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Whether thyroid gland is functioning sufficiently or not is demonstrated by "TSH hormone" released from a section of a brain called "pituitary". This substance ensures providing more hormone production and delivering to blood by stimulating structures secreting hormone in the thyroid gland. When thyroid gland works less, delivery of TSH from pituitary to blood increases. Thyroid hormone in sufficient amount is not secreted for body tissues. In this case called as "hypothyroidism", level of thyroid hormones decreases and hormone in sufficient amount does not reach tissues. 

Hypothyroidism is a disease arising from partial or whole failure of formation of thyroid gland hormones from thyroid glands and failure of delivering to blood circulation. Hypothyroidism seen in infants and children causes retardation in growth and development. The most serious of these is mental retardation and this case cannot be fixed with delayed treatment. In adult, a general slow-down may be witnessed in the activities of organism. However, symptoms in adults are greatly fixed with thyroid hormone treatment.

The most important causes

 • Destruction of thyroid gland by antibodies (antimatter) generated by organism against thyroid gland.
 • Surgical removal of all and almost all of thyroid gland with different reasons.
 • Destruction of nearly all thyroid gland after treatment of toxic goiter 
 • Other reasons can be listed as insufficient intake of iodine with diet, congenital absence of thyroid hormones, congenital defects in formation of thyroid hormones.

Symptoms and Findings

Development and growth retardation in children is an important symptom. In the adults, weakness, drowsiness, weakness, headache, weight gain, chills, irregularity in menstrual bleeding in women, skin dryness, loss of hair, eyebrows and other hairs, breaking nails, constipation, hoarseness, decreased sweating, tingling in hands and feet, swelling in the arms and legs and face, decreased hearing, joint and muscle pain, reduction in memory, mood changes may occur. 

Course of the disease with treatment usually yields good results. However, rarely coma due to hypothyroidism may be seen in elderly patients who have quitted their treatment for a while and having received no treatment at all. 

In summary;

 • Hypothyroidism is a disease resulting from deficiency or rarely ineffectiveness of thyroid hormone on tissue level.
 • Primary hypothyroidism is due to reasons arising from thyroid gland deficiency. 
 • Secondary hypothyroidism is a type of hypothyroidism depending on TSH deficiency. 
 • Tertiary hypothyroidism is a type of hypothyroidism depending on TRH deficiency.
 • Diagnosis of primary hypothyroidism is placed according to TSH levels. 
- TSH 0.5- 4 mIU/L normal (except for pregnancy)
- TSH >4 mIU/L T3, T4 normal: subclinical hypothyroidism\n
- TSH >10 mIU/L T4 and/or T3 lower: overt hypothyroidism 
- TSH >10 mIU/L, T3, T4 low and organ failure: myxedema coma

 • Average replacement dose is 1.6 mcg/kg, (1.4-1.8 µg/kg) doses suitable for a person may vary. Initial dose in treatment depends on age of a patient, duration of disease, severity of disease.

Subclinical Hypothyroidism 
This is a case where T3 and T+ levels are normal and TSH level is high TSH levels (>4 mIU/L) and there is no clinical findings of overt hypothyroidism. 

Slight subclinical hypothyroidism: TSH: 4-10 mIU/L
Heavy subclinical hypothyroidism: TSH >10 mIU/L

What should be done? 

 • TSH should be measured in people over 35 years of age once in 5 years.
 • TSH value should be measured minimum twice within quarterly period in diagnosis of subclinical hypothyroidism.
 • If there is risk factor in pregnancy and in those considering pregnancy, TSH measurement should be conducted at all times.
 • Even if there is no Risk Factor, TSH measurement at least once is recommended in pregnant women or planning pregnancy.
 • TSH > 10 mIU/L and T3 –T4 all normal cases should be treated at all times.
 • TSH: Patients with normal 4-10 mIU/L and T3-T4 should be treated in the following cases:
Pregnant women and those planning pregnancy and patients with positive thyroid antibodies (anti-TPO and / or anti-Tg).

Patients are treated by administering thyroxine (T4) hormone. Note that T4 hormone will be taken lifetime except for cases of transient hypothyroidism cases. Patients should understand well that T4 hormone they use is in fact not a substance foreign to the organism and hormone deficiency not produced by thyroid gland is replaced by this. Control examination of patients under treatment is determined by their doctors. However, check-ups and hormone measurements should be conducted at least annually.

Prımary Hypothyroidism 
Hypothyroidism and subclinical hypothyroidism is a frequent occurrence. Its frequency increases with ageing and in women gender. The most common cause of primary hypothyroidism is chronic autoimmune thyroiditis (Hashimoto's thyroiditis), thyroidectomy or radioactive iodine therapy, some medications and thyroiditis.  

Hypothyroidism is diagnosed with laboratory data. High TSH, low T4 is essential for overt hypothyroidism. Clinical symptoms and findings depend on the severity and duration of hypothyroidism. Weakness, fatigue, weight gain, forgetfulness, difficulty in concentrating, dry skin, loss of hair, chills, constipation, hoarseness, irregular and heavy menstruation, infertility, muscle stiffness, muscle pain, carpal tunnel syndrome, depression, dementia may be seen. Dry, pale skin, infrequent coarse hair, rough hoarse voice, bradycardia, myxedema (nonpitting), carpal tunnel syndrome, goiter can be determined. Rare pericardial fluid can be encountered. In hypothyroidism developed depending on chronic autoimmune thyroiditis (Hashimoto), autoimmune diseases may be accompanied with such as vitiligo, Pernicious anaemia, rheumatoid arthritis, Type 1 diabetes mellitus Addison's disease.

Anti-TPO antibodies should be measured for diagnosis in regard to etiology of diagnosed cases of hypothyroidism/subclinical hypothyroidism. It is useful in determining the prognosis. TSH levels may be normal or low in secondary and tertiary hypothyroidism (pituitary or hypothalamic).
Treatment is lifetime in those with persistent disease.