Biology of Disease

The management of GH excess cannot reverse any clinical changes that have

taken place prior to treatment. However, management is important because

it improves survival and reduces deaths due to heart disease or stroke. Growth

hormone secreting tumors are treated in one of three ways, the clinical decision

depending on severity of disease, the age of the patient and the response

to treatment. Surgical removal of the pituitary tumor may be attempted

depending on its size and location with steps taken to minimize damage to

the anterior pituitary. If GH levels do not normalize, other treatments must

be considered. Radiotherapy is usually performed over a 4–6 week period by

external irradiation using a cobalt source. Its effects are slow and, in some

cases, hyposecretion of other pituitary hormones may occur. Drugs, such as

bromocriptine, suppress GH release in acromegalics and are often used in

patients too old to undergo surgery or radiotherapy. Somatostatin analogs, for

example octeotride, have also been used to inhibit GH release. Some patients

require all three modes of treatment but are not always successfully treated. To

detect recurrence, serum IGF-I needs to be monitored at regular intervals.

7.7 Thyroid Hormone Disorders

The thyroid gland is the largest endocrine gland in the body, weighing about

20 g. It is a bilobular organ that consists of microscopic spherical follicles

with secretory cells (Figure 7.16) that synthesize the thyroid hormones.

Thyroid hormones (Figure 7.17 (A)and(B)) consist of triiodothyronine (T 3 )

and thyroxine (T 4 ). Triiodothyronine is the most active, four times that of T 4 ,

and has a half-life of 1.5 days compared with 9 days for T 4. However in most

tissues, particular the liver, T 4 can be readily converted to T 3. More than 99% of

T 3 and T 4 are transported in the serum as complexes: 70% to thyroxine binding

globulin (TBG) and about 20% to albumin and around 10% to prealbumin.

The remaining small free portion is the metabolically active fraction.

X]VeiZg,/ DISORDERS OF THE ENDOCRINE SYSTEM

&,' W^dad\nd[Y^hZVhZ

Follicular cell

Follicle containing

thyroglobulin

Figure 7.16 The histological structure of the

thyroid gland. Courtesy of Dr A.L. Bell, University of

New England, College of Osteopathic Medicine, USA.

Figure 7.17 The major thyroid hormones (A) T 3 ,

(B) T 4 and (C) the regulation of the secretion of T 4

and T 3 by a negative feedback mechanism.

CH 2

NH 2

CH COOH

I I

I I

I I

I

HO O

CH 2

NH 2

HO O CH COOH

A)

B)

TRH

TSH

Thyroid

gland

T 4 + T 3

Binding

protein

Hypothalamus

Anterior

pituitary

C)