Lecture 16

Thyroid Disorders

Thyroid Anatomy

• Macroscopically

The thyroid has two lobes joined by an isthmus

Embedded into the upper and lower poles of both lobes are the parathyroid glands.

Thyroid Structure

• Microscopically

The gland is formed of follicles, which are rings of thyroid cells enclosing a colloid which contains the stored hormones and thyroglobulin the precursor

Thyroid Hormone Formation

• Essential requirements are the protein Thyroglobulin, Iodine and enzymes which join the two together

• The hormones are composed of tyrosine ( a subcomponent of thyroglobulin) and 1, 2, 3 or 4 Iodine molecules,

• The active hormones are the tri form = tri-iodo thyronine and thyroxine, which has 4 Iodine molecules

Control of Thyroid hormone production

• Thyroid releasing hormone (TRH) from the Hypothalamus causes the pituitary to release Thyroid Stimulating Hormone (TSH)

• There is a negative feedback loop between the Thyroid hormones and these control hormones

• Iodine is also required

Action of Thyroid Hormones

• Increase sweating• Increase cardiac output• Increase pulse pressure ( higher syst. and lower

dias.)due to an increase in the stroke volume and a reduction in peripheral vascular resistance

• Increased utilization of CHO, Protein and Fat• Increased excitation of Nervous system• Plus Thyroid hormones are essential for the growth

and development of the Skeleton, Teeth, Epidermis and CNS

Thyroid Disorders

Common disorders are

• Goitre = an enlarged thyroid

• Hypothyroidism = symptoms of reduced levels of thyroid hormones

• Hyperthyroidism = symptoms of increased levels of thyroid hormones

• Less common are thyroid tumours

Terms and their definitions

• Goitre = enlarged thyroid (does not refer to the function of the thyroid)

• Toxic = increased thyroid hormone output

• Nodule = palpable lump in thyroid

Goitres

• Can be either simple or toxic

• Diffuse or Multinodular

Simple (non-toxic)Goitre

= thyroid is enlarged ( either diffusely or in a multinodular form) but there is no excess thyroid hormone production

• Aetiology

Inadequate iodine,

Excessive amounts of goitrogens

Pregnancy ( higher need for Iodine)

Drugs

Clinical Features of Simple Goitre

• Enlarged thyroid which is soft and symmetrical

• No changes in any of the hormones = euthyroid

• Usually no treatment is required and the goitre resolves

Simple Multinodular Goitre

• A gradually enlarging thyroid which develops localised areas of hyperplasia resulting in palpable nodules

• The gland can become large enough to compress nearby structures but it does not usually produce excessive levels of hormones

• Unless causing pressure problems or early hyperthyroidism the thyroid is merely reviewed annually

Multinodular goitre

Hypothyroidism

• Most common thyroid disorder• Is the clinical syndrome that results form a

deficiency of thyroid hormones• It can develop in utero (cretinism) or as an

adult• If It occurs in the latter situation it can

result in deposition of glycosaminoglycans (GAGS) and thus produce myxedema

Pre tibial Myxedema

Causes of Hypothyroidism

Primary = failure of thyroid gland to produce thyroid hormone. Can be caused by,

• Hashimoto’s thryoiditis (+/- goitre)• End stage Graves disease• Surgical removal of thyroid or radioactive ablation

of thyroid as treatment of Graves

Secondary = failure of pituitary to produce TSH

Can also be tertiary

Hashimoto’s Thyroiditis

• Most common cause of hypothyroidism and also causes a goitre

• Aetiology• Cause by an auto-immune reaction to

thyroid but mechanism is still unclear• There are auto-antibodies to thyroglobulin,

etc but their levels do not correlate well with the severity of the disease

Hashimoto’s cont’d

• Pathology

The thyroid undergoes infiltration by lymphocytes and fibrosis follows resulting in an initially enlarged gland shrinking

Lymphoid follicles at right and centre

Hashimoto’s thyroiditis

Clinical Features of Hypothyroidism

• There is usually a goitre and variable levels of thyroid hormones

• Some are due to reduced levels of thyroid hormones eg slowness, cold intolerance

• Others are due to a build up of GAGS eg non-pitting oedema of hands, eyes

Complications

• 5% develop thyroid cancer

• As it is usually a disease of the elderly it can co-exist with CAD so treatment can unmask and stress the previously protected coronary vessels

Thyrotoxicosis

• Definition

• Clinical features of excess thyroid hormone

Aetiology of Thyrotoxicosis

• Graves Disease, which is associated with autoantibodies which act to stimulate TSH receptors and thus cause the production of greater amounts of thyroid hormone

• A nodule in a multinodular goitre which produces excess thyroid hormone ( ie becomes toxic)

• Toxic adenoma

Clinical Features of Thyrotoxicosis

• General = heat intolerance, warm moist skin, LOW in spite of increased appetite

• GIT = malabsorption and diarrhoea

• CVS = palpitations, tachycardia

• Neuromuscular = tremor, irritability, proximal myopathy

• Ocular = wide, staring gaze, lid lag

Graves Disease

• Is the most common form of thyrotoxicosis (90%)

• F:M = 4-5:1, 30-50 yoa

• Aetiology

autoantibodies which stimulate the thyroid ( in contrast with Hashimoto’s)

Graves Disease

• Thyrotoxicosis and goitre due to increased stimulation

• Infiltrative opthalmopathy = exopthalmos

• Dermopathy ( pretibial myxedema) due to accumulation of GAGS

Toxic Adenoma

• A benign tumour which secretes excess thyroid hormones

• Note there can also be malignant thyroid tumours but these are uncommon