Thyroid Gland – Class 11 | Chapter – 22 | Biology Short Notes Series PDF

Thyroid Gland: The thyroid gland is a ductless endocrine gland situated in the anterior/front portion of the neck. It roughly resembles the shape of a butterfly. It is also one of the largest endocrine glands, weighing an average of 25 – 30 g. This gland has two lobes on either side of the trachea, with each lobe measuring 4 – 6 cm in length and 1.3 – 1.8 cm in width.

The primary function of the thyroid gland is to secrete two hormones, namely, Triiodothyronine (T3) hormone and the Thyroxine hormone (T4). Both T3 and T4 hormones play a very important role and affect almost every tissue in the body.

Location of Thyroid Gland

As discussed, the thyroid gland is located in front of our neck. More specifically, it lies in front of our trachea, or “windpipe”. When the structure of the thyroid gland is inspected more closely, it has a reddish-brown coloration. The color is due to the fact that the thyroid gland is highly innervated and has its blood supplied by the superior and inferior thyroid arteries and the external carotid artery. The two-lobed structure will be linked by a bridge called the isthmus that lies in the middle of the lobes.

Thyroid Gland Anatomy

Hormones of Thyroid Gland

The thyroid gland primarily makes and releases T3 and T4 hormones and the levels at which either thyroxine (T4) or triiodothyronine (T3) are released can be modulated to slow things down or speed things up. Thyroid hormones are made with the iodine flowing in our blood after a meal, which is then integrated into the physical structure of the hormones.

The thyroid cells that make up the gland have a special trait of being highly absorbent to iodine. Every remaining cell in the body will rely on the thyroid gland to manage its metabolism. Regardless, the levels of T3 to T4 will be eighty to twenty percent in a normal, functional thyroid gland.


The hypothalamus and the pituitary gland are the main controls that regulate the actual thyroid’s activity. When T3 or T4 hormone levels become too low, the hypothalamus will respond by secreting TSH-releasing hormone (or TRH). TRH signals the pituitary gland to create more thyroid stimulating hormone (or TSH). The thyroid gland, in turn, will respond by making more thyroid hormone in a feedback loop. The levels will be finely tuned to maintain a balance of T3 to T4 hormones.

TSH release will have a direct impact on the hormones’ respective levels. The role of TSH can be summed up as a stimulus that will lead the thyroid gland to release more hormone. Abnormally high levels of TSH can indicate an underactive thyroid. This is a condition called hypothyroidism. The related symptoms of having T3 and T4 in excess are listed below:

  • Anxiety
  • Hair loss
  • Irritability
  • Hyperactivity
  • Hand trembling


On the other hand, when the T3 and T4 levels fall under the functional amounts, the body will undergo changes in the opposite direction. Chronically high thyroid hormone levels will lead to hyperthyroidism. The high T3 and T4 levels will signal the pituitary gland to release less TSH in the system. Common symptoms may include the following:

  • Insomnia
  • Fatigue and tiredness
  • Depression
  • Difficulty concentration
  • Muscle pain

Normal Levels of Thyroid Activity

In summary, a high TSH will strongly suggest that the thyroid is underactive, or not making the right amount of thyroid hormone to sustain the body’s functions. The reason the levels of TSH are so high is that the body is trying to make the thyroid gland produce hormones to compensate for its lacking. A low TSH, in contrast, will infer a very active thyroid gland that is making too much thyroid hormone. In this case, the pituitary gland will try to inhibit TSH release to stop the thyroid gland from making more of it.

There is some variance over the reference range of TSH levels in the blood, but when the test reveals a level 0.5 or below, it is a possible sign of hyperthyroidism. When levels are above the 3-5.0 range, there is a good chance of hypothyroidism. This scale is hotly contested in the medical community but is still a parameter used to diagnose thyroid disorders. In addition to a blood test, an iodine thyroid scan will reveal if the origin of the thyroid hormone imbalance is a single nodule or the entire thyroid gland.

Symptoms of Thyroid

Sometimes, symptoms of a thyroid disorder are not very obvious. This is because there are various other factors that can induce similar symptoms and usually, the treatment is given according to the symptoms. For instance, excessive tiredness may be associated with sleep apnea, narcolepsy etc, but the underlying symptoms may actually be thyroid related. Some of the common symptoms of the thyroid are:

  • Nervousness
  • Poor concentration and knowledge retention
  • Change in the menstrual cycle
  • Increased heart rate
  • Muscle aches
  • Weight gain
  • High level of cholesterol

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By Team Learning Mantras