conversely, as the levels of the two hormones in the bloodstream fall the pituitary releases more TSH resulting in an increase of T3 and T4 secretion.
How is hypothyroidism treated?
Impairment of thyroid gland function and the attendant decrease in thyroid hormone levels in the body causes profound changes in many body systems yielding the clinical syndrome of hypothyroidism (too little thyroid hormone). Hypothyroidism may be brought about by disease of the thyroid gland itself or by removal of the gland for malignant tumors or enlargements (goiters) causing pressure on surrounding structures resulting in difficulty swallowing and breathing. Hypothyroidism may also result from treatment of hyperthyroidism caused by Graves’ disease with radioactive iodine, external beam radiation to treat other kinds of cancers in the head and neck, and insufficient dietary iodine. The failure of the gland to function due to problems with the gland itself is termed primary hypothyroidism. Conditions that interfere with the pituitary gland regulating thyroid secretion due to failure of TSH secretion are termed secondary hypothyroidism. Finally, disruption of the signals from the area of the brain (hypothalamus) signaling the pituitary to secrete TSH is known as tertiary hypothyroidism. Congenital hypothyroidism is a form of primary hypothyroidism where the infant has failed to develop the thyroid gland during fetal development. The older, no longer used term for this condition is “cretinism”. If unrecognized and untreated, these unfortunate children develop severe mental retardation, growth problems and heart problems.
In adults who become hypothyroid, symptoms and signs may include: weight gain, fluid retention, muscle cramps, constipation, depression, cold intolerance, slowing of the heart rate (bradycardia), enlargement of the thyroid gland (goiter) and fatigue. Hypothyroidism rarely reaches the point where it is clinically apparent, as tests for thyroid function are routinely included in blood tests for annual checkups. Even prior to clinical symptoms, the diagnosis is based on blood tests indicating below normal levels of thyroid hormone.
What is hyperthyroidism?
Hypothyroidism, regardless of the cause, is treated with thyroid hormone replacement therapy. Most patients do well with chemically identical, man-made (synthetic) hormone (T4) for which the dose of hormone the patient is receiving can be precisely regulated and monitored with simple blood testing. In rare instances, some patients feel better taking a combination of both hormones (T3 and T4). A so-called bioidentical type of thyroid hormone containing a variable proportion of T3 and T4 can be obtained from desiccated animal extracts (usually from pig thyroid glands). However, administering this type of hormone replacement may require more frequent testing as the exact quantities of each component is less regulated.
If an individual has lost the function of their thyroid gland to disease or surgery, and is being treated with the correct replacement dose of thyroid hormone, there is no decrease in lifespan or general health and well-being compared to persons who have functional thyroid glands.
How is hyperthyroidism treated?
Hyperthyroidism (too much thyroid hormone) or an overactive thyroid gland is the condition of excessive secretion of thyroid hormone into the bloodstream. It is the opposite of hypothyroidism. Causes of hyperthyroidism include the following which is not an exhaustive list:
- Graves’ Disease (an autoimmune disease affecting the thyroid gland)
- Thyroiditis (inflammation of the thyroid gland)
- Toxic Multinodular Goiter (multiple small nodules in an enlarged gland each secreting hormone)
- Toxic Thyroid Adenoma (a single, benign, thyroid hormone secreting tumor)
- Pituitary Adenoma (a benign tumor of the pituitary gland secreting excess thyroid stimulating hormone-TSH)
- Amiodarone (a drug used to treat an abnormal heartbeat or arrhythmia)
- Thyroid Hormone Abuse ( ingesting excessive amounts of thyroid hormone usually in the mistaken belief it will lead to weight loss)
- Iodine-Rich Over the Counter (OTC) Supplements (eg. Kelp tablets)
Since thyroid hormone levels are routinely assessed in general medical practice, the incidence of clinical hyperthyroidism is low. The condition is diagnosed and treated before symptoms and signs occur. However, in individuals who become symptomatic, symptoms and signs consist of the following which is again not an exhaustive list:
- Anxiety and apathy
- Tachycardia (rapid heartbeat)
- Tremors (shaking of the hands)
- Weight Loss
- Hypoglycemia (low blood sugar)
- Polyuria (excessive urination)
- Polydipsia (excessive thirst)
- Pretibial Myxedema (swelling over the shin bones)
- Loose Stools
- Thinning Hair/Loss
The end stage of long-standing hyperthyroidism or with very high levels of thyroid hormone in the circulation is the life-threatening condition of thyroid storm or crisis which may usually occurs in response to illness or other physical stress. Some of the symptoms and signs of this rare condition are as follows:
- Increased body temperature over 104 degrees Fahrenheit
- Tachycardia (heart rate greater than 120 beats per minute)
- Abnormal heart contractions (arrhythmias)
What are the different types of thyroid cancer and what is the prognosis (outlook)?
The initial treatment of hyperthyroidism is with drugs to suppress symptoms and decrease thyroid hormone output. Beta-blockers are drugs that are used to suppress the cardiac effects of too much thyroid hormone as well as other adrenaline-like effects throughout the body. Metoprolol or Propranalol are the agents most commonly employed. Drugs used to inhibit thyroid hormone production are methimazole and propylthiouracil (PTU).
Once the symptoms of hyperthyroidism are under control, most patients will have definitive treatment with radioactive iodine ablation of the gland. An oral pill containing iodine, complexed (bound) with radioactive material is given and enters only thyroid cells (having and iodine pump or symporter). This treatment effectively destroys the thyroid gland from within without damaging other body cells. Surgical removal is reserved for patients with large glands, those who do not desire radioactive therapy, have iodine allergies or for women on child bearing age who wish to conceive without waiting many months before it is safe for the baby.
Malignant tumors of the thyroid gland or cancer are of different cell types and carry a much different prognosis for cure. There are three main histologic (tissue) types: differentiated (including papillary, follicular and Hürthle), medullary and anaplastic (aggressive, undifferentiated tumor). Differentiated thyroid cancers (papillary, follicular and Hürthle) account for 95% of all thyroid cancer types. Although thyroid cancer occurs more often in women, mortality rates are higher in men, probably due to a more advanced age at the time of diagnosis. Fortunately, patients with differentiated thyroid cancer survive for 10 years or more after initial diagnosis and treatment (papillary 93%, follicular 85%, Hürthle 76%). More than 95% of patients with early stage disease can be cured. Although a number of factors determine the outcome for patients with differentiated thyroid cancers, age is the single most important prognostic variable. Patients over the age of 40 years are more likely to die of their disease than younger patients, and this risk increases for each decade. However, tumor recurrences are more common in patients younger than 20 years or older than 60 years of age. The risk of recurrence of thyroid cancer also depends on the completeness of the surgical removal, the size and biological aggressiveness of the tumor noted on the microscopic analysis (eg. Invasion of cancer cells either outside of the thyroid gland into surrounding anatomy, inside blood or lymphatic vessels with lymph node metastases or dissemination throughout the thyroid gland itself).