Hyperthyroidism: Causes, Signs & Treatments Video

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  • 0:03 Thyroid Anatomy
  • 3:18 Symptoms & Etiology
  • 5:41 Diagnosis & Treatment
  • 7:24 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

The thyroid gland weighs only about half an ounce, but it drives the metabolism of every cell in our body. In this lesson we'll look at the effects of hyperthyroidism, in which the thyroid gland and its effects on our metabolism are kicked into overdrive.

Thyroid Anatomy and Hormones

'Metabolism' is a word that you hear in many diet ads. But we don't often think about what that means. More than just burning calories, metabolism is all of the biochemical processes that we need to stay alive. All cells - bacteria, plant, and animal - use biochemical pathways to provide them with energy. In humans, metabolism is largely under control of the thyroid gland.

The thyroid gland is a butterfly-shaped gland located in the neck, on top of the larynx. It's divided into two lobes, one on either side of the neck, and connected by an isthmus.

The thyroid is under the control of both the pituitary gland and the hypothalamus. The hypothalamus makes thyrotropin-releasing hormone, or TRH. TRH goes to the pituitary to tell it to make thyroid-stimulating hormone (TSH). TSH, in turn, speeds to the thyroid, signaling it to release thyroid hormones. The production of thyroid hormones stops via negative feedback. In other words, when there are high concentrations of thyroid hormones in the blood, the whole system stops.

A good way to help you remember this is to think of this hormone system like a house's heating system. In this example, the thyroid gland is the furnace, the pituitary gland is the thermostat, and the hypothalamus is the person living in the house. When the house is cold, the person sets the thermostat to signal the furnace to make heat. When the house grows warmer, the system shuts off.

In hyperthyroidism, the system never shuts off, causing the release of too many thyroid hormones. For this lesson, we'll focus on two of them: T3 (tri-iodothyronine) and T4 (thyroxine).

The thyroid needs iodine to make T3 and T4. Thyroid cells absorb iodine and combine it with the amino acid tyrosine to make these hormones. About 80% is T4, and 20% is T3.

Too much of T3 and T4 results in thyrotoxicosis. There is a subtle difference between hyperthyroidism and thyrotoxicosis - hyperthyroidism means having an overactive thyroid gland, while thyrotoxicosis is the condition of having increased levels of T3 and T4 in the blood. Thyrotoxicosis can be caused by taking too much thyroid medication, for example, or having thyroiditis (an inflamed thyroid).

T3 and T4 exert their effects in cells all over the body. These effects include:

  • An increase in basal metabolic rate (or, how quickly we burn energy at rest)
  • Regulation of body temperature
  • Control of heart and respiration rates
  • An increase in sensitivity to catecholamines, like epinephrine. (Recall that these chemicals play a large role in your 'fight or flight' response)
  • Regulation of metabolism of protein, fat, and carbohydrates

Hyperthyroidism Symptoms and Etiology

Recalling that 'hyper' means 'high,' imagine that that the output of the thyroid gland suddenly increases sharply. What symptoms would it make sense to see - that is, what might happen when there is a sudden increase in metabolism and an increase in our 'fight or flight' response? Although they vary from individual to individual, symptoms of hyperthyroidism generally include:

  • Nervousness and excitability
  • Muscle weakness and tremor
  • Tachycardia (that is, a heart rate of over 100 beats per minute) or irregular heart beat
  • An inability to adjust to changes in temperature (called heat intolerance) coupled with heavy perspiration
  • Weight loss

In some cases, patients present with goiter, a somewhat generic term for an enlarged thyroid. Goiter can interfere with breathing and swallowing.

Let's dig into what causes these symptoms.

Graves' disease is the most common cause of hyperthyroidism. Graves' is an autoimmune disease, a disease in which the body mistakes its own cells for pathogens. In this case, the body produces an antibody that attaches to the TSH receptors on the thyroid gland. Because TSH isn't actually triggering the release of T3 and T4, the negative feedback system the body has in place doesn't work. Instead, these antibodies continually tell the thyroid gland to take in iodine, pump out T3 and T4, and ultimately increase the thyroid gland's size, causing goiter.

Thyroid nodules are lumps of tissue on the thyroid. Often caused simply by advancing age, these lumps can sometimes produce thyroid hormones and cause hyperthyroidism. One nodule is called toxic adenoma, while more than one are termed toxic multinodular goiter.

Other less common causes for hyperthyroidism include thyroid cancer and pituitary adenoma. In adenoma, a non-cancerous hormone-secreting tumor forms on the pituitary gland, causing it to release too much TSH. This is a form of secondary hyperthyroidism, because the thyroid gland itself remains normal.

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