**1. Physiological considerations**

Thyroid disorders are most common endocrine problem next to diabetes mellitus. Thyroid disorders affect women more compared to men. Thyroid glands secrete, store, and release triiodothyronine (T3) and thyroxine (T4). The hormone T4 gets converted into T3 at tissue level and produces its effect. Iodine is necessary for production of thyroid hormone. Iodine in food is trapped by thyroid gland and is utilized in hormone production. Pituitary and hypothalamus control thyroid gland hormone secretion. TRH from hypothalamus modulates through pituitary to produce TSH, which in turn controls thyroid hormone production. If T4 and T3 are low, TSH level increases to stimulate thyroid gland to secrete more hormone. T3 and T4 hormones have profound effect on the body. Almost all the tissues are stimulated, and body metabolism is increased. T3 and T4 affect cardiovascular system, GI tract, brain, metabolism, weight, bone, etc. With increased T4 and T3, there is tachycardia, diarrhea, hyperglycemia, lowering of cholesterol, increased growth rate in infant, normal brain development, and sexual function. Undiagnosed hypothyroidism in infants not only affects physical and bony growth but also damages brain growth. If untreated, it leads to permanent damage.

Thyroid functions are affected by congenital absence of thyroid glands, autoimmune thyroid disease, surgical removal, infiltrative diseases, and after radiation to neck. Drugs like amiodarone, lithium, interferon alpha, and interleukin 2 prevent thyroid glands from making hormone and causing hypothyroidism.

Pituitary damage by tumor radiation or surgery can affect thyroid glands and cause secondary hypothyroidism.

For brain maturation and brain function, thyroid hormone is necessary. Thyroid diseases like hypothyroid can cause lethargy, hyporeflexia, depression, memory impairment, weight gain, dry

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

skin, and constipation along with dyslipidemia. Hyperthyroidism produces weight loss, tremors, irritability, and hyperreflexia. Glucose intolerance can also be caused by hyperthyroidism.

**5. Thyroid carcinoma**

fer to fetus.

involved which is termed as thyroid ophthalmopathy.

**6. Thyroid disorders in pregnancy**

**7. Thyroid function test in pregnancy**

• Less than 3 μg in the second trimester

• Less than 5 μg in the third trimester

drugs are not needed in this situation.

**2.** Increase in estrogen leads to increased TBGT level.

**2.** TSH level varies with trimester

• 2.5 μg in the first trimester

• T4 = 1.5 times higher

type 3 deiodinase leading to decreased thyroxin level

**8. Factors influencing thyroid functions in pregnancy**

**3.** Altered immune reaction leads to autoimmune phenomenon.

**4.** Increased thyroid hormone metabolism occurs in placenta.

dysfunctions can affect neurointellectual development of fetus.

Thyroid carcinoma like anaplastic, follicular, and adeno may eventually cause hypothyroidism. Diabetes mellitus is associated more with hypothyroidism. In Grave's disease, eyes are

Introductory Chapter: Introduction to Thyroid Disorders http://dx.doi.org/10.5772/intechopen.82044 3

Thyroid diseases affect both mother and child before and after delivery. Untreated thyroid

During pregnancy, human chorionic gonodotrophin is produced from placenta. This is structurally similar to TSH. Hence, this stimulates thyroid gland, leading to increase in T4 and T3 and decrease in TSH. Thyroid binding globulin is increased due to estrogen and in turn binds T4 and increases T4 level and increases T4 to T3 conversion. There is increased iodine consumption and increased renal iodine clearance. There is increase of iodine trans-

**1.** Increased TBG, T4, and iodine clearance, decreased thyrotropin, and increased placental

**1.** Increase in human chorionic gonodotrophin in the first trimester stimulates TSHR. This leads to transient gestational hyperthyroidism or hyperemisisgravidorum. Antithyroid

Thyroid hormone acts through T3 with nuclear receptors and regulation of gene expression. Hormone deficiency can cause retarded brain maturation and neurological impairment. Thyroid hormone deficiency is caused by congenital and maternal hypothyroidism. Hypothyroidism causes lethargy, hyporeflexia, poor motor coordination, and memory impairment. Hypothyroidism is also associated to bipolar affective disorders, depression, or loss of cognitive functions, especially in the elderly. Thyroid hormone deficiency, even of short duration, may lead to irreversible brain damage.

There is suggestion that reduction in circulating thyroid hormone concentrations is one of the factors mediating impaired neurological development in intrauterine growth retardation and premature babies. More research is required to resolve these questions and ultimately shows if thyroid hormone or iodide supplementation in hypothyroid mothers in the antenatal period and in premature neonates can reduce the prevalence of neurodevelopmental delay.
