Bma Illustrated Medical Dictionary

this duct runs from the base of the

tongue to the thyroid gland. Abnormal

development may cause the duct to

persist in its entirety or partly as a cyst.

A cyst usually becomes infected and

swollen, which may lead to formation of

a fistula. The cyst and any remaining

parts of the duct are removed.

thyroid cancerRare tumoursof the

thyroid gland.In most cases the cause

is unknown, although exposure to radio-

active fallout increases the risk of

developing the condition. There are sev-

eral types, depending on the type of

cells involved. In all of them, however,

the first sign is a firm nodule in the

neck, which may grow slowly or rapidly.

In many cases, the cancer is painless

and symptoms such as difficulty swal-

lowing, and hoarseness or loss of voice,

only develop when the tumour presses

on other structures. A diagnosis is made

by thyroid scanningand needle aspiration

or a biopsy. A thyroidectomyis usually

followed by treatment with radioactive

iodineto destroy any residual cancer.

Cure rates depend on the cell type and

on the size and spread of the tumour

when diagnosed. Patients need to take

thyroxinefor the rest of their lives.

thyroidectomySurgical removal of all

or part of thethyroid gland, performed

to treat thyroid cancer, some cases of

hyperthyroidism, goitre, or a noncancer-

ous tumour of the thyroid gland.

thyroid-function tests A group of

blood tests used to evaluate the func-

tion of the thyroid glandand to detect

or confirm any thyroid disorder. The

thyroid hormones T 3 and T 4 are mea-

sured, as well as thyroid-stimulating

hormone (TSH), the pituitary gland hor-

mone that stimulates the thyroid gland.

thyroid glandOne of the main endo-

crine glands, which helps to regulate

the rate of all the body’s internal pro-

cesses. The thyroid gland is situated in

the front of the neck, just below the lar-

ynx(voice box). It consists of 2 lobes,

one on each side of the trachea(wind-

pipe), joined by a portion of tissue

called the isthmus. Thyroid tissue is

composed of follicular cells, which sec-

rete the iodine-containing hormones

thyroxine (T 4 ) and triiodothyronine (T 3 ),

and parafollicular cells (or C cells),

which secrete the hormone calcitonin.

T 4 and T 3 are important in controlling

the body’s metabolism. Calcitonin

helps to regulate calcium balance in

the body. (See also thyroid gland, dis-

orders of; thyroid hormones.)

thyroid gland, disorders ofDisorders

of the thyroid gland may cause hyperthy-

roidism, hypothyroidism, or enlargement

or distortion of the gland. Myxoedema,

Graves’ disease, and Hashimoto’s thy-

roiditisare common disorders. Goitremay

sometimes occur with no accompanying

abnormality of thyroid function. In rare

cases, the gland is absent at birth, pro-

ducing severe cretinism. Sometimes it

develops in an abnormal position in the

neck, causing, in rare cases, difficulty in

swallowing or breathing.

A genetic disorder may impair the thy-

roid’s ability to secrete hormones and

goitre may result. Thyroid infection is

uncommon and leads to thyroiditis. Viral

infection can cause extreme pain and

temporary hyperthyroidism. Hormonal

changes during puberty or pregnancy

may cause a degree of goitre temporar-

ily. Hyperthyroidism due to excessive

production of TSH by the pituitary

gland is rare but can occur as a result of

a pituitary tumour.

Because iodine is necessary for the

production of thyroid hormone, its defi-

ciency may lead to goitre. Severe iodine

deficiency in children may cause myx-

oedema. (See also thyroid cancer.)

THYROID CANCER THYROID GLAND, DISORDERS OF

T

THYROID GLAND

Thyroid cartilage

LOCATION

Thyroid gland

Trachea