REVIEW
SA JOURNAL OF DIABETES & VASCULAR DISEASE
14
VOLUME 8 NUMBER 1 • MARCH 2011
Diabetes and thyroid disorders
GABrIElA BrENTA
Abstract
I
t has long been recognised that thyroid hormones have
markedeffects onglucosehomeostasis. Glucose intolerance
is associated with hyperthyroidism and most recently it
was shown that hypothyroidism is characterised by insulin
resistance. Although autoimmune thyroid disease is more
prevalent in type 1 diabetes as a result of their common
origin, in patients with type 2 diabetes the prevalence of
hypothyroidism and hyperthyroidism is similar to that of the
general population. However, in type 2 diabetic patients,
the presence of the highly frequent sub-clinical forms of
hyperthyroidism and hypothyroidism should be ruled out
since they may be associated with higher cardiovascular
risk. While there are no doubts about the therapeutic impact
of normalising hypothyroidism and hyperthyroidism, the
information available about the benefit of treating sub-
clinical thyroid disease in diabetes remains insufficient.
Keywords:
diabetes, hyperthyroidism, hypothyroidism, insulin
resistance, thyroid.
Introduction
The term ‘thyroid diabetes’ was coined in the early literature to
depict the influence of thyroid hormone excess in the deterioration
of glucose control,
1
and for nearly a century many publications
focused on the relationship between diabetes and thyroid disease.
The literature concerning the effects of thyroid hormones on
glucose metabolism in normal and diabetic states has been
evaluated in detail.
2
This review will therefore address only specific
issues of a broad field. It is intended to illustrate some aspects of
the prevalence of thyroid disorders in the general population and
in diabetic patients, the pathological mechanisms underlying both
diseases and the use, or potential use, of pharmacological therapies
to treat diabetic and thyroid patients. A brief overview of long-term
mortality or morbidity studies in patients with thyroid dysfunction
and diabetes will also be presented.
Prevalence of thyroid disorders in the general
population and in diabetic patients
Both hyperthyroidism and hypothyroidism are graded phenomena,
ranging from very mild cases in which biochemical abnormalities
are present without any symptoms or signs of thyroid hormone
excess or deficiency, to very severe cases that may end up as a
life-threatening thyrotoxicosis crisis or myxoedema coma. Their
prevalence varies according to the studied population. The
Whickham survey, conducted in the north of England, revealed
a prevalence of overt thyrotoxicosis or hypothyroidism of at least
2% in females and 0.2% in males.
3
In the NHANES III study it was
shown that 4.6% of the US population had hypothyroidism (0.3%
clinical and 4.3% sub-clinical) and 1.3% had hyperthyroidism
(0.5% clinical and 0.7% sub-clinical).
4
The incidence of progression
from sub-clinical to overt hypothyroidism is 5–15% per year;
women with positive thyroid antibodies are especially at risk.
3
Sub-clinical hypothyroidism, the most prevalent form of thyroid
diseases, is more common in females and in the elderly, reaching
a prevalence of up to 20% in women over 60 years old.
5
This
increased prevalence in the elderly was recently questioned by Surks
et al
.
6
whose re-analysis of the NHANES data revealed that TSH
serum values might be shifted toward higher levels with increasing
age. Accordingly, TSH levels up to 7.5
μ
U/ml would be considered
normal in a patient 80 years and older and about 70% of the
Department of Endocrinology and Metabolism, Cesar Milstein Hospital,
Buenos Aires, Argentina.
Correspondence to: Dr Gabriela Brenta
Virrrey del Pino 3370 3ºA, CABA 1426, Argentina.
Tel: +5411 4555 7593
E-mail:
S Afr J Diabetes Vasc Dis
2011;
8
: 14–18.
Abbreviations and acronyms
AITD
autoimmune thyroid disease
ATP
adenosine-5
′
-triphosphate
BMI
body mass index
HbA
1C
glycated haemoglobin A
1C
HIF-1
a
hypoxia-inducible factor 1
a
MHC
major histocompatibility complex
NHANES
National Health and Nutrition Examination Survey
T3
triiodothyronine
TSH
thyroid stimulating hormone/thyrotropin
Gabriela Brenta
