Thyroid Gland

This article is currently under review and may not be up to date. Please come back soon to see the finished work! (12/02/2022)

Original Editor - Niha Mulla

Top Contributors - Niha Mulla, Lucinda hampton, Kim Jackson and Carina Therese Magtibay  

Introduction

Thyroid gland is a part of human endocrine system. Thyroid gland works together with the nervous system and the immune system to regulate the body metabolism by secreting hormone and releasing it into the blood stream .

Anatomy

Thyroid gland is a butterfly shaped organ and is situated at the front of the neck in the lower part of the throat and consists of two connected lobes on either side of the wind pipe. Lower end of the lobes is connected by thyroid isthmus which is a thin band of connective tissue.

The thyroid weighs between 20 and 60 grams on average. With each lobe being about 5 cm long, 3 cm wide, and 2 cm thick. The thyroid gland is comparatively larger in women as compared to men and also increases in size during pregnancy. The infrahyoid muscles lie in front of the gland and the sternocleidomastoid muscle to the side. Behind the outer wings of the thyroid lie the two carotid arteries whereas the trachea, larynx, lower pharynx and esophagus all lie behind the thyroid. Four parathyroid glands, two on each side, lie on each side between the two layers of the thyroid capsule, at the back of the thyroid lobes.

There is loose connective tissue between the inner and the outer capsule, so the thyroid can move and change its position when we swallow.

Thyroid Gland- location

Thyroid gland- anterior view

Blood supply of thyroid gland

The thyroid gland is supplied with arterial blood from:

• The superior thyroid artery - a branch of the external carotid artery.
• The inferior thyroid artery - a branch of the thyrocervical trunk.
• And sometimes, by an anatomical variant the thyroid ima artery which has a variable origin.
• The superior thyroid artery splits into anterior and posterior branches supplying the thyroid, and the inferior thyroid artery splits into superior and inferior branches.
  

The venous blood is drained via superior and middle thyroid veins, which drain to the internal jugular vein and via the inferior thyroid veins. Both arteries and veins form a plexus between the two layers of the capsule of the thyroid gland.

Nerve supply of thyroid gland

The gland receives sympathetic nerve supply from the superior, middle and inferior cervical ganglion of the sympathetic trunk whereas the gland receives parasympathetic nerve supply from the superior laryngeal nerve and the recurrent laryngeal nerve.

Lymphatic drainage

Lymphatic drainage frequently passes through:

• Pre-laryngeal lymph nodes which is located just above the isthmus
• Pre-tracheal lymph nodes and
• Paratracheal lymph nodes.

Function of thyroid gland

Thyroid gland produces three hormones:

1. Triiodothyronine, also known as T3
2. Tetraiodothyronine, also called thyroxine or T4
3. Calcitonin

Physiology of thyroid hormones:

• T3 and T4 are the only proper thyroid hormones, they are made made in follicular epithelial cells of the thyroid gland.
• The third hormone produced by thyroid gland calcitonin is made by c-cells.
• Iodine is the building block of both T3 and T4. Hence, dietary intake of this trace mineral iodine is vital.
• Amount of thyroid hormone required by the human body at a particular time varies; and to make the perfect quantity of thyroid hormone at any given time, the human body needs help from the pituitary gland.

Thyroid hormones affect your:

1. T3 and T4 increase the basal metabolic rate.
2. T3 and T4 promote physical and mental growth in children.
3. Calcitonin is involved in calcium and bone metabolism.
4. Thyroid hormones activate the nervous system which improves concentration.
5. Thyroid hormone affects your: body temperature and circulation, appetite, energy levels, growth and bone development, muscle tone and suppleness, heart rate, blood sugar levels, central nervous system and bowel function, cholesterol levels, fat, carbohydrate and protein metabolism.
6. Higher the amount of T3 and T4 in the body, higher is the body metabolism. If the level of T3 and T4 drops in the body, so does the rate of metabolism.

Iodine for thyroid hormone

• Human body need iodine to create the thyroid hormone. As we know iodine is the building block of both T3 and T4.
• This trace mineral cannot be produced by the human body.
• Hence, dietary intake of iodine is vital.
• Iodine is absorbed into the blood stream from food and bowel, from where it is carried to the thyroid gland to be used by the thyroid gland in due course.

The pituitary gland and thyroid hormone

• The pituitary is an endocrine gland that is located at the base of your brain.
• It controls the endocrine system.
• It effects the thyroid by producing a hormone called thyroid stimulating hormone (TSH).
• TSH stimulates the thyroid gland to produce right amount T3 and T4.
• If there is more amount of T4 present in ones blood stream, the pituitary gland will produce less TSH which will slow down the thyroid activity leading to less production of T4. Whereas, if the level of T4 hormone is less then required amount in they body, the pituitary gland will produce higher quantity of TSH to boost the metabolism and production of thyroid hormone.

Hormone imbalances: Overactive and underactive thyroid gland

Overactive thyroid ( hyperthyroidism) occurs if the thyroid gland produces thyroid hormones in excess. And an underactive thyroid (hypothyroidism) is where the thyroid gland does not produce enough thyroid hormones.

Overactive thyroid (hyperthyroidism):

1. a
2. b

Underactive thyroid (hypothyroidism):

1. a
2. b