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Hypothyroidism

Definition

The diagnosis of 'Hypothyroidism' should be used for the diagnosis of primary, secondary or tertiary hypothyroidism ONLY

  • Primary Hypothyroidism - means the thyroid gland doesn't produce an adequate amount of thyroxine (T4).
  • Secondary Hypothyroidism - develops when the pituitary gland does not release enough of the thyroid-stimulating hormone (TSH) that prompts the thyroid to manufacture more of the thyroid hormones T4 and triiodothyronine (T3). IMPORTANT: TSH is a pituitary hormone NOT a thyroid hormone!
  • Tertiary Hypothyroidism - results from a malfunction of the hypothalamus, the part of the brain that controls the endocrine system.
  • Type 1 Hypothyroidism - is associated with insufficient secretion by the thyroid, the pituitary or the hypothalamus gland, thereby reducing the mainly inactive T4 available for conversion, to the active hormone T3 to energise the body. In the majority of cases, Type 1 Hypothyroidism may be corrected by using levothyroxine (T4-only) replacement.
  • Type 2 Hypothyroidism or more correctly called Euthyroid Hypometabolism - which is defined as deficiencies in the peripheral conversion of T4 to T3, the subsequent reception of the active thyroid hormone T3, and the use of T3 by the body's cells. Type 2 hypothyroidism reduces the amount of the active thyroid hormone T3 in the body, producing the same sort of symptoms that Type 1 hypothyroidism does. Environmental toxins may also cause or exacerbate the problem. The pervasiveness of Type 2 hypothyroidism has yet to be recognised by mainstream medicine, but already is in epidemic proportions. Type 2 hypothyroidism can be corrected by T3 or T4/T3 hormone replacement therapy - and not by T4 only therapy.

Epidemiology

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Incidence: overt (symptomatic) - 2% women, 0.2% men; sub-clinical (non-symptomatic) - 6-8% women, 3% men; during pregnancy - 2.5% women, 10% men. OK, probably not quite so many men. Increases with age; commonest around 60 years old. Iodine deficiency is the biggest cause worldwide.

Pathophysiology

Clinical Features

Emergency.gif
Medical Emergency - Myxoedema coma is where the body's adaptation to a lack of thyroxine fail. Coma is the main symptoms. Other are general symptoms of hypothyroidism (below).

Usually, with insidious onset with non-specific symptoms making it hard to pick up clinically. The symptoms are often the opposite of what happens in hyperthyroidism.

Investigations

  • TFTs - if you forget to do this in suspected hypothyroidism, you are an idiot (aka an F1 in their 1st week/a medical student). Essentially, it works like this:
    • If the TSH is high - it's primary hypothyroidism. The thyroid gland isn't working so the TSH is having no effect. The low thyroxine means that the pituitary axis is churning out TSH in an attempt to get the (now broken) thyroid gland going.
    • If the TSH is low - it's probably secondary hypothyroidism. The hypothalamus or pituitary is broken and not producing enough TSH. This results in the thyroid gland not being stimulated to produce thyroxine - even though it's probably fine.

There's probably other stuff. Try a knee X-ray. That might help. Doubt it though.

Management

THYROXINE. They don't have enough of it. You give them some. They theoretically and often actually get better. Start with 100-150μg (25-50μg).

In sub-clinical hypothyroidism (high TSH, normal T3 & T4), you geneerally do not treat. Exceptions are previous radio-iodide treatment and positive thyroid autoantibody. Monitor these patient 6-12 monthly.

Prognosis

Iodine can be round in rocks. So if you are in the developing world and suffering from iodine-deficiency hypothyroidism, don't eat rocks if tempted because that's silly.