THE THYROID

The Thyroid (shield like) is one of the larger endocrine glands, situated in the lower part of the front side of the neck. It regulates the basal metabolic role, stimulates somatic and psychic growth and plays an important role in calcium metabolism.

ThyroidThe gland consists of right and left lobes that are joined to each other by isthmus and third, pyramidal lobe may project upwards from the isthmus. Sometimes a fibrous or fibro-muscular band descends from the body of the hyrid bone to the isthumus or to the pyramidal lobe. The thyroid glands controls how quickly the body uses energy, makes proteins & controls how sensitive the body is to other hormones. It participates in these processes by producing thyroid hormones; the principal ones being trioiodothyonine (T3) and thyroxine (T4), These hormones regulate the growth and rate of function of many other systems in the body. T3 & T4 are synthesized from iodine and thyroxine. The Thyroid also produces calcitonin which plays a role in calcium homeostasis.

Hormonal output from the thyroid is regulated by thyroid stimulating hormone (TSH) produced by the anterior pituary which itself is regulated by thyotropin-releasing hormone (TRH) produced by the hypothalamus.

 


 

STRUCTURE

The gland lies against vertebrae C5, C6, C7 & T embracing the upper part of trachea. Each lobe extends from the middle of the thyroid cartilage to the 4th or 5th tracheal ring. The isthmus extends from the 2nd to the 4th tracheal ring or and average, the gland weighs about 25g. However, it is larger in females than in males and further increases in size during menstruation and pregnancy.

The Thyroid gland is butterfly shaped organ covered by a thia fibrous sheath, the capsula grandulae thyreoideae composed of an internal and external layer. The gland is covered exteriorly continuous with infrahyoid muscles. “On the posterior side, the gland is fixed to the cricoid and tracheal cartilage and cricopharyngeus muscle by a thickening of fascia to form the posterior suspensory ligament of thyroid gland also known as Berry’s ligament. The Thyroid glands’ firm attachment to the underlying trachea is the reason behind its movement with swallowing the thyroid gland is supplied by the superior and inferior thyroid arteries. The Thyroid is drained by the superior middle of inferior thyroid veins.

 


 

CLINICAL ANATOMY

In a health patient the gland is not visible yet can be palpated as a soft mass. Any swelling of the thyroid gland (goitre) should be palpated from behind examination of the thyroid gland is carried out by locating the thyroid cartilage and passing the fingers up and down, examining for abnormal masses and overall thyroid size. The two lobes of the gland should be compared for size and texture using visual inspection as well as manual or bimanual palpation. Finally ask patient to swallow to check for mobility of the gland. in a healthy state, the gland is mobile when swallowing occurs due to its fascial encasement. Thus when the patient swallows, the gland more spuriously as does the whole lorynce.

Removal of thyroid with true capsule may be necessary in hyper-thyroidism. Hypo-thyroidism causes cretinism in infants and myxedema in adults. Benign tumours of the gland may displace and even compress neighbouring structures like the carotid sheath and trachea etc. Malingnent growth tend to invake and erode neighbouring structures. Pressure symptoms and nerve involvements are common in carcinoma of the glands.

Tumours of the para-thyroid gland lead to excessive secretion of parathormone (hyper parathyroidism). This leads to increased removal of calcium from bone making them weak and liable to fracture. Calcium levels in blood increase (hypercalcemia) and increased urinary excretion of calcium can lead to the formation of urinary tract cells of the developing brain are a major target for the thyroid hormones T3 and T4. Thyroid hormones play a particularly crucial role in brain maturation during fatal development.
Hypoparathoridism may occur spontaneously or from accidental removal of the gland during thyroidectomy. This result in hypocalcaemia leading to increased neuromuscular inevitability causing muscular spasm and convulsions (tetany).

 


 

CLINICAL SIGNIFICANCE

Thyroid disorders include

  • hyperthyroidism (abnormally increased activity),
  • hypothyroidism (abnormally decreased activity)
  • thyroiditis, inflammation of the thyroid
  • thyroid nodules, which are generally benign thyroid neoplasms (tumours), but may be thyroid cancers.

All these disorders may give rise to a goiter, that is, an enlarged thyroid.

 


 

HYPERTHYROIDISM

Hyperthyroidism, or overactive thyroid, is due to the overproduction of the thyroid hormones T3 and T4, which is most commonly caused by the development of Graves’ disease, an autoimmune disease in which antibodies are produced which stimulate the thyroid to secrete excessive quantities of thyroid hormones. The disease can result in the formation of a toxic goiter as a result of thyroid growth in response to a lack of negative feedback mechanisms. It presents with symptoms such as a thyroid goiter, protruding eyes (exopthalmos), palpitations, excess sweating, diarrhea, weight loss, muscle weakness and unusual sensitivity to heat. The appetite is often increased. 

 


 

HYPOTHYROIDISM

Hypothyroidism is the underproduction of the thyroid hormones T3 and T4.

Hypothyroid disorders may occur as a result of

  • congenital thyroid abnormalities (Thyroid deficiency at birth. See congenital hypothyroidism),
  • autoimmune disorders such as Hashimoto’s thyroiditis,
  • iodine deficiency (more likely in poorer countries) or
  • the removal of the thyroid following surgery to treat severe hyperthyroidism and/or thyroid cancer.

The gland consists of right and left lobes that are joined to each other by isthmus and third, pyramidal lobe may project upwards from the isthmus. Sometimes a fibrous or fibro-muscular band descends from the body of the hyrid bone to the isthumus or to the pyramidal lobe. The thyroid glands controls how quickly the body uses energy, makes proteins & controls how sensitive the body is to other hormones. It participates in these processes by producing thyroid hormones; the principal ones being trioiodothyonine (T3) and thyroxine (T4), These hormones regulate the growth and rate of function of many other systems in the body. T3 & T4 are synthesized from iodine and thyroxine. The Thyroid also produces calcitonin which plays a role in calcium homeostasis.

Hormonal output from the thyroid is regulated by thyroid stimulating hormone (TSH) produced by the anterior pituary which itself is regulated by thyotropin-releasing hormone (TRH) produced by the hypothalamus.

 


 

THYROIDITIS

There are two types of thyroiditis where initially hyperthyroidism presents which is followed by a period of hypothyroidism; (the overproduction of T3 and T4 followed by the underproduction of T3 and T4). These are Hashimoto’s thyroiditis and postpartum thyroiditis.

Hashimoto’s thyroiditis or Hashimoto’s Disease is an autoimmune disorder whereby the body’s own immune system reacts with the thyroid tissues in an attempt to destroy it. At the beginning, the gland may be overactive, and then becomes underactive as the gland is damaged resulting in too little thyroid hormone production or hypothyroidism. Some patients may experience “swings” in hormone levels that can progress rapidly from hyper-to-hypothyroid (sometimes mistaken as severe moodswings, or even being bipolar, before the proper clinical diagnosis is made). Some patients may experience these “swings” over a longer period of time, over days or weeks or even months. Hashimoto’s is more common in females than males, usually appearing after the age of 30, and tends to run in families, meaning it can be seen as a genetic disease. Also more common in individuals with Hashimoto’s thyroiditis are type 1 diabetes and celiac disease.

 


 

CANCERS

In most cases, thyroid cancer presents as a painless mass in the neck. It is very unusual for thyroid cancers to present with symptoms, unless they have been neglected. One may be able to feel a hard nodule in the neck. Diagnosis is made using a needle biopsy and various radiological studies.