Hypothyroidism

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Contents
Introduction
Hashimoto's Thyroiditis
Clinical Features
Investigations
Treatment

Introduction

Hypothyroidism is also known as myxoedema and is the clinical syndrome that results when circulating levels of thyroxine are inappropriately low.
Of these causes, Hashimoto's thyroiditis, atrophic thyroiditis, iatrogenic aetiologies (drugs and surgery) account for many cases. Globally, iodine deficiency is an important case.

Atrophic thyroiditis affects older people and exhibits chronic inflammation of the thyroid gland accompanied by atrophy of the follicles and fibrosis of the remaining tissue. In contrast with Hashimoto's thyroiditis, there is no goitre.

Riedel's thyroiditis is very rare. The thyroid gland is diffusely replaced by fibrous tissue and forms a goitre. The process of fibrosis can extend beyond the thyroid gland. Riedel's thyroiditis may be associated with retroperitoneal fibrosis.

The antiarrhythmic drug amiodarone contains iodine and is able to induce hypothyroidism or hyperthyroidism.

Hyperthyroidism may initially occur in acute thyroiditis, due to the release of excessive amounts of thyroxine secondary to the inflammation but after the initial surge has subsided the thyroid may become underactive while it recovers from the inflammatory attack.
Rarely, the thyroid gland can fail to develop properly. Dyshormogenesis is another rare cause of hypothyroidism in which one of several enzyme defects prevents the gland from synthesising thyroxine.

Hashimoto's Thyroiditis

Hashimoto's thyroitis is an autoimmune disease that presents between the ages of 30 to 50 years and is considerably more common in women (the male : female ratio may reach 1 : 20). The condition is associated with HLA-DR3 and DR5.

The thyroid gland is enlarged due to a widespread chronic inflammatory cell infiltrate. Lymphoid follicles are formed. The follicular epithelial cells of the thyroid gland are enlarged and have increased eosinophilia of their cytoplasm due to the accumulation of mitochondria. This is known as Hurtle cell change.

A rare complication of Hashimoto's thyroiditis is the development of a B cell non-Hodgkin lymphoma in the thyroid, usually extranodal marginal zone lymphoma.

Clinical Features

The clinical features of hypothyroidism generally reflect the expected consequences of a deficit of thyroxine and are in many cases the opposite of the corresponding features in the same organ systems to hyperthyroidism.

As with hyperthyroidism, patients who suffer from hypothyroidism tend to have a characteristic facial appearance. The face appears puffy, particularly around the eyes. Alopecia occurs; loss of the outer third of the eyebrows is often quoted. The skin is pale, dry and dough-like. The tongue can be enlarged.

The thyroid gland may or may not be enlarged, depending on the cause of the hypothyroidism. Two important causes that produce enlargement of the thyroid gland are Hashimoto's thyroiditis and iodine deficiency.

The pulse is slow, in the form of a sinus bradycardia. A pericardial effusion may develop,

The patient's appetite is reduced but their weight may actually increase because the low metabolic rate means that there is a decrease in the rate of use of ingested food for energy transduction.

Constipation may develop.

The effects of hypothyroidism on the brain can generally be conceptualised as a slowing down and a loss of drive. Concentration and memory are impaired. The patient may sleep more. Depression can supervene.

Muscle weakness may develop. The reflexes are reduced and may be slow to relax. The patient's voice can become hoarse and deep. Rarely, deafness or cerebellar ataxia occur.

Whereas hyperthyroid patients are intolerant of warm temperatues, patients who have hypothyroidism deal with the cold badly due to their impaired ability to generate heat.

Hypothyroidism can disturb menstruation and produce infrequent, heavy periods. Infertility may be a problem.

Myxoedema Coma

A myxoedematous coma is an acute, severe presentation of hypothyroidism. The patient has depressed consciousness and is often hypothermic, bradycardic and has a slow respiratory rate. The condition tends to occur in older patients who are just about coping with undiagnosed hypothyroidism but then suffer an intercurrent illness such as an infection which overwhelms their reserves.

Investigations

Confirmation of hyporthyroidism is achieved through thyroid function tests. In untreated hyporthyroidism the T4 (and T3) levels are reduced. In almost all cases the levels of TSH are high because the lack of endogenous thyroxine removes the feedback inhibition on the release of TSH.

Other blood tests are related to excluding background complications or exarcerbating factors.

FBC Anaemia can exacerbate some features of hypothyroidism.
Autoimmune pernicious anaemia can be associated with autoimmune hypothyroidism
U+E Hyponatraemia can complicate hypothyroidism.
LFT Liver function tests may be mildly disturbed in hypothyroidism.
Haematinics Pernicious anaemia may co-exist with hypothyroidism. There is also often a macrocytosis in hypothyroidism and therefore determination of the levels of folate and vitamin B12 is prudent.
Creatinine Kinase A myopathy can be part of the muscular features of hypothyroidism.
Lipids Hypothyroidism is sometimes associated with hyperlipidaemia.

Blood can also be analysed for antithyroid antibodies.

Treatment

Hypothyroidism can be corrected by giving oral thyroxine. The dose is adjusted until the patient's TSH levels are normal. This correction of the TSH levels implies that the hypothalamic-pituitary system perceives the thyroxine levels to be adequate.

A myxoedematous coma is an emergency that requires treatment of any underlying precipitating condition, T3/T4 replacement and intravenous hydrocortisone (due to the risk of co-existing Addison's disease).