Thyroid gland

Hypothyroidism is a common condition where the thyroid doesn't create and release enough thyroid hormone into your bloodstream. It can occur congenital (develops prenatally and is present at birth) or as an acquired defect.

Acquired hypothyroidism (primary hypothyroidism)

Causes of acquired hypothyroism:

• thyroidectomy (surgical removal);
• ablation of the gland with radiation for treatment of hyperthyroidism;
• lithium carbonate, and the antithyroid drugs propylthiouracil and methimazole in continuous dosage can block hormone synthesis and produce goiter with hypothyroidism
• Iodine deficiency (rare)
• Autoimmune disorder (Hashimoto thyroiditis) (most common). Predominant disease in women

Manifestastion of disorder are related largely to two factors:

1. hypometabolic state (due to thyroid hormone deficiency)
2. myxedematous involvement of body tissus

Hypometabolic state associated with:

• gradual onset of weakness and fatigue;
• tendency to gain weight despite a loss of appetite;
• cold intolerance.

In more progressed conditions:

• dry and rough skin;
• hair becomes coarse and brittle;
• face becomes puffy with edematous eyelids;
• thinning of the outer third of the eyebrows;
• Gastrointestinal motility is decreased, producing constipation^[1], flatulence^[2], and abdominal distention^[3].

[1] constipation - condition when people unable to completely empty bowel

[2] flatulence is passing gas from the digestive system out of the back passage. It's more commonly known as "passing wind", or "farting".

Myxedematous state 

• Myxedematous fluid is usually most obvious in the face;
• enlargement of tongue;
• the voice becomes hoarse and husky;

Thyroidectomy (surgical removal) or ablation of the gland with radiation for treatment of hyperthyroidism → Destruction or dysfunction of the thyroid gland (primary hypothyroidism) → Thyroid hormone deficiency due to autoimmune disorder (Hashimoto thyroiditis) → hypometabolic state → symptoms:

• gradual onset of weakness and fatigue;
• tendency to gain weight despite a loss of appetite;
• cold intolerance;
• dry and rough skin;
• coarse, dry, brittle hair;
• periorbital edema (edematous eyelids), and puffy face;
• gastric atrophy: constipation, flatulence, abdominal distention;
• CNS involement: mental dullness, lethargy, impaired memory; 
• Tongue enlargement;
• Deep, coarse voice

Mucopolysacharide deposits in the heart → generalized cardiac dilation, bradycardia, and other signs of altered cardiac output 

Hyperthyroidism (overactive thyroid) occurs when thyroid gland produces too much of the hormone thyroxine.

• The most common cause of hyperthyroidism is Graves disease. 
• Other causes are: multinodular goiter; adenoma of the thyroid; thyroiditis.
• Ioding-containin agents can induce hyperthyroidism

Many of the manifestations of hyperthyroidism are related to the increase in
oxygen consumption and use of metabolic fuels associated with the hypermetabolic state, as well as increase in symphathetic nervous system activity. 

Hypermetabolic state → complaints of nervousness, irritability, and fatigability, weight loss despite a large appetite. Other manifestations include:

• tachycardia;
• palpitations;
• shortness of breath;
• excessive sweating;
• muscle cramps;
• heat intolerance;
• fine muscle tremor;
• exophthalmos (bulging eyes) and infrequent blinking (staring);

Graves disease is an autoimmune disorder characterized by abnormal stimulation of the thyroid gland by thyroid-stimulating antibodies (TSH receptor antibodies) that act through the normal TSH receptors.

*TSH receptor - thyrotropin receptor is a receptor (and associated protein) that responds to thyroid-stimulating hormone