Thyroid

Anatomy

Structure

The thyroid is found in the neck in front of the larynx, around C5,6,7 and T1. It weighs about 25g. The term thyroid means shield, though it looks more like a butterfly. There are two lobes, joined in the middle by the isthmus.

Functions

Produces three hormones, the thyroid ones released in response to TSH from the pituitary:

• thyroxine (T4) inert storage hormone
• triiodothyronine (T3) active hormone used by cells
• calcitonin helps preserve calcium, released when levels of Ca²⁺ are high.

Blood supply

Blood supply is, unsurprisingly, from the thyroid arteries, and drains to the thyroid veins. Check out this picture of the larynx blood supply for more info.

Clinical Conditions

Hyperthyroidism

Leads to excessive levels of T3 and T4. This leads to symptoms such as:

• Hair loss
• Anxiety
• Goitre
• Weight loss
• Tremor
• Tachycardia
• Diarrhoea
• Menorrhagia

Long term, it can lead to osteoporosis.

Graves disease

The commonest type, usually associated with a big goitre. It happens when an IgG antibody is produced, which mimics the action of TSH, thus causing growth in gland tissue and overproduction. Look out for exophthalmos (bulgy eyes).

Toxic nodular goitre

Rare, where one of more of the lumps in a goitre becomes hypersecretory. It's unlikely you will see one of these, but doesn't hurt to be in the know!

Check out the hyperthyroidism page for more!

Hypothyroidism

Also known as myxoedema, results from lack of circulating T3 and T4. Presents with:

• Loss of outer third of eyebrows (IMPORTANT)
• Weight gain
• Constipation
• Bradycardia
• May have goitre
• General slowing down physically and mentally

If it happens in the newborn, it can lead to permanent cretinism, unless therapy is undergone immediately.

Hasimoto's thyroiditis

Commonest cause of hypothyroidism. Basically the thyroid is attacked by the body. At first the gland may hypertrophy (grow), but soon it will atrophy and fibrose. Treatment is by Levothyroxine (T4) tablets, and in 14% of patients, treatment is with liothyronine (T3)

Iatrogenic

Certain drugs, such as Lithium, can cause hypothyroidism, and surgical removal of the tissue has a similar effect.

Check the hypothyroid page for more on this bad boy

Tumour

Patients presenting with a solid mass in the thyroid is a common problem. Usually it will prove to be a particularly large node in a multinodular goitre, but it can also be neoplastic.

Benign

Tumours of the thyroid are generally benign, with the only real danger being potential malignancy, or of tracheal compression/patient discomfort.

Malignant

Carcinoma of the thyroid is a fairly rare tumour, and has a good prognosis, since it presents early – patients tend to notice a lump on their neck – and is usually well differentiated. It is known to be very associated with radiation exposure.

Main types are:

• Papillary, under 45s, slow growing, excellent prognosis

• Follicular, (FCT)20-55s, FCT can spread by blood vessels and metastases outside of the neck. Unlikely to spread to the lymph nodes. Can be targeted by (RAI)radioactive iodine, good prognosis.

• Hurthle Cell,(HCC) Generally presents at the fifth and sixth decade in patients with a solitary nodule or multi goitre. Classified a "Variant" by WHO world health orginasation. It has a 3% incidence of all thyroid cancers and treated as FCT. More aggressive type of carcinoma Hurthle cell may metastatise to the lymph nodes. Spreads by invasion into blood vessels, which may give rise to metastases outside of the neck. Good prognosis and receptive to RAI therapy.

• Medullary Carcinoma (MTC) 75% are Sporadic and isolated. Hereditary MTC 25% occurs with multiple endocrine neoplasia. Tumour marker "Serum Calcitonin"(CT)screening is more sensitive than a (FNAB) Fine needle aspiration gives an earlier diagnosis of an unsuspected malignancy with a better outcome. Surgery is recommended.

• Anaplastic, Elderly, aggressive, poor prognosis.