Effects of caffeine on human health

Coffee and health: a review of recent human research

Coffee is a complex mixture of chemicals that provides significant amounts of chlorogenic acid and caffeine. Unfiltered coffee is a significant source of cafestol and kahweol, which are diterpenes that have been implicated in the cholesterol-raising effects of coffee. The results of epidemiological research suggest that coffee consumption may help prevent several chronic diseases, including type 2 diabetes mellitus, Parkinson's disease and liver disease (cirrhosis and hepatocellular carcinoma). Most prospective cohort studies have not found coffee consumption to be associated with significantly increased cardiovascular disease risk. However, coffee consumption is associated with increases in several cardiovascular disease risk factors, including blood pressure and plasma homocysteine. At present, there is little evidence that coffee consumption increases the risk of cancer. For adults consuming moderate amounts of coffee (3-4 cups/d providing 300-400 mg/d of caffeine), there is little evidence of health risks and some evidence of health benefits. However, some groups, including people with hypertension, children, adolescents, and the elderly, may be more vulnerable to the adverse effects of caffeine. In addition, currently available evidence suggests that it may be prudent for pregnant women to limit coffee consumption to 3 cups/d providing no more than 300 mg/d of caffeine to exclude any increased probability of spontaneous abortion or impaired fetal growth.

Beverage caffeine intakes in young children in Canada and the US

PURPOSE

Throughout childhood there is a shift from predominantly milk-based beverage consumption to other types of beverages, including those containing caffeine. Although a variety of health effects in children and adults have been attributed to caffeine, few data exist on caffeine intake in children aged one to five years.

METHODS

Because beverages provide about 80% of total caffeine consumed in children of this age group, beverage consumption patterns and caffeine intakes were evaluated from two beverage marketing surveys: the 2001 Canadian Facts study and the 1999 United States Share of Intake Panel study.

RESULTS

Considerably fewer Canadian children than American children consume caffeinated beverages (36% versus 56%); Canadian children consume approximately half the amount of caffeine (7 versus 14 mg/day in American children). Differences were largely because of higher intakes of carbonated soft drinks in the US.

CONCLUSIONS

Caffeine intakes from caffeinated beverages remain well within safe levels for consumption by young children.

Black tea--helpful or harmful? A review of the evidence

OBJECTIVE

To consider whether consumption of black tea has a positive or negative impact on health.

DESIGN

Databases were searched for relevant epidemiological and clinical studies published between 1990 and 2004.

RESULTS

Clear evidence was found for coronary heart disease (CHD), where an intake of > or = 3 cups per day related to risk reduction. The mechanism could involve the antioxidant action of tea polyphenols. While experimental models have suggested that flavonoids attenuated cancer risk, epidemiological studies failed to demonstrate a clear effect for tea, although there is moderate evidence for a slightly positive or no effect of black tea consumption on colorectal cancer. Studies on cancer were limited by sample sizes and insufficient control of confounders. There is moderate evidence suggestive of a positive effect of black tea consumption on bone mineral density although studies were few. There is little evidence to support the effect of tea on dental plaque inhibition but evidence to support the contribution of tea to fluoride intakes and thus theoretical protection against caries. There was no credible evidence that black tea (in amounts typically consumed) was harmful. Normal hydration was consistent with tea consumption when the caffeine content was < 250 mg per cup. A moderate caffeine intake from tea appeared to improve mental performance, although sample sizes were small. There was no evidence that iron status could be harmed by tea drinking unless populations were already at risk from anaemia.

CONCLUSIONS

There was sufficient evidence to show risk reduction for CHD at intakes of > or = 3 cups per day and for improved antioxidant status at intakes of one to six cups per day. A maximum intake of eight cups per day would minimise any risk relating to excess caffeine consumption. Black tea generally had a positive effect on health.

Modifiable risk factors for impaired fertility in women: What nurse practitioners need to know

PURPOSE

To provide an overview of impaired fertility in childbearing-aged women, to review the current research on modifiable lifestyle risk factors implicated in its development, and to suggest strategies for nurse practitioners (NPs) to assist women in behavioral changes that will allow them to protect their fertility.

DATA SOURCES

Original research articles and comprehensive review articles identified through Medline, CINAHL, and OVID databases.

CONCLUSIONS

Research has shown that advancing age, a history of a sexually transmitted infection and/or pelvic inflammatory disease, extremes of body weight, and tobacco and caffeine use are potentially modifiable risk factors in the development of impaired fertility.

IMPLICATIONS FOR PRACTICE

NPs must be aware of the link between these behaviors and the development of impaired fertility in order to assist women in preserving their fertility. Individual counseling, education, and community-wide education strategies are discussed.

Consumption of coffee is associated with reduced risk of death attributed to inflammatory and cardiovascular diseases in the Iowa Women's Health Study

BACKGROUND

Coffee is the major source of dietary antioxidants. The association between coffee consumption and risk of death from diseases associated with inflammatory or oxidative stress has not been studied.

OBJECTIVE

We studied the relation of coffee drinking with total mortality and mortality attributed to cardiovascular disease, cancer, and other diseases with a major inflammatory component.

DESIGN

A total of 41,836 postmenopausal women aged 55-69 y at baseline were followed for 15 y. After exclusions for cardiovascular disease, cancer, diabetes, colitis, and liver cirrhosis at baseline, 27,312 participants remained, resulting in 410,235 person-years of follow-up and 4265 deaths. The major outcome measure was disease-specific mortality.

RESULTS

In the fully adjusted model, similar to the relation of coffee intake to total mortality, the hazard ratio of death attributed to cardiovascular disease was 0.76 (95% CI: 0.64, 0.91) for consumption of 1-3 cups/d, 0.81 (95% CI: 0.66, 0.99) for 4-5 cups/d, and 0.87 (95% CI: 0.69, 1.09) for > or =6 cups/d. The hazard ratio for death from other inflammatory diseases was 0.72 (95% CI: 0.55, 0.93) for consumption of 1-3 cups/d, 0.67 (95% CI: 0.50, 0.90) for 4-5 cups/d, and 0.68 (95% CI: 0.49, 0.94) for > or =6 cups/d.

CONCLUSIONS

Consumption of coffee, a major source of dietary antioxidants, may inhibit inflammation and thereby reduce the risk of cardiovascular and other inflammatory diseases in postmenopausal women.

Maternal Exposure to Caffeine and Risk of Congenital Anomalies

BACKGROUND

Caffeine is teratogenic in animal studies when administered at high concentrations. Previous review articles have concluded that maternal caffeine consumption does not influence the risk of congenital anomalies. These reviews were narrative rather than systematic. The objective of the current systematic review is to provide a critical appraisal of epidemiologic evidence.

METHODS

A search of the MEDLINE/PUBMED database (1966-October 2004) was conducted for all published epidemiologic studies with maternal intake of caffeine as an exposure and major malformations as an outcome. Study characteristics were abstracted, internal validity evaluated, and study findings summarized.

RESULTS

Twenty-five papers met the initial criteria for inclusion, of which 18 were subsequently excluded as a result of other limitations. Effect estimates for the remaining 7 studies were generally close to null. Specific subgroup analyses were summarized across studies (associations between coffee and cardiovascular malformations, coffee and oral clefts, and tea and cardiovascular malformations). Summary point estimates ranged from 1.0 to 1.2; the upper limits of all confidence intervals were less than 1.7.

CONCLUSIONS

There is no evidence to support a teratogenic effect of caffeine in humans. Current epidemiologic evidence is not adequate to assess the possibility of a small change in risk of congenital anomalies resulting from maternal caffeine consumption.

Chronic intake of caffeine during gestation down regulates metabotropic glutamate receptors in maternal and fetal rat heart

Caffeine is the most widely consumed substance in the world which antagonizes adenosine effects. Adenosine acting through A(1) receptors inhibits glutamate release which binds to metabotropic glutamate receptors (mGluRs). Recently, we have shown that maternal caffeine intake during gestation causes down-regulation of A(1) and metabotropic glutamate receptors in the brain of both rat mothers and fetuses. In the present work we provide evidence that caffeine also affects receptors in hearts, causing a decrease in mGluRs from both maternal and fetal hearts. A decrease in G(q/11) and PLC beta(1) proteins level was also observed in both tissues. However, phospholipase C activity was only affected in fetal heart, being significantly decreased. These results suggest an in vivo cross-talk mechanism between adenosine and glutamate receptors in peripheral tissues. Therefore, special attention should be paid to caffeine ingestion during gestation.

Maternal caffeine consumption and fetal death: a case-control study in Uruguay

The objective of this study was to examine the association between caffeine intake during pregnancy and fetal mortality in Montevideo, the capital city of Uruguay, taking into account several potential confounding factors. A population-based case-control study was conducted between 1 August 2002 and 31 December 2003. A total of 382 cases and 792 controls were recruited. Cases consisted of women hospitalised with a medically confirmed diagnosis of spontaneous antepartum fetal death, in all maternity hospitals during the study period. Antepartum fetal death was defined as a fetal death in which the attending doctor certified that the death occurred prior to the onset of labour. Fetal deaths were included if they were of at least 20 weeks' gestational age or weighed >350 g. Controls were women who had a live, vigorous and term adequate-for-gestational-age newborn. Multiple gestations and fetuses/newborns with evident congenital malformations were excluded. Only a small proportion of the mothers (8.1% of the cases and 9.5% of the controls) did not consume caffeine during pregnancy. Among consumers, mate drinking was the most frequent source of caffeine in both cases and controls. After controlling for mother's and her partner's education, history of abortions and/or fetal deaths, vomiting/nausea during the first trimester of gestation and attendance for prenatal care, the category of mean caffeine intake of > or = 300 mg/day showed a significantly increased risk of fetal death (OR 2.33 [1.23; 4.41]) compared with no caffeine consumption during pregnancy. The study also found that less-educated women, mothers who did not attend for prenatal care and women with a history of abortions and fetal death were at an increased risk of fetal death. As mate drinking is highly consumed among pregnant women in Uruguay, the association found with fetal death makes it a preventable risk factor.

A new proposed guidance system for beverage consumption in the United States

The Beverage Guidance Panel was assembled to provide guidance on the relative health and nutritional benefits and risks of various beverage categories. The beverage panel was initiated by the first author. The Panel's purpose is to attempt to systematically review the literature on beverages and health and provide guidance to the consumer. An additional purpose of the Panel is to develop a deeper dialog among the scientific community on overall beverage consumption patterns in the United States and on the great potential to change this pattern as a way to improve health. Over the past several decades, levels of overweight and obesity have increased across all population groups in the United States. Concurrently, an increased daily intake of 150-300 kcal (for different age-sex groups) has occurred, with approximately 50% of the increased calories coming from the consumption of calorically sweetened beverages. The panel ranked beverages from the lowest to the highest value based on caloric and nutrient contents and related health benefits and risks. Drinking water was ranked as the preferred beverage to fulfill daily water needs and was followed in decreasing value by tea and coffee, low-fat (1.5% or 1%) and skim (nonfat) milk and soy beverages, noncalorically sweetened beverages, beverages with some nutritional benefits (fruit and vegetable juices, whole milk, alcohol, and sports drinks), and calorically sweetened, nutrient-poor beverages. The Panel recommends that the consumption of beverages with no or few calories should take precedence over the consumption of beverages with more calories.

Effects of caffeine on olfactory and visual learning in the honey bee (Apis mellifera)

Although caffeine is known to improve alertness and arousal in humans and other mammals, its impacts on specific behaviours, including complex cognitive processes, remain controversial. We reasoned that the availability of an easily manipulable, but behaviourally complex invertebrate organism with a simpler nervous system would be beneficial to this field of research. We used a popular behavioural model, the honeybee, to evaluate the effects of caffeine on (1) the development of olfactory learning and (2) the performance in complex learning paradigms, including a 'delayed-match-to-sample' task and visual associative learning. To evaluate the efficacy of caffeine treatment, a variety of doses (0.4-400 ng/1 mg of body mass) were applied topically to tethered individuals. Behavioural testing was performed with either tethered or free-flying adult honeybees. We show that caffeine has marked cognitive effects in this species. In young honeybees, it reduces the age at which restrained individuals are able to learn an olfactory associative task, whereas in older, free-flying bees, caffeine improves both motivation and cognitive performance in complex learning tasks. Our results suggest that the honeybee model may be useful in explaining caffeine-related behavioural changes not only in this species, but also in mammalian systems.

Does caffeine consumption during pregnancy increase the risk of fetal mortality? A literature review

The aim of this study was to evaluate the available epidemiological evidence of the effect of caffeine consumption during pregnancy on fetal mortality. A systematic qualitative review of observational studies that referred to any source of exposure to caffeine from food in pregnancy and to fetal mortality as the outcome was conducted in the databases MEDLINE and LILACS. Studies published between January 1966 and September 2004 were searched. The following descriptors were used: "caffeine", "coffee", "tea", "cola", and "cacao" to define the exposure and "fetal death", "stillbirth", "fetal demise", and "fetal loss" to define the outcome. The search strategy retrieved 32 publications, but only six met the inclusion criteria and three were included. One more article was found using "see related articles" feature in PubMed. A total of four publications were included in the review. The small number of publications addressing this subject, methodological limitations, inaccurate exposure assessment in all the studies, overall risks only marginally significant in most cases, and the possibility of publication bias preclude stating with certainty that caffeine consumption is actually associated with fetal death.

Influence of caffeine on frequency of hypoglycemia detected by continuous interstitial glucose monitoring system in patients with long-standing type 1 diabetes

OBJECTIVE

The aim of this study was to investigate the effect of caffeine (in doses equivalent to normal daily ingestion) on rates and severity of hypoglycemia in patients with long-standing type 1 diabetes to determine the relationship between caffeine, autonomic function, and hypoglycemia.

RESEARCH DESIGN AND METHODS

Using a double-blinded randomized study, we investigated the effect of caffeine versus placebo in 19 patients with long-standing type 1 diabetes using continuous glucose sensing technology and simultaneous assessment of autonomic function using Holter monitoring.

RESULTS

Caffeine reduced the duration of nocturnal hypoglycemia with a mean duration of 49 minutes (range 0-235) versus 132 (0-468) minutes (P = 0.035). The reduction in duration of nighttime hypoglycemia was due to a decline in the number of episodes of moderate hypoglycemia at the expense of mild hypoglycemic episodes (P = 0.04). There was no overall correlation between reduced heart rate variability (a marker of autonomic dysfunction) and hypoglycemic events (r(s) = 0.12, P = 0.62).

CONCLUSIONS

Our results suggest that caffeine is associated with a significant reduction in nocturnal hypoglycemia. The reduction in nocturnal hypoglycemia was not linked to the concomitant rise in parasympathetic activity associated with caffeine.

Coffee consumption and human health--beneficial or detrimental?--Mechanisms for effects of coffee consumption on different risk factors for cardiovascular disease and type 2 diabetes mellitus

Coffee is probably the most frequently ingested beverage worldwide. Especially Scandinavia has a high prevalence of coffee-drinkers, and they traditionally make their coffee by boiling ground coffee beans and water. Because of its consumption in most countries in the world, it is interesting, from both a public and a scientific perspective, to discuss its potential benefits or adverse aspects in relation to especially two main health problems, namely cardiovascular disease and type 2 diabetes mellitus. Epidemiological studies suggest that consumption of boiled coffee is associated with elevated risk for cardiovascular disease. This is mainly due to the two diterpenes identified in the lipid fraction of coffee grounds, cafestol and kahweol. These compounds promote increased plasma concentration of cholesterol in humans. Coffee is also a rich source of many other ingredients that may contribute to its biological activity, like heterocyclic compounds that exhibit strong antioxidant activity. Based on the literature reviewed, it is apparent that moderate daily filtered, coffee intake is not associated with any adverse effects on cardiovascular outcome. On the contrary, the data shows that coffee has a significant antioxidant activity, and may have an inverse association with the risk of type 2 diabetes mellitus.

Chronic caffeine or theophylline intake during pregnancy inhibits A1 receptor function in the rat brain

The aim of this work was to study whether caffeine or theophylline chronically consumed during pregnancy affect inhibitory adenylyl cyclase pathway mediated by adenosine, in rat brain of both mothers and full-term fetuses. Immunoblotting analysis revealed a significant decrease in alphaGi(1,2) subunit level (27-29% in mothers, 15-18% in fetuses), associated with a significant increase in the mRNA level coding alphaGi(1) in both maternal and fetal rat brain (12-22%) after methylxanthine intake. No significant differences in alphaGi(3) level were detected in any case. On the other hand, forskolin- and forskolin plus guanosine-5'-O(3-thiotriphosphate) tetralithium salt-stimulated adenylyl cyclase activity was significantly decreased (30-36%) in maternal brain. Moreover, adenylyl cyclase inhibition elicited by N(6)-cyclohexyladenosine, specific adenosine A(1) receptor agonist, was also significantly decreased in caffeine- (40.5%) and theophylline- (55.0%) treated mothers, suggesting a desensitization of adenosine A(1) receptor/adenylyl cyclase pathway in maternal brain. However, no significant differences were detected in fetal brain between control and treated animals. Therefore, caffeine or theophylline chronic intake during pregnancy differently modulates inhibitory adenylyl cyclase pathway mediated by adenosine in maternal and fetal brain causing a loss of the system responsiveness only in maternal brain but down-regulating Gi(1) protein in both mother and fetus brain.

Beverage caffeine intake in US consumers and subpopulations of interest: estimates from the Share of Intake Panel survey

Concerns exist about the potential adverse health effects of high consumption of dietary caffeine, especially in children and pregnant women. Recommended caffeine intakes corresponding to no adverse health effects have been suggested recently for healthy adults (400-450 mg/day), for women contemplating pregnancy (300 mg/day), and for young children age 4-6 years (45 mg/day). To determine whether current caffeine intake approaches these levels, intake from major dietary sources (coffee, tea and carbonated soft drinks) were measured in 10,712 caffeinated beverage consumers in the 1999 US Share of Intake Panel, a targeted beverage survey. Mean caffeine intakes in adult caffeinated beverage consumers ranged from 106 to 170 mg/day (90th percentile intake was 227-382 mg/day). In children 1-5 and 6-9 years, mean caffeine intakes were 14 and 22 mg/day, respectively; corresponding 90th percentile intakes were 37 and 45 mg/day. Pregnant women consumed an average of 58 mg/day (157 mg/day at the 90th percentile), and women of reproductive age ingested 91-109 mg/day (229-247 mg/day at the 90th percentile). These data show that while mean caffeine intakes are within recommended safe levels, heavy consumers of certain subpopulations, including young children and women contemplating pregnancy, might benefit from dietary advice.

Associations between coffee consumption and inflammatory markers in healthy persons: the ATTICA study

BACKGROUND

The effect of coffee consumption on the cardiovascular system is conflicting. Inflammation is important to the development of cardiovascular disease (CVD), and several dietary factors are thought to exert significant effects on inflammation and thus on the risk of CVD.

OBJECTIVE

We aimed to investigate the associations between coffee consumption and inflammatory markers.

DESIGN

The cross-sectional survey enrolled 1514 men (x +/- SD age: 46 +/- 13 y; range: 18-87 y) and 1528 women (aged 45 +/- 13 y; range: 18-89 y). Five percent of men and 3% of women were excluded for history of CVD. Fasting blood samples were collected. Dietary habits (including consumption of various types of coffee) were evaluated by using a validated food-frequency questionnaire.

RESULTS

Compared with coffee nondrinkers, men who consumed >200 mL coffee/d had 50% higher interleukin 6 (IL-6), 30% higher C-reactive protein (CRP), 12% higher serum amyloid-A (SAA), and 28% higher tumor necrosis factor alpha (TNF-alpha) concentrations and 3% higher white blood cell (WBC) counts (all: P < 0.05). Women who consumed >200 mL coffee/d had 54% higher IL-6, 38% higher CRP, 28% higher SAA, and 28% higher TNF-alpha concentrations and 4% higher WBC counts (all: P < 0.05) than did coffee nondrinkers. The findings were significant even after control for the interactions between coffee consumption and age, sex, smoking, body mass index, physical activity status, and other covariates.

CONCLUSIONS

A relation exists between moderate-to-high coffee consumption and increased inflammation process. This relation could explain, in part, the effect of increased coffee intake on the cardiovascular system.

Pharmacotherapy for excessive daytime sleepiness

Excessive daytime sleepiness (EDS) has recognized detrimental consequences such as road traffic accidents, impaired psychological functioning and reduced work performance. EDS can result from multiple causes such as sleep deprivation, sleep fragmentation, neurological, psychiatric and circadian rhythm disorders. Treating the underlying cause of EDS remains the mainstay of therapy but in those who continue to be excessively sleepy, further treatment may be warranted. Traditionally, the amphetamine derivatives, methylphenidate and pemoline (collectively sympathomimetic) psychostimulants were the commonest form of therapy for EDS, particularly in conditions such as narcolepsy. More recently, the advent of modafinil has broadened the range of therapeutic options. Modafinil has a safer side-effect profile and as a result, interest in this drug for the management of EDS in other disorders, as well as narcolepsy, has increased considerably. There is a growing school of thought that modafinil may have a role to play in other indications such as obstructive sleep apnea/hypopnea syndrome already treated by nasal continuous positive airway pressure but persisting EDS, shift work sleep disorders, neurological causes of sleepiness, and healthy adults performing sustained operations, particularly those in the military. However, until adequately powered randomised-controlled trials confirm long-term efficacy and safety, the recommendation of wakefulness promoters in healthy adults cannot be justified.

Caffeine ingestion before an oral glucose tolerance test impairs blood glucose management in men with type 2 diabetes

Caffeine ingestion negatively affects insulin sensitivity during an oral glucose tolerance test (OGTT) in lean and obese men, but this has not been studied in individuals with type 2 diabetes. We examined the effects of caffeine ingestion on insulin and glucose homeostasis in obese men with type 2 diabetes. Men (n = 12) with type 2 diabetes (age = 49 +/- 2 y, BMI = 32 +/- 1 kg/m(2)) underwent 2 trials, 1 wk apart, in a randomized, double-blind design. Each trial was conducted after withdrawal from caffeine, alcohol, exercise, and oral hypoglycemic agents for 48 h and an overnight fast. Subjects randomly ingested caffeine (5 mg/kg body weight) or placebo capsules and 1 h later began a 3 h 75 g OGTT. Caffeine increased (P < 0.05) serum insulin, proinsulin, and C-peptide concentrations during the OGTT relative to placebo. Insulin area under the curve was 25% greater (P < 0.05) after caffeine than after placebo ingestion. Despite this, blood glucose concentration was also increased (P < 0.01) in the caffeine trial. After caffeine ingestion, blood glucose remained elevated (P < 0.01) at 3 h postglucose load (8.9 +/- 0.7 mmol/L) compared with baseline (6.7 +/- 0.4 mmol/L). The insulin sensitivity index was lower (14%, P = 0.02) after caffeine than after placebo ingestion. Overall, despite elevated and prolonged proinsulin, C-peptide, and insulin responses after caffeine ingestion, blood glucose was also increased, suggesting an acute caffeine-induced impairment in blood glucose management in men with type 2 diabetes.

Synergistic interaction between excess caffeine and deficient iodine on the promotion of thyroid carcinogenesis in rats pretreated with N-bis(2-hydroxypropyl)nitrosamine

The combined effects of caffeine (1,3,7-trimethylxanthine) with iodine deficiency (ID) were examined in a rat two-stage thyroid carcinogenesis model using N-bis(2-hydroxypropyl)nitrosamine (DHPN). Male F344 rats were divided into 6 groups each consisting of 10 animals, and received a single s.c. injection of 2800 mg/kg DHPN. From 1 week after the DHPN initiation, the rats were respectively fed a basal diet in which the protein was exchanged for 20% gluten, containing 1500 ppm caffeine + ID, 300 ppm caffeine + ID, 60 ppm caffeine + ID, 1500 ppm caffeine or ID or a basal diet alone for 12 weeks. Relative thyroid weights were significantly (P < 0.05) increased due to the development of proliferative lesions induced by the ID diet as compared to the DHPN-alone group value, which was enhanced by caffeine, albeit without statistical significance. Relative pituitary weights were significantly (P < 0.05) increased with 300 or 1500 ppm caffeine + ID as compared to the DHPN-alone group value. Serum thyroid stimulating hormone (TSH) levels were slightly increased by ID, an effect which was further enhanced by 300 or 1500 ppm caffeine. Serum thyroxine (T(4)) levels were slightly increased by caffeine or ID alone, but decreased by caffeine with ID. Histopathologically, thyroid follicular carcinomas were found only in the 1500 ppm caffeine + ID group, although thyroid follicular adenomas were detected in all the ID-treated groups. The multiplicity of focal thyroid follicular hyperplasias was significantly (P < 0.05) increased by 1500 ppm caffeine. These results indicate that caffeine may synergistically promote thyroid carcinogenesis with ID partially through a pituitary-dependent pathway in rats, implying the possible implication of routine caffeine intake in the promotion of thyroid carcinogenesis.