Hyperthyroid Disorders

Lesson 2: Signs and Symptoms of Hyperthyroidism

Hyperthyroidism and Thyrotoxicosis

Hyperthyroidism refers to any condition in which an overactive thyroid gland produces excess thyroid hormone. Excess thyroid hormone refers to a situation in which more thyroid hormone is secreted into the blood circulation than the body requires for its needs.

Hyperthyroidism can occur for a number of different reasons. For instance, in toxic multinodular goiter, thyroid cells cluster together and trap increased amounts of iodine. Consequently, these cells produce excess thyroid hormone. In this condition, toxic refers to the effects caused by excess thyroid hormone.

The term hyperthyroidism covers any disease that results in the thyroid gland producing excess thyroid hormone. The autoimmune disorder, Graves’ disease, is the most common cause of hyperthyroidism. The various causes of hyperthyroidism are discussed in Lesson Four. Overall, hyperthyroidism occurs about seven times more often in women than in men. Although hyperthyroidism can affect people of all ages, it is more likely to occur after age 15.

Thyrotoxicosis refers to the clinical, physiologic and biochemical syndromes that result when the body’s tissues are exposed to high levels of thyroid hormone. Thyrotoxicosis occurs in people with hyperthyroidism and in people with thyroiditis when inflamed cells release thyroid hormone into the blood circulation. Thyrotoxicosis also occurs in people who purposefully or accidentally ingest too much thyroid hormone or foods contaminated with thyroid hormone. Thyrotoxicosis may also occur when excess thyroid hormone is produced by ectopic thyroid tissue. Ectopic thyroid tissue is thyroid tissue located in sites other than or away from the thyroid gland.

Therefore, thyrotoxicosis can occur in people who do not have hyperthyroidism. However, because the symptoms of thyrotoxicosis occur in people who are hyperthyroid, the term thyrotoxicosis is often used as if it is the same as hyperthyroidism.

Thyrotoxicosis usually develops insidiously, developing slowly and gradually and causing barely noticeable symptoms. Most patients have mild transient symptoms for as long as 3-6 months before thyrotoxicosis becomes clinically apparent. Almost every system in the body is affected and patients with thyrotoxicosis often visit various medical specialists before they’re properly diagnosed. For instance, patients with palpitations or an increased heart rate may first consult a cardiologist.

As we learned in the first lesson the body normally produces a stable amount of thyroid hormone, an amount that’s just right to cover the body’s needs. This level varies from person to person, and we all have our own optimal levels of thyroid hormone. When a large group of normal people is studied, their thyroid hormone levels can be averaged into a mean or average number.

The mean value can be used to devise a normal or reference range, in which half of the people have levels below the mean, and half of the people have levels above the mean. For instance, the FT4 normal or reference range is usually 0.8-1.8 ng/dl, and the mean or average level is 1.3 ng/dl.

People who normally have an FT4 level of 1.2 may experience symptoms and signs of thyrotoxicosis when their FT4 level rises to 1.4 ng/dl even though that level falls within the normal range. If the patient in this example who had a 1.4 ng/dl FT4 went to the doctor with symptoms of thyrotoxicosis, his condition would be difficult to diagnose especially if he never had thyroid function tests before.

In most cases the TSH level would be helpful because the pituitary gland would slow down on TSH secretion as soon as FT4 began rising. However, TSH lags behind thyroid hormone levels. The pituitary gland would wait to see if the slight elevation in FT4 was a temporary occurrence or a sign that hyperthyroidism was developing. Typically, TSH wouldn’t change to reflect this rise in FT4 for at least 6 weeks. TSH typically lags 6 weeks behind thyroid hormone levels.

From this you can see, that even though it lags behind thyroid hormone levels, in most cases TSH will drop to abnormally low levels, often to <.01 mu/L, before FT4 levels rise higher than the normal range. For this reason, the TSH test can be the first indicator that hyperthyroidism is developing. Exceptions would include thyroid hormone resistance in which the pituitary doesn’t respond to rising thyroid hormone levels and TSH remains normal despite high thyroid hormone levels and conditions in which TSH-secreting tumors are the cause of the hyperthyroidism.