The normal thyroid gland is a fairly homogenous structure, but nodules often form within its substance. These nodules may be only the growth and fusion of localized colloid-filled follicles, or more or less discrete adenomas, or cysts. Nodules larger than 1 cm may be detected clinically by palpation. Careful examination discloses their presence in at least 4% of the general population. Nodules less than 1 cm in diameter and not clinically detectable unless located on the surface of the gland, are much more frequent. The terms adenomatous goiter, nontoxic nodular goiter, and colloid nodular goiter are used interchangeably as descriptive terms when a multinodular goiter is found.

INCIDENCE

The incidence of goiter, diffuse and nodular, is very much dependent on the status of iodine intake of the population. In areas of iodine deficiency, goiter prevalance may be very high and especially in goiters of longstanding, multinodularity developes frequently (see Chapter 20). The incidence of multinodular goiter in areas with sufficient iodine intake has been documented in several reports. In a comprehensive population survey of 2,749 persons in northern England, Tunbridge et al.¹ found obvious goiters in 6.9% with a female/male ratio of 13:1.Single and multiple thyroid nodules were found in 0.8% of men and 5.3% of women, with an increased frequency in women over 45 years of age. Routine autopsy surveys and the use of sensitive imaging techniques produces a much higher incidence. In three reports nodularity was found in 30% to 60% of subjects in autopsy studies, and in 16% to 67% in prospective studies of randomly selected subjects on ultrasound.² In Framingham the prevalence of multinodular goiter as found in a population study of 5234 persons over 60 years was 1%.³. Recent results from Singapore shows a prevalence of 2.8%.⁴ In an evaluation in 2,829 subjects, living in southwestern Utah and Nevada (USA between 31 and 38 years) of age, 2.3% had non-toxic goiter, including, 18 single nodules, 3 cysts, 38 colloid goiters and 7 without a histological diagnosis. No mention was made of multinodular goiters, although some might have been present in the colloid and unidentified group.⁵ In general, in iodine sufficient countries the prevalence of multinodular goiter goiter is not higher than 4 %.⁶

CAUSE

The first comprehensive theory about the development of multinodular goiter was proposed by David Marine and studied further by Selwyn Taylor, and can be considered one of the classics in this field. Nodular goiter may be the result of any chronic low-grade, intermittent stimulus to thyroid hyperplasia. Supporting evidence for this view is circumstantial. David Marine first developed the concept, that in response to iodide deficiency the thyroid first goes through a period of hyperplasia as a consequence of the resulting TSH stimulation, but eventually, possibly because of iodide repletion or a decreased requirement for thyroid hormone, enters a resting phase characterized by colloid storage and the histologic picture of a colloid goiter. Marine believed that repetition of these two phases of the cycle would eventually result in the formation of nontoxic multinodular goiter.⁸ Studies by Taylor of thyroid glands removed at surgery led him to believe that the initial lesion is diffuse hyperplasia, but that with time discrete nodules develop.⁹ By the time the goiter is well developed, serum TSH levels and TSH production rates are usually normal or even suppressed.¹⁰ For example, Dige-Petersen and Hummer evaluated basal and TRH-stimulated serum TSH levels in 15 patients with diffuse goiter and 47 patients with nodular goiter.¹¹ They found impairment of TRH-induced TSH release in 27% of the patients with nodular goiter, suggesting thyroid autonomy, but in only 1 of the 15 with diffuse goiter. Smeulers et al. ²², studied clinically euthyroid women with multinodular goiter and found that there was an inverse relationship between the increment of TSH after administration of TRH, and size of the thyroid gland (Figure 17-1). It was also found that, while being still within the normal range, the mean serum T3 concentration of the group with impaired TSH secretion was significantly higher than the normal mean, whereas the mean value of serum T4 level was not elevated.¹² These and other (1) results ¹²a are consistent with the hypothesis that a diffuse goiter may precede the development of nodules. They are also consistent with the clinical observation that, with time, autonomy may occur, with suppression of TSH release, even though such goiters were originally TSH dependent.

Figure 17-1. Relationship of TSH (after 400 m g TRH i.v.) and thyroid weight (g) in 22 women with clinically euthyroid multinodular goiter (with permission ref. 12).

Comprehensive reviews about insights into the evolution of multinodular goiter have been published by Studer et al.¹³-¹⁶ An adapted summary of the major factors that are discussed by these authors is presented in Table 17-1 and will be referred to in the discussion that follows.

PRIMARY FACTORS

Genetic heterogeneity of normal follicular cells and acquisition of new inheritable qualities by replicating epithelial cells.

It has been shown that cells of many organs, including the thyroid gland, are often polyclonal, rather than monoclonal of origin. Also from a functional aspect it appears that through developmental processes the thyroid epithelial cells forming a follicle are functionally polyclonal and possess widely differing qualities regarding the different biochemical steps leading to growth and to thyroid hormone synthesis like e.g. iodine uptake (i.e. transport), thyroglobulin production and iodination, iodotyrosine coupling, endocytosis and dehalogenation. As a consequence there is some heterogeneity of growth and function within a thyroid and even within a follicle (Fig. 17-2). Studer et al ¹⁶a demonstrated the existence of monoclonal and polyclonal nodules in the same multinodular gland. They analyzed 25 nodules from 9 multinodular goiters and found 9 to be polyclonal and 16 monoclonal. Three goiters contained only polyclonal nodules and 3 contained only monoclonal nodules. In 3 goiters poly- and monoclonal nodules coexisted in the same gland.

Figure 17-2. Heterogeneity of morphology and function in a human multinodular goiter. Autoradiographs of two different areas of a typical multinodular euthyroid human goiter excised after administration of radioiodine tracer to the patient. There are enormous differences of size, shape and function among the individual follicles of the same goiter. Note also that there is no correlation between the size or any other morphological hallmark of a single follicle and its iodine uptake. (with permission ref. 15).

Newly generated cells may acquire qualities not previously present in mother cells. These qualities could subsequently be passed on to further generations of cells. A possible example of this process is the acquired abnormal growth pattern that is reproduced when a tissue sample is transplanted into a nude mouse.¹⁶b Other examples are acquired variable responsiveness to TSH.¹³ These changes may be related to mutations in oncogenes such as ras, or others which do not produce malignancy per se, but that can alter growth and function. An example of acquisition of genetic qualities is the identification in the last few years of constitutively activating somatic mutations not only in solitary toxic adenoma, but also in hyperfunctioning nodules of toxic multinodular goiters. So far these mutations in MNG have only been found in the TSH-receptor (TSHR) gene, and not in the Gs-alpha gene. Different somatic mutations are found in exon 9 and 10 of the TSHR gene and the majority of mutations that are present in toxic adenomas are also found in toxic nodules in multinodular goiter. Sometimes different toxic nodules in the same multinodular gland harbor different mutations.¹⁶c,d An important fact is the finding of a germline mutation of codon 727 of the TSHR gene that is specifically associated with MNTG (16e). Also evidence was found for linkage of familial euthyroid goiter to the recently identified locus for familial multinodular nontoxic goiter (MNG-1) on chromosome 14q ¹⁶f. Perhaps MNTG constitutes a heterogeneous group consisting of MNG due to multiple autonomously functioning nodules generated by somatic mutations of the TSHR (and Gs-alpha?) gene, and MNG caused by a germline mutation(s) of this gene. TSH-R mutations have also been detected in microscopically hot areas in thyroids of patients living in an iodine deficient region ¹⁷. Three dominant MNG loci have been identified in familial MNG, i.e. MNG1, 2 and 3. In MNG1 a major locus was identified on chromosome 14q by a genomic search on a single large Canadian family with 18 cases of nontoxic multinodular goiter. Although the gene for TSHR is located on this chromosome, it was excluded as a candidate gene. In the analysis of an Italian three-generation pedigree with familial MNG2, including 10 affected females and 2 affected males, a novel MNG locus was searched for. Because no male-to-male transmission was present in the study pedigree, an X-linked autosomal dominant pattern of inheritance was hypothesized and confirmed. A significant LOD score was observed in the Xp22 region (17b). A third locus, MNG3, for a dominant form of familial multinodular goiter was detected on 3q26.1-q26.3, in 2 independent Japanese families. This variant however was characterized by congenital hypothyroidism (17c). No inborn error of thyroid hormone synthesis could be detected, but this entity should probably be still categorized in congenital hypothyroidism with secondary MNG, rather than primary MNG.

Subsequent functional and structural abnormalities in growing goiters

Follicles of second and following generations are less well formed and compartmentalization of key enzymes may become altered. Intercellular communication may become disrupted. As a consequence inter- and intrafollicular growth and function may become poorly integrated resulting in further heterogeneity.¹³,¹⁸

SECONDARY FACTORS

The secondary factors discussed below stimulate thyroid cell growth and/or function and, because of differences in cellular responsiveness that are presumed to exist, aggravate the expression of heterogeneity which leads to further growth and focal autonomic function of the thyroid gland. Local necrosis, cyst formation sometimes with bleeding and fibrosis may be the anatomical end stage of such processes.¹³ (see pathology)

Iodine Deficiency

Stimulation of new follicle generation seems to be necessary in the formation of simple goiter. Evidence accumulated from many studies indicates that iodine deficiency or impairment of iodine metabolism by the thyroid gland, perhaps due to congenital biochemical defects, may be an important mechanism leading to increases in TSH secretion. Since in experimental animals the level of iodine per se may modulate the response of thyroid cells to TSH, this is an additional mechanism by which relatively small increases in serum TSH level may cause substantial effects on thyroid growth in iodine-deficient areas. Koutras et al.¹⁹ found that the thyroidal iodine clearance of patients with nontoxic nodular goiter in Scotland was, on average, higher than that in normal persons (Fig. 17-3). This finding was interpreted as a reflection of a suboptimal iodine intake by such patients. Similar observations have been made in Belgium and France but not in the United States. When data published from various major cities in Western Europe, regarding thyroid volume and iodine excretion are put together,²⁰ an inverse relation is found between urinary iodine excretion and thyroid volume (Fig. 17-4). Physiologic stresses, such as pregnancy, may increase the need for iodine and require thyroid hypertrophy to increase iodine uptake that might otherwise satisfy minimal needs. An elevated renal clearance of iodine occurs during normal pregnancy.²¹,²² It has been suggested that in some patients with endemic goiter there are similar increases in renal iodine losses.²³,²⁴ Increased need for thyroxin during pregnancy may also lead to thyroid hypertrophy when iodine intake is limited. Iodide need in pregnancy is increased by increased iodide loss through the kidneys, but also because of significant transfer of thyroid hormone from the mother to the fetus.²⁵ Glinoer and co-workers showed that, especially in areas of moderate iodine intake, thyroid volume increase is predominantly effected by a higher HCG serum concentration during the first trimester of pregnancy, and by a slightly elevated serum TSH level present at delivery.²⁰

Figure 17-3.Relationship between nontoxic goiter and thyroidal iodine clearance (with permission ref. 19).

Fig. 17-4. Correlation between thyroid volume and urinary iodine excretion in normal populations from various areas (with permission ref. 20).

Fig. 17-4. Correlation between thyroid volume and urinary iodine excretion in normal populations from various areas (with permission ref. 20).

Dietary Goitrogens

Patients occasionally have thyroid enlargement either because of goitrogenic substances in their diet²⁷ or because of drugs that have been given for other conditions. Peltola²⁸ has shown this response experimentally by feeding rats minute doses of a natural goitrogen over many months. Similar results have been obtained by Langer ²⁹,³⁰ using combinations of the three most prevalent goitrogens contained in cabbage. The explanation for the effect of such substances is that the goitrogen is much more effective at the level of iodothyronine synthesis than at earlier steps in hormone production such as iodide trapping. Thus, the RAIU may be high, but with a block in hormone synthesis the stage would be set for the production of a goiter. This possibility remains to be proved in humans, but one might surmise that, if true, it would operate most effectively in a situation of borderline iodine supply. As discussed in Chapter 5, the goitrogen KSCN potentiates the effect of severe iodine deficiency in endemic areas of Africa.

Inherited Defects in T4 Synthesis

An intriguing clue to the cause of nontoxic goiter in some patients is that it is familial. No particular pattern of inheritance has been found in these situations, although the condition can often be traced through several generations. Occasionally, other members of the family may have Graves' disease. One might propose that patients with nontoxic goiter are heterozygous for genes that in the homozygous state may lead to clinically apparent hypothyroidism. Some investigators have evaluated iodide transport in patients with multinodular goiter and found it to be normal.³¹ Parker and Beierwaltes³² observed that many relatives of patients with defective iodine organification had goiter, but the results of iodine-binding studies were normal in these subjects. Some relatives of patients with the iodotyrosine halogenase defect have goiter and are euthyroid. In the latter instance, it has been possible to demonstrate that relatives have a deficiency in the deiodinating enzyme system, but this deficiency is not severe enough to cause hypothyroidism. Similar results were reported by McGirr.³³

R.S., 42-Year-Old Man: Multinodular Goiter Produced by a Congenital Metabolic Defect.

The birth and early development of this clerk were entirely normal. His progress in school was slow, but he managed to complete trade school. A goiter was first noted at age 14; because of slowly increasing pressure symptoms, it necessitated a subtotal thyroidectomy 9 years later. The specimen showed hyperplasia and involution. By age 37 a goiter again was present. The patient ultimately sought help because of difficulty in passing a preemployment examination. His mother had a goiter, and one sister died at age 13 at operation for removal of a goiter. Two other siblings were well.

The patient appeared well developed and adequately nourished. The thyroid was four times the normal size, lobulated, and without a bruit or thrill. The patient's intelligence was less than average. His physical examination was otherwise normal. The RAIU was 60% at 2 hours and 82% at 24 hours. The PBI concentration in serum was 4.8 m g/dl (normal, 4-8 m g/dl).

A second subtotal thyroidectomy was performed. Slices from the specimen deiodinated MIT and DIT normally. The tissue was fractionated by centrifugation, and 99% of labeled iodine was found in the 1000,000 x g supernatant fraction. On ultracentrifugation, 51% of the protein was present as a 19.4 S component and 49% as a 4.3 S component. The latter fraction, which would contain lightweight proteins such as albumin, was much increased over normal values. On enzymatic digestion of the supernatant fraction (presumably containing TG), 40% of the 131I, administered prior to surgery, was DIT; 21%, MIT; and 3%, iodothyronine. Twenty-one percent resisted hydrolysis. Electrophoresis of the supernatant fraction showed that 131I was associated with a visible TG band, and in addition, a protein moving in the position of albumin, present in high concentration. This protein behaved immunologicaly as albumin. It was estimated that the thyroid contained approximately 5 g/dl of this iodinated albumin or albumin-like protein.

The pathologic diagnosis was multinodular goiter with multiple adenomas. Numerous adenomas of the fetal and embryonal type were interspersed with colloid nodules; the picture suggested prolonged stimulation of the thyroid gland.

After the second operation, the patient was maintained on thyroid hormone replacement and did well. There was no further recurrence of the goiter. This case illustrates the development of multinodular goiter in a patient with an inherited defect in thyroid hormone biosynthesis.

The appearance of the goiter by age 14, the strong familial tendency to thyroid disease, and the mental retardation, all suggested that this patient had a congenital goiter that may have been associated with hypothyroidism during early life. The tendency of the goiter to recur, as well as the histologic pattern of the second surgical specimen, supported the interpretation that the goiter grew in response to an abnormality in hormone synthesis. Metabolic compensation was apparently achieved during adult life by means of hypertrophy of the gland. The thyroid appeared to form and release into the serum an abnormal iodoprotein. This iodoprotein was metabolically inactive, and it indicated inefficient use of iodide by the thyroid. Formation of this protein presumably was secondary to some metabolic block in hormone synthesis.

Despite the possibility that inherited defects are involved in some patients with multinodular goiter, most have been completely normal when examined specifically for such defects. Major problems of analysis are the low sensitivity to identify recessive states and the marked heterogeneity of function that exists within a single gland.¹³ For example, Niepomniszcze and co-workers³⁴ evaluated peroxidase function in 13 patients with nontoxic multinodular goiter. Both "cold" and "warm" nodules were identified by scintiscanning before thyroidectomy. The iodide peroxidase activity of cold nodules was in general reduced, whereas in 10 warm nodules studied, 7 had normal activity and 3 decreased activity. Thus, one may conclude that the cold nodules of these multinodular glands, were peroxidase deficient. However, in the same glands, normal activity could be found in other nodules that were active in concentrating RAI. Heterogeneity of iodide organification was confirmed in the studies of Peter et al.³⁵ and summarized by Studer.³⁶ By autoradiography two types of cold follicles were found, namely those that failed to accumulate iodide because of deficient trapping and those that could transport iodide but could not organify it, suggesting failure of apical membrane peroxidase.³⁷ Three TSH dependent enzymic activities, i.e. peroxidase, NADPH-cytochrome-c reductase and monoamine oxidase, showed dissimilar activity within a single tissue sample and among different tissues of multinodular goiters³⁷a As in multinodular goiter not only distinct nodules are discernable, but thyroid tissue is in general goitrous, it would seem reasonable to assume that some form of a partial biosynthetic abnormality is the most likely explanation for sporadic multinodular goiter. This concept appears to be borne out in the reported family in which goiter was associated with a mutation in the TG gene in the area of a "hormonogenic" thyroxin residue.³⁸ Apart from this phenomenon affecting the whole thyroid gland, somatic mutations of the TSH-R additionally cause growth and autonomous function of some nodules present in this type of goiter.

Other Thyroid-Stimulating Factors

Other substances that could be involved in stimulating thyroid enlargement are epidermal growth factor (EGF) and insulin-like growth factors (IGF). EGF stimulates the proliferation of thyrocytes from sheep, dogs, pigs, calves, and humans.⁴² While stimulating growth, EGF reduces trapping and organification of iodide, TSH receptor binding, and release of thyroglobulin, T3 and T4. On the other hand TSH may modulate EGF binding to thyroid cell membranes and thyroid hormone may stimulate EGF production and EGF receptor number.⁴² In a study on adenomatous tissue, obtained from patients with multinodular goiter, it was found, by immunohistochemistry, that expression of EGF was increased.⁴³ IGF-2 interacts with trophic hormones to stimulate cell proliferation and differentiation in a variety of cell types. The interaction between TSH and IGF-2 is synergestic.⁴⁴ Increased IGF-I expression may contribute to goiter formation.⁴⁵ A similar synergistic effect may exist between IGF-I and TSH⁴⁶. This synergism on DNA synthesis is mediated by complex interactions including the secretion of one or more autocrine amplification factors. Non-functioning nodules in patients with multinodular goiter contain the same IGF-1 receptors that are present in the normal adjacent extra-nodular follicles but are expressed in higher concentrations.⁴⁷ Fibroblast growth factor (FGF)- 1, stimulates colloid accumulation in thyroids of rats but only in the presence of TSH.⁴⁸ Expression of FGF-1 and -2 and FGF-receptor- 1 accompany thyroid hyperplasia and may play a role in development of multinodular goiter.⁴⁸a Fancia et al.⁴⁸b found that in goiters with aneuploid components growth rate was higher than when euploid components were present (48c.) Other factors promoting cell growth and differentiation have been identified in the past decade. These include cytokines, acetylcholine, norepinephrine, prostaglandins, substances of neural origin like vasoactive intestinal peptide, and substances of C-cell origin. It is however not known to what extent these compounds play a role in the genesis of multinodular goiter. These substances are discussed in Chapter 1.

The hypothesis that the development of thyroid autonomy is due to a gradual increase in the numbers of cells having relatively autonomous thyroid hormone synthesis is supported by the 27% prevalence of impaired TSH responses to TRH in patients with nodular goiter as opposed to such responses in only 1 of 15 patients with diffuse goiter.¹¹ Such partial autonomy may appear only with time and could possibly be prevented by TSH-suppressive therapy. The fact that it is possible to induce hyperthyroidism in some patients with multinodular goiters by administration of iodide suggests that certain of the nodules in the multinodular gland are autonomous but unable under normal iodine intake to concentrate sufficient quantities of iodide to cause hyperthyroidism.⁴⁹ Presumably iodide administration provides sufficient substrate for generation of excessive amounts of hormone, although it does not readily account for the long persistence of the hyperthyroidism in some of those cases.

Thus, there may be several etiologic factors in simple and nodular goiter, and some of these factors may act synergistically. The end result is a collection of heterogeneously functioning thyroid follicles, some of which may be autonomous and produce sufficient amounts of thyroid hormone to cause hyperthyroidism.