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Endocrine System - Part Two - The thyroid, pineal and parathyroid glands

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VOL: 103, ISSUE: 22, PAGE NO: 26

Brendan Docherty, MSc, PGCE, RN is clinical stream manager, cardiology and critical care, South Eastern Sydney and Illawarra Health Service, Australia

The first article in this four-part series looked at the role and functions of the endocrine system and its related hormones. In this article we explore the role and functions of the thyroid, pineal and parathyroid glands. Part three will focus on the thymus and adrenal glands, and the final part will examine the pancreas and the sex glands (gonads).

Thyroid gland

The thyroid gland (Fig 1) is located in the middle of the lower neck, below the larynx cartilage (voice box) and just above the clavicles (collar bones).

It consists of right and left lobes that are connected by a piece of thyroid tissue called the isthmus. The gland is often referred to as being ‘butterfly’ or ‘bow tie’ in shape.

The isthmus lies in front of the trachea, with the lobes curving around the sides of the trachea.

The thyroid gland is responsible for cell metabolism, growth and controlling the body’s basal metabolic rate.

It is made up of thyroid follicles that are mainly microscopic. When inactive these are cuboidal, and when they are stimulated to secrete they become columnar in shape.

The presence of thyroid stimulating hormone (TSH), which is secreted from the pituitary gland in the brain, is required to trigger these follicles to secrete hormones.

The thyroid gland produces two hormones: thyroxine (also known as tetraiodothyronine or T4 as it has four iodine atoms) and triiodothyronine (or T3 as it has three iodine atoms).

Thyroid cells are the only cells in the human body that can absorb iodine.

Parafollicular cells are less in number and lie between the thyroid follicles. They produce the hormone calcitonin, which is responsible for the regulation of the body’s calcium levels (Marieb and Hoehn, 2006; Saladin, 2006).

Thyroid hormone levels are regulated through the hypothalamic-pituitary-thyroid pathway by a feedback inhibition system (Wood et al, 2006) (see box).

Low levels of thyroid hormones in the blood stream are detected by the hypothalamus, which secretes thyrotropin releasing hormone, leading to the secretion of TSH. This results in the thyroid being stimulated to produce its hormones (Saladin, 2006).

When blood levels of the thyroid hormones become normalised, the hypothalamus then switches off the production of thyrotropin-releasing hormone.

Hormone underproduction can lead to delayed growth and intellectual ability in children if not corrected (cretinism); a lower metabolic rate (myxoedema); and enlargement of the thyroid.

Hormone overproduction can lead to Graves’ disease (an autoimmune disease) and osteoporosis (Wood et al, 2006).

Hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid) are the most common problems of the thyroid.

Pineal Gland

The pineal gland, which is sometimes referred to as the pineal body or epiphysis (Fig 2), lies towards the back of the roof of the third ventricle of the brain, between the two hemispheres.

It is a small pine cone-shaped structure (hence its name) and forms part of the mid-brain called the epithalamus (Marieb and Hoehn, 2006).

The pineal gland is approximately 1cm long. In adults it becomes calcified, and can therefore be identified on a skull X-ray in the midline position.

The pineal gland consists of the following (Wood et al, 2006):

- Pinealocytes - specialised secretory cells;

- Interstitial cells - surrounding the pinealocytes;

- Neurons and neuroglial cells;

- Pia mater - the covering capsule.

The pineal secretory cells receive stimulation from the retina (stimulated by darkness and inhibited by light), resulting in the hormone melatonin being directly secreted from the gland into the cerebrospinal fluid, from where it is then transported to the bloodstream.

Melatonin is thought to have a role in reproductive development (in children the gland is bigger and then shrinks after puberty); and in affecting daily physiological cycles, for example circadian rhythms (Harrower, 2005).

The role of melatonin in humans is still not fully understood and there are very few clinical conditions related to this gland.

Parathyroid glands

The parathyroid glands are four small structures that are usually situated at the back of the thyroid gland (Fig 1), although they can sometimes be found anywhere in the neck from just below the jaw to further down the chest near the heart.

Although the parathyroid and thyroid glands have similar names they have different functions.

Each parathyroid gland is approximately the size and shape of a single grain of rice. They produce parathyroid hormone, which is the endocrine system regulator of both calcium and phosphorus levels contained in the blood (Marieb and Hoehn, 2006).

The hormone’s major target cells are in the kidneys and in bones. It increases the concentration of blood calcium:

- By accessing the calcium stored in bones to elevate low plasma calcium;

- To alter renal filtration to encourage an increased reabsorption of calcium in the distal renal tubules;

- To absorb calcium direct from the gastrointestinal tract, thereby elevating the levels in blood (Wood et al, 2006).

High levels of circulating plasma calcium results in a reduction in parathyroid hormone secretion, thereby returning the calcium level to an optimal level.

Another effect of parathyroid hormone on the kidneys is to increase the loss of phosphate ions in the urine, thereby reducing the blood level of phosphate (Wood et al, 2006).

This phenomenon is reversed when the plasma phosphate level is low and the kidneys reabsorb phosphate in the tubules to restore homeostasis.

The oversecretion of parathyroid hormone (hyperparathyroidism) by the parathyroid glands results in high calcium levels, which can in turn lead to cardiac arrhythmias, pancreatitis and confusion.

When this happens the surgical removal of the oversecreting gland is usually required (Wood et al, 2006).

Underproduction of the hormone (hypoparathyroidism) results in low calcium levels, which can cause tremors, agitation and seizures.

When this happens it is very important that calcium supplements are given to the patient in order to restore normal calcium levels in the blood (Wood et al, 2006).

This article has been double-blind peer-reviewed

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