Download THYROID GLAND The thyroid gland releases thyroid hormones. It is

THYROID GLAND
The thyroid gland releases thyroid hormones. It is under the influence of thyroidstimulating hormone (TSH or thyrotropin) from the pituitary gland, which is itself
regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus.
Thyroid hormones, stored as iodide-rich 'thyroid colloid', are:
o thyroxine (T4; tetraiodothyronine) mainly, which has a half life of one week,
and is converted to
o triiodothyronine (T3) a small amount only - the active form, which has a half
life of one day.
Thyroid hormones feed back to the hypothalamus and pituitary to regulate TSH
release. Thyroid hormones act on:
1. metabolism by regulating protein synthesis via effects on gene transcription
and mRNA stabilization.
2. They have profound effects on the sensitivity of tissues to catecholamines.
3. mitochondrial oxidative activity.
4. synthesis and degradation of proteins.
5. differentiation of muscle fibres.
6. capillary growth.
7. levels of antioxidant compounds.
Thyroid function tests include the following:
 Levels of free T4 and free T3 in serum provide a better assessment of the
thyroid status than do total T4 and T3. About 95% of thyroid hormones are
bound to plasma proteins, especially thyroid-binding globulin (TBG) and
thyroid-binding prealbumin (TBPA).
 Thyroid-stimulating hormone (TSH) levels can be assayed.
 Thyroid antibody tests.
ultrasound and radio-iodine uptake using 131I or 123I are the other common
LINGUAL THYROID
The thyroid normally develops as a downgrowth from the foramen caecum at the
junction of the posterior third with the anterior two-thirds of the tongue.
Rarely, ectopic thyroid tissue remains in this tract and may be seen as a lump
anywhere in the midline between the foramen caecum and epiglottis, but has also
been recorded in the oropharynx, infra-hyoid region, larynx, oesophagus, heart
and mediastinum.
A lingual thyroid is seen mainly in females, and is often asymptomatic but may
cause dysphagia, airway obstruction or ewn haemorrhage.
Hypothyroidism may be associated in about one-third of cases and, occasionally,
the lingual thyroid becomes malignant. There is a raised incidence of thyroid
disease in relatives.
I
A lingual thyroid may not be suspected until the lump in the tongue has been
biopsied or excised and examined histologically.
The diagnosis can be confirmed by iodine-123 or -131, or technetium-99 uptake in
the tongue or by biopsy or by CT scanning without contrast or by MRI.
Treatment depends on the size of the lingual thyroid but thyroxine may be needed
and if the lump does not regress sufficiently, the lingual thyroid can be ablated,
best by surgery, or if the patient is unfit, by iodine-131, if normal functioning
thyroid tissue is identified in the neck.
GOITRE
A goitre is an enlarged thyroid gland, usually a consequence of hyperplasia
secondary to excessive TSH levels caused by low circulating thyroid hormone.
Most goitres are acquired and seen in Graves' disease, or thyroiditis, but a few are
congenital and seen in cretinism. Thyroid cancer is another possible cause. H The
cause of the goitre should be sought.
Thyroid function is assessed to determine whether it is:
 normal (euthyroid)
 hyperactive (hyperthyroid)
 hypoactive (hypothyroid).
Most goitres do not require surgery but this is indicated if there is a danger of
airways obstruction (cough, voice changes, dyspnoea, tracheal deviation or
dysphagia), or for cosmetic reasons.
Dental management in goitre may be influenced by abnormal thyroid function, by
the underlying cause of the goitre, or by complications such as respiratory
obstruction.
A rare cause of goitre is a medullary carcinoma of the thyroid, which can be part
of a multiple endocrine adenomatosis syndrome (MEA II and MEA III). In the
latter, numerous small plexiform neuromas form in the oral mucosa, lips, eyelids
and skin. The patient may also have a Marfanoid habitus and diarrhoea.
THYROID NODULES
A nodule in the thyroid gland may represent a benign or malignant neoplasm.
Patients are examined with radio-iodine thyroid scans and thyroid function tests.
A nodule that takes up radio-iodine is termed a hot nodule and unlikely to be
malignant, and more usually is an adenoma.
A nodule that fails to take up the radio-iodine is termed a cold nodule, and may be
malignant, usually a papillary, follicular or medullary cell carcinoma. Needle
biopsy is indicated.
II
HYPERTHYROIDISM
General aspects
Hyperthyroidism is associated: usually, with a diffuse goitre due to autoimmune
disease (Graves' disease, primary hyperthyroidism) when there are thyroidstimulating autoantibodies against thyroid TSH receptor (TRAbs) and thyroid
microsomal antibodies (TMAbs) sometimes, with a hyperfunctioning (toxic)
multinodular goitre or nodule due to one or more thyroid adenomas producing
excess thyroxine. 90% of the swellings are benign rarely, with thyroiditis, thyroid
hormone overdosage, or ectopic thyroid tissue.
Clinical features
Hyperthyroidism mimics the effects of cpinephrine, and can cause: anorexia,
vomiting or diarrhea, weight loss, anxiety and tremor, sweating and heat
intolerance, cardiac disturbances, particularly in older patients: These include
tachycardia, dysrhythmias (especially atrial fibrillation) or cardiac failure,
exophthalmos, eyelid lag and eyelid retraction.
Thyrotoxic periodic paralysis comprises attacks of mild to severe weakness,
during which serum potassium levels are generally low.
Myasthenia gravis may occasionally be associated.
III
General management
The diagnosis of hyperthyroidism should be confirmed by:
1) raised serum levels of T3 and T4
IV
2) circulating TMAbs and TRAbs in 55% of patients with Graves' disease.
3) a radioactive iodine uptake test or thyroid scan. Radioactive iodine (RAI)
uptake can differentiate causes of hyperthyroidism: subacute thyroiditis (low
uptake) versus Graves' disease (high uptake).
Hyperthyroidism can be treated with: beta blockers. These achieve rapid control of
many of the signs and symptoms by moderating sympathetic overactivity.
However, suddenly stopping beta-blocker treatment can precipitate a thyroid crisis
within 4 h. Carbimazole - the usual antithyroid drug but it can suppress the bone
marrow and rarely cause rashes. Nearly 50% of patients have a relapse.
Propylthiouracil is an alternative. 131 Iodine effective, but can result in
hypothyroidism. There appears to be no risk of neoplastic change surgery. This is
effective, but leads to hypothyroidism in about 30%. Hypoparathyroidism or
recurrent laryngeal nerve palsy are rare complications. In untreated patients with
hyperthyroidism, pain, anxiety, trauma, general anaesthesia, or premature
cessation of antithyroid treatment may precipitate a thyroid (thyrotoxic) crisis.
Thyroid crisis, characterized by anxiety, tremor and dyspnoea, is dangerous and
can go on to ventricular fibrillation. Medical assistance is essential as treatment
requires the use of potassium iodide and propylthiouracil, and propranolol or
chlorpromazine.
Dental aspects
Patients with untreated hyperthyroidism can be difficult to deal with as a result of
heightened anxiety and irritability. The sympathetic overactivity may lead to
fainting. Local anaesthesia is the main means of pain control. The risk from
epinephrine exacerbating symphathetic overactivity is only theoretical: prilocaine
with felypressin is not known to be safer than lidocaine. Conscious sedation is
frequently desirable to control excessive anxiety. Benzodiazepines may potentiate
antithyroid drugs, and therefore nitrous oxide, which is more rapidly controllable,
is probably safer.
Povidone-iodine and similar compounds are best avoided.
Carbimazole occasionally causes agranulocytqsis, which may cause oral or
oropharyngeal ulceration.
Otherwise the treated thyrotoxic patient presents no special problems in dental
treatment. However, after treatment of hyperthyroidism the patient is at risk from
hypothyroidism, which may pass unrecognized. This point must especially be
borne in mind if a general anaesthetic is required.
V
Hyperthyroidism
Hypothyroidism
Heat intolerance
Cold intolerance
Excess sweating Warm moist skin No Decreased sweating Dry cold skin
hair loss
Hair loss
Increased appetite Weight loss
Decreased appetite Weight gain
Tachycardia (atrial fibrillation) Heart Bradycardia Angina Hoarseness Slow
failure No voice change Tremor
reactions
Diarrhoea Irritability Psychosis Atrial Constipation Slow cerebration, poor
fibrillation
memory Psychosis Ischaemic heart
disease
Exophthalmos in some Amenorrhoea Periorbital oedema Menorrhagia
Gynaecomastia
Serous effusions
HYPOTHYROIDISM
General aspects hypothyroidism may be:
 primary (due to thyroid disease)
 secondary (to hypothalamic or pituitary dysfunction).
The common causes of hypothyroidism are:
 thyroid loss from surgical removal of too much thyroid tissue in a previously
hyperthyroid patient
 destruction by irradiation of the neck or thyroid gland
 autoimmune disease (Hashimoto's thyroiditis), associated with autoantibodies to
thyroglobulin and thyroid microsomes
 drugs such as amiodarone, carbimazole, lithium or radio-iodine.
 Rare cases in the developed world are caused by iodine deficiency.
Clinical features
Subclinical hypothyroidism, with raised TSH but normal T4 levels, may be found in
up to 10% of postmenopausal females.
Hypothyroidism is often unrecognized but can cause:
 weight gain
 lassitude
 dry skin, myxoedema, loss of hair
 cardiac failure or ischaemic heart disease, bradycardia, anaemia
 neurological or psychiatric changes, hypotonia, cerebellar signs of ataxia,
tremor, and dysmetria, polyneuropathy, cranial nerve deficits, entrapment
VI
neuropathy (e.g. carpal tunnel syndrome), myopathic weakness, dementia, apathy,
mental dullness, irritability, sleepiness
 hoarseness
 hypothermia and may be complicated by coma.
Sjogren's syndrome may be associated Congenital hypothyroidism (cretinism) has
similar features, together with an enlarged tongue and learning impairment.
General management
-The diagnosis of hypothyroidism is confirmed by demonstrating.
-Serum T4 and T3 levels (low in hypothyroidism).
TSH levels. The TSH-immunometric assay can discriminate directly between
normal TSH and low levels without requiring the use of the thyrotropin-releasing
hormone (TRH) infusion test, serum TSH is raised in primary hypothyroidism
depressed in secondary hypothyroidism, serum antibodies in patients with
Hashimoto's thyroiditis, thyroid microsomal antibodies (TMAb) in 95% thyroglobulin antibodies (TGAbs) in 60%.
-Symptomatic patients are managed with daily oral thyroxine sodium. Treatment is
started slowly but, especially if there is evidence of ischaemic heart disease, angina,
MI or sudden death may be precipitated.
Dental aspects
The main danger is of precipitating myxoedema coma by the use of sedatives
(including diazepam or midazolam), opioidanalgesics (including codeine), or
tranquillizers. These should, therefore, be either avoided or given in low dose.
Local anaesthesia is satisfactory for pain control. Conscious sedation can be carried
out with nitrous oxide and oxygen. Diazepam or midazolam may precipitate coma.
General anaesthesia may be complicated because of possible ischaemic heart
disease and the danger of coma, and the respiratory centre is also hypersensitive to
drugs such as opioids or sedatives. General anaesthesia, if unavoidable, should be
delayed if possible until thyroxine has been started.
Associated problems may include hypoadrenocorticism, anaemia, hypotension,
diminished cardiac output and bradycardia. Occasional associations include
hypopituitarism and other autoimmune disorders such as Sjogren's syndrome.
Povidone-iodine and similar compounds are best avoided.
VII