Genetic Factors

The thyroid gland contains a series of enzymes that are essential for the biosynthesis and secretion of thyroid hormones. A defect in any of these hormones can result in diminished hormone synthesis and a condition of goiter formation known as dyshormonogene sis. Because the defects are inherited disorders, dyshormonogenesis is also known as familial goiter. These enzyme defects may be partial or complete. Patients with a more severe enzymatic defect may develop goiter and cretinism early in life. When the defect is partial or less severe, goiter often develops during adolescence or later in life, and these individuals are usually euthyroid. Although familial clustering of goiter is well recognized, no simple mode of inheritance has been recognized. Familial euthyroid goiter has recently been linked to a multinodular nontoxic goiter (MNG1) locus on chromosome 14q.6,7 Concordance rates for simple goiter in female monozygotic twins have been reported higher than in female dizygotic twins (42% and 13%, respectively).8 The age-adjusted cumulative risk for simple goiter from birth to age 43 years was 0.53 for female monozygotic twins and 0.18 for female dizygotic twins.9 These facts provide evidence of a genetic component of the etiology of goiter.

Tissue refractoriness to thyroid hormones due to a thyroid-stimulating hormone receptor (TSHR) defect is a rare cause of familial goiter. A germline mutation on codon 727 of the TSHR gene on chromosome 14q31 is specifically

TABLE 4-1 Classification of Goiter

Epidemiology Endemic Sporadic Familial Etiology Morphology Diffuse Nodular Multinodular Solitary nodule Activity of the thyroid gland Toxic Nontoxic Location Neck

Thorax (substernal) Malignant vs, benign associated with toxic multinodular goiter.10'11 Similar somatic cell mutations may activate an intrinsic growth control system leading to goiter.

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