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12.02 Thyroid Gland

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Overview

  1. Thyroid gland
    1. Location: bilobed “H”-shaped gland below larynx on anterior trachea
    2. Hormones
      1. Triiodothyronine (T3)
      2. Tetraiodothyronine (T4) (thyroxine)
      3. Calcitonin – NOT under control of anterior pituitary gland like T3 and T4
  2. Parathyroid glands (4)
    1. Location: in thyroid on posterior side
    2. Hormone
      1. Parathormone (parathyroid hormone)
    3. Controlled by calcium concentration in blood

Nursing Points

General

  1. T3 and T4
    1. Control of T3/T4 secretion
      1. Hypothalamus makes thyroid releasing hormones (TRH)
        1. Enters HHP system
        2. Goes to anterior pituitary gland
      2. Anterior pituitary responds and secretes TSH into blood
      3. Thyroid gland responds and secretes T3 and T4
      4. If T3 and T4 are too high:
        1. Inhibits TRH release from hypothalamus
        2. Therefore ↓ TSH
        3. Negative feedback loop
          1. Allows for tight control
      5. T3
        1. Short duration of activity
        2. Fast reactions
      6. T4
        1. Long duration
        2. Slow reactions
    2. Actions of T3 and T4
      1. Stimulate carbohydrate metabolism in most cells
      2. Stimulate production of body heat
        1. Helps maintain normal temperature
      3. Stimulate protein synthesis
  2. Calcitonin
    1. Maintain calcium concentration in blood
    2. Controlled by calcium concentration
    3. If calcium concentration in blood is increased:
      1. Stimulates osteoblasts to make bone from blood calcium
      2. Inhibits osteoclasts from adding calcium to blood by breaking down bone
      3. Inhibits calcium uptake from small intestine
        1. Excreted in stool
      4. Inhibits calcium uptake from PCT of kidney
        1. Excreted in urine
      5. Effect = decreased calcium levels in blood
  3. Parathyroid Hormone (parathormone
    1. Controlled by calcium concentration
    2. Functions
      1. Opposes activity of Calcitonin
  4. Clinical Notes:
    1. Hyperthyroidism – see individual lesson
      1. Excess secretion of T3/T4
      2. Low levels of TSH
      3. Hypermetabolic state
    2. Hypothyroidism – see individual lesson
      1. Insufficient secretion of T3/T4
      2. High levels of TSH
      3. Hypometabolic state
    3. Hyperparathyroidism
      1. Excess secretion of parathormone
      2. Increased calcium levels in blood
      3. Decreases bone density
    4. Hypoparathyroidism
      1. Insufficient secretion of parathormone
      2. Decreased calcium levels in blood
      3. Muscle twitching, convulsions, respiratory muscle failure

References:

Betts, J.G., et al. (2017). Anatomy and physiology. Houston, TX: OpenStax, Rice University. Retrieved from https://openstax.org/details/books/anatomy-and-physiology?Book%20details

Study Tools

Video Transcript

The first thing that we have to look at is that the thyroid gland is a by load, meaning it’s got two lobes, H shaped gland that sits below the larynx. It’s just in front of or anterior to the trachea. The thyroid gland is responsible for producing several different hormones. These hormones are T3, or triiodothyronine, T4, or Tetraiodothyronine, or calcitonin. It’s important to note that calcitonin is not controlled by the anterior pituitary gland and we’ll get into that later.

The parathyroid is also part of the thyroid gland but they’re their own particular gland. There are four glands and they are located on the posterior side of the thyroid gland. The main hormone that they produce is called parathormone or the parathyroid hormone. And that’s controlled by the calcium concentration in the blood.

So how does T3 and T4 work in the body? Well first there’s a signal that’s received by the hypothalamus that stimulates the release of thyrotropin-releasing hormone, or trh. That then signals to the anterior pituitary gland to secrete thyroid stimulating hormone, which travels to the thyroid gland. At that point the thyroid gland produces T3 and T4 until it’s not necessary anymore. Once that signaling stops, and negative feedback signal is sent back to the anterior pituitary gland and the hypothalamus to tell them to stop producing trh and TSH.

T3 is a short-acting hormone and has a fast reaction so if there’s a need for faster metabolism and T3 is released. T4 is just the opposite. If long-duration of metabolism or a slow reaction is needed, T4 is released. The main functions of these two hormones are to stimulate carbohydrate metabolism and the production of body heat. It also helps to maintain a normal temperature and stimulates protein synthesis.

So let’s Circle back to calcitonin. Calcitonin is produced by the thyroid gland but unlike the other thyroid hormones it is not controlled by the pituitary gland. What actually controls calcitonin production is the calcium level in the blood. So let’s look at this diagram. If calcium levels are elevated, what happens is it calcitonin is released by the thyroid gland. This vindas several things and it tells osteoblast to make new bone, which pulls calcium from the blood. More importantly it tells of osteoclasts to stop breaking down bone and releasing calcium into the bloodstream. It also inhibits calcium uptake from the small intestine and allows the calcium to be excreted in the stool, and finally it inhibits calcium uptake in the proximal convoluted tubule in the kidney. this allows the body to have the blood calcium return to normal.

Parathormone is the exact opposite and opposes the activity of calcitonin. If blood calcium is low, parathormone is released by the parathyroid glands. This in turn stimulates osteoclast production and calcium is broken down from bone and released into the bloodstream. This increases blood calcium. So they work together to help maintain balance of calcium in the blood.

Sometimes we can have access or deficient amount of a particular hormone. With the thyroid gland we can either have access thyroid hormone production, or we can have changes in the amount of parathormone produced by the parathyroid gland.

Starting with thyroidism, we are looking specifically at the production of T3 and T4. In hyperthyroidism, there is an excess production of T3 and T4, this in turn turns down the response of the pituitary gland in producing thyroid stimulating hormone. This has an increase in the metabolic state. If we look at hypothyroidism, this is a decrease production of T3 and T4, and there’s going to be this constant stimulation of thyroid stimulating hormone from the pituitary gland. But because there’s a decrease production of those hormones, that’s going to lead to a decreased metabolic State.

We talked about the parathyroid gland, and excess production of parathyroid hormone causes an increased calcium in the blood. Remember that the increase in the parathormone is going to stimulate those osteoclasts to break down the bone and that’s going to release the calcium into the bone but it also weakens the bone and causes a decrease in bone density. If we look at hypoparathyroidism, this is a decrease in parathormone. This means that there is a decreased level of calcium in the blood and that leads to muscle twitching and failure of the respiratory muscles.

Okay so let’s recap.

The thyroid gland is located just below the larynx And in front of the trachea.

The parathyroid gland are four glands located on the posterior side of the thyroid.

The thyroid hormones are produced are T3 T4 and calcitonin. Remember that calcitonin is not stimulated by the pituitary gland.

Parathormone is the hormone that is secreted by the parathyroid gland.

And finally calcitonin and parathyroid hormone are directly related to the amount of calcium in the blood and respond depending on if the calcium in the blood is too high or too low
And that’s it for our lesson on the thyroid gland. Make sure you check out all the resources attached to this lesson. Now go out, and be your best self today, and is always happy nursing.

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