The thyroid gland controls how quickly the body uses energy, makes proteins, and controls how sensitive the body is to other hormones. It participates in these processes by producing thyroid hormones, the principal ones being triiodothyronine (T3) and thyroxine (sometimes referred to as tetraiodothyronine (T4)). 

These hormones regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body. 

There are five general types of thyroid disorders which can influence the function of the gland:

1.  Hypothyroidism 

Hypothyroidism means the thyroid gland is underactive and fails to secrete enough hormones into the bloodstream. Hypothyroidism is the most common thyroid disorder, and it is thought to affect around six to 10 per cent of women. The prevalence rises with age - up to a quarter of women over the age of 65 years may be affected. Men are also affected, but less frequently.

Hypothyroidism can be either primary or secondary. Primary hypothyroidism means that the thyroid gland itself is diseased, while secondary hypothyroidism is caused by problems with the pituitary gland, the brain structure that supervises the thyroid gland. The most common cause of primary hypothyroidism is the autoimmune condition Hashimoto’s disease (an autoimmune disease in which the thyroid gland is gradually destroyed).

Common hypothyroid symptoms include fatigue, low energy, slow heart rate and dry skin. Hypothyroid individuals may have weight gain despite poor appetite, cold intolerance, constipation and lethargy. However, these symptoms are often non-specific and make diagnosis difficult.

2.  Hyperthyroidism

Hyperthyroidism means the thyroid gland is overactive. This condition tends to affect women more than men. Around two in every 100 women will experience some degree of hyperthyroidism. The most common cause is Graves' disease, which is due to an immune system abnormality. Other causes of an overactive thyroid include local inflammation (thyroiditis), nodules or lumps. There is no cure for hyperthyroidism, but it can be successfully managed with treatments such as anti-thyroid medication.

For hyperthyroidism, the most common symptoms are associated with adrenergic (working on adrenaline (epinephrine) or noradrenaline (norepinephrine)) stimulation: fast heartbeat, palpitations (the perceived abnormality of the heartbeat characterized by awareness of cardiac muscle contractions in the chest: hard, fast and/or irregular beats), nervousness, tremor (an unintentional and uncontrollable rhythmic movement of one part or one limb of your body), increased blood pressure, weight loss, diarrhea and heat intolerance. Clinical manifestations are often related to hypermetabolism (increased metabolism), enlargement of the thyroid, and ocular changes (exophthalmos) (Pic. 1). 

3.  Structural abnormalities, most commonly an enlargement of the thyroid gland

A struma (Pic. 2, 3, 4) or goitre or goiter is a swelling of the neck or larynx resulting from enlargement of the thyroid gland (thyromegaly), associated with a thyroid gland that is not functioning properly.

Regarding morphology, goitres may be classified either as the size of the growth:

  • Class I (palpation goitre): in normal posture of the head, it cannot be seen; it is only found by palpation.
  • Class II: the goitre is palpable and can be easily seen.
  • Class III: the goitre is very large and is retrosternal; pressure results in compression marks.

Goitre, associated with hypothyroidism or hyperthyroidism, may be present with symptoms of the underlying disorder. 

4.  Tumors which can be benign or cancerous

Thyroid cancer is the most common endocrine malignancy. Most often the first symptom of thyroid cancer is a nodule in the thyroid region of the neck. However, many adults have small nodules in their thyroids, but typically under 5% of these nodules are found to be cancerous. Sometimes the first sign is an enlarged lymph node. Later symptoms that can be present are pain in the anterior region of the neck and changes in voice due to an involvement of the recurrent laryngeal nerve.

Thyroid cancer is usually found in a euthyroid (the state of having normal thyroid gland function) patient, but symptoms of hyperthyroidism or hypothyroidism may be associated with a large or metastatic well-differentiated tumor.

Thyroid nodules are of particular concern when they are found in those under the age of 20. The presentation of benign nodules at this age is less likely, and thus the potential for malignancy is far greater.

5.  Abnormal thyroid function tests without any clinical symptoms

Sometimes, the origin of thyroid dysfunction is unknown.

Anovulation

Anovulation (when the ovaries do not release an oocyte during a menstrual cycle), and infertility are common symptoms of both types of thyroid disorders and result from both direct and indirect effects on the endometrium (the inner layer of uterus) and ovaries. 

Direct binding of thyroid hormones to receptors on ovarian surface epithelium, ovarian follicles, and the endometrial lining influences proliferation (phase of the menstrual cycle in which ovarian follicles mature and prime themselves for ovulation) and regulation of menstruation. 

Indirectly, alterations in thyroid hormone levels cause changes in serum levels of sex hormone binding globulin (SHBG), sex hormones (estrogen and testosterone), and their metabolism which are neccessary for regular menstrual cycle and ovulation.

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Sources

Anovulation ―sourced from Wikipedia licensed under CC BY-SA 3.0
Thyroid disease ―sourced from Wikipedia licensed under CC BY-SA 3.0
Thyroid cancer ―sourced from Wikipedia licensed under CC BY-SA 3.0
Goitre ―sourced from Wikipedia licensed under CC BY-SA 3.0
Overview of the Thyroid Gland ―sourced from Boundless licensed under CC BY-SA 4.0
Hypothyroidism ―by Queensland Government licensed under CC BY 3.0
Hyperthyroidism ―by Queensland Government licensed under CC BY 3.0
Goitre ―by Ed Uthman licensed under CC BY 2.0
Struma ―by Drahreg01 licensed under CC BY- SA 3.0
Goitre ―by Martin Finborud licensed under CC0
Exophtalmos ―by The Armed Forces Institute of Pathology licensed under CC0
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