The thyroid is a highly vascular ductless gland found deep in the neck region and overlapped by the anterior neck muscles.
Location of the thyroid gland:
It situated in front and at the sides of trachea opposite 5th, 6th, 7th cervical and 1st thoracic vertebrae in the neck region.

Shape of thyroid gland:
It is H-shaped mass and weighs about 25 gms.

Coverings of thyroid gland: 
It is enclosed by fibrous capsule (true capsule). It is also invested by the pretracheal layer of cervical fascia (false capsule).

Above, pretracheal fascia is attached to the hyoid bone and thyroid cartilage on each side. Below, it enters the thoracic cavity and blends with fibrous pericardium of heart. The pretracheal fascia is thickened to form the ligament (of Berry) which connects each lobe of thyroid with the cricoids cartilage (of larynx). These attachments of thyroid gland make it move up and down with swallowing.

Presenting parts of Thyroid gland:
It consists of two lateral lobes and an isthmus connecting them across.

Lateral lobes:
Each lobe is roughly pyramidal. The apex is directed upward and towards the thyroid cartilage and is sandwiched between the inferior constrictor (of pharynx) and sternothyroid muscles.
The superior thyroid artery and external laryngeal nerve are closely related to the apex of the thyroid lobe. The artery lies superficial and nerve passes deep to the apex. This is why the artery is ligated away from the thyroid gland during surgery.

The base extends up to 5th or 6th ring of the trachea and is related to the inferior thyroid artery and recurrent laryngeal nerve. Near the base of the lateral lobe, the recurrent laryngeal nerve is closely related to inferior thyroid artery and its branches, nerve crossing the artery either anteriorly or posteriorly or may pass between the branches of artery. Due to this close proximity, the artery is ligated some distance away laterally to the thyroid gland to avoid the injury to recurrent laryngeal nerve.

Superficial surface is overlapped by the neck muscles whereas the deep surface is related with the larynx and trachea, pharynx and oesophagus and parathyroid glands.

In the surgical treatment of hyperthyroidism, the posterior part of each lobe of the enlarged gland is usually preserved to protect superior and recurrent laryngeal nerves and parathyroid gland.

Pyramidal lobe of the thyroid gland
In addition to its normal presenting parts, approximately 50% of the thyroid glands present a pyramidal lobe that varies in size. The lobe extends superiorly from the isthmus of thyroid, usually to the left of the median plane. A band of connective tissue may connect the apex of the pyramidal lobe to the hyoid bone.

Arterial supply to thyroid gland:
Following arteries and their branches supply the thyroid gland:
Superior thyroid artery, a branch of external carotid artery
Inferior thyroid artery, a branch of thyrocervical trunk of subclavian artery

These arteries lie between the true capsule and pretracheal layer of deep cervical fascia.
The superior thyroid artery descend to the superior pole of the gland and divides into anterior and posterior branches after piercing the pretracheal fascia. The anterior branch of superior thyroid arterydescends along the anterior border of thyroid gland and supplies the anterior surface. the anterior branches of right and left sides anastomose across the midline. The posterior branch of superior thyroid artery descends along the posterior surface of the gland and anastomose with the inferior thyroid artery.
The inferior thyroid artery divides into several branches that pierce the pretracheal fascia and supply the inferior pole (or base) of the gland.

Thyroid gland is also supplied by:
Arteria thyroidea ima, if present, branch of arch of aorta or bracheocephalic trunk
Accessory thyroid arteries, from oesophageal and tracheal branches.

Venous drainage of thyroid gland:
It is important to understand that the veins do not accompany the arteries. They arise from the venous plexus which is present deep to the true capsule and are drained by three pairs of veins:
superior thyroid veins (drain the superior pole of the gland)
middle thyroid veins (drain the middle of the lobe) and
inferior thyroid veins (drain the inferior pole of the gland)

Superior and middle thyroid veins drain into internal jugular vein whereas the inferior drains mostly into the left bracheocephalic vein.

Lymphatic drainage of thyroid gland:

The lymphatic vessels of the thyroid gland run in the interlobular connective tissue, usually accompanying the arteries and form a capsular network of lymphatic vessels. The lymphatic vessels then pass initially to prelaryngeal, pretracheal, and paratracheal lymph nodes, which drain in turn to the superior (from the prelaryngeal nodes) and inferior deep cervical nodes (from the pretracheal and paratracheal nodes). Laterally, lymphatic vessels located along the superior thyroid veins pass directly to the inferior deep cervical lymph nodes.

Nerve Supply of thyroid gland:

The nerves of the thyroid gland are derived from the superior, middle, and inferior cervical sympathetic ganglia. The nerve fibers reach the gland through the cardiac and superior and inferior thyroid periarterial plexuses that accompany the thyroid arteries. These fibers are vasomotor and cause constriction of blood vessels. The secretory function or endocrine function of thyroid gland is regulated by hormones from the pituitary gland.

Structure of Thyroid gland:
The thyroid gland is divided into a number of lobules by numerous internal septa projecting from the fibrous capsule. Each lobule contains 40 to 60 thyroid follicles and a stroma in which blood vessels and lymphatics ramify.
Thyroid follicle is filled with colloid that contains iodinated thyroglobulin as storage. The follicle is surrounded by a single layer of follicular cells. Depending on the state of activity, the cells are flattened  with colloid abundant (resting state) or cuboidal (secretory state) with colloid diminished.

Following text books are used while preparing this post (readers are strongly recommended to go through them for more details):

Gray's Anatomy
K. L. Moore's Clinically Oriented Anatomy
R. Snell's Clinical Anatomy
Wheater's Functional Histology