Thyroid disease and diabetes: a complex relationship

Diabetes and thyroid disease can have similar signs and symptoms which means the presence of one should alert a healthcare provide to look out for the other. Dr Angela Murphy expands on this.

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Thyroid disease and diabetes are both common conditions and so it’s not unexpected that many people may be diagnosed with both. There are genetic and pathological reasons why the conditions may co-exist. In addition, the presence of one condition may make it difficult to diagnose and control the other. This is the challenge the healthcare professional must tackle.

The thyroid gland

The thyroid is a small, butterfly-shaped gland found in the neck that secretes two hormones: T4 (90%) and T3. Most of T3 is made by conversion of T4 in the tissues where it is to be used; thus, T3 is considered the active hormone.

The production of thyroid hormones is controlled by the pituitary gland in the brain which secretes thyroid stimulating hormone (TSH). This feedback loop ensures that all the levels are in the normal range.

The role of thyroid hormones is to regulate all the functions in the body, including heart rate and blood pressure, gut function, and cognition.

When a disease process alters the thyroid hormone levels, specific conditions occur: hyperthyroidism (over active thyroid function) or hypothyroidism (underactive thyroid function).

Thyroid disease

	HYPERTHYROID	HYPOTHYROID
DEFINITION: Blood tests	TSH < 0.01mIU/mL   LOW	TSH >10mIU/mL   HIGH
	T4 > 22ug/dL             HIGH	T4 < 9ug/dL           LOW
CAUSES	Autoimmune thyroiditis (Graves’ disease) *	Autoimmune thyroiditis (Hashimoto’s thyroiditis) *
	Other thyroiditis**	Other thyroiditis
	Single, toxic nodule or multi-nodular goitre	Surgical or radiation damage/removal
SYMPTOMS	Weight loss, hunger,	Weight gain, fatigue,
	tremors, palpitations,	cold intolerance, hair loss, dry skin, depression
	heat intolerance,
	muscle weakness,
	anxiety, insomnia
TREATMENT	NeoMercazole	Levothyroxine
	Radioactive iodine
	Surgical removal

*Named after the doctors who first described the conditions.   **Viral, environmental toxin.

Autoimmune thyroid disease

Autoimmune disease occurs when circulating antibodies inadvertently attack normal tissue. In the case of autoimmune thyroid disease, different antibodies cause an inflammatory response in the different cells of the thyroid gland. This is called a thyroiditis.  The damage from some antibodies blocks thyroid hormone production causing an underactive thyroid. Other antibody damage results in overproduction of thyroid hormone causing an overactive gland.

Type 1 diabetes

Type 1 diabetes is an autoimmune condition where antibodies attack the insulin producing cells of the pancreas. When considering its association with thyroid disease we must remember:

• Up to 25% of children with Type 1 diabetes develop autoimmune thyroid disease. Adult females with Type 1 diabetes have a two times increased risk of developing hypothyroidism and males with Type 1 diabetes have a four times higher risk. The combination of autoimmune thyroid disease and Type 1 diabetes is part of the Polyglandular syndrome.
• Patients should be screened (with blood tests) for abnormal thyroid function annually.
• All pregnant women should be screened.
• Hypothyroidism causes hypoglycaemia.
• Hyperthyroidism worsens glucose control.

Type 2 diabetes

Type 2 diabetes develops when insulin secretion decreases from the pancreas and at the same time peripheral tissues (liver and muscle) becomes resistant to insulin. Thyroid disease has an impact in patients with Type 2 diabetes and so:

• Screening should start age 45 years or earlier if person is overweight, has had gestational diabetes, has co-morbidities like high cholesterol and hypertension. Screening should be done every three to five years depending on risk.
• Increase glucose monitoring until normal thyroid hormone levels achieved with levothyroxine.
• Adjust diabetes regimen as necessary.

Effect of thyroid hormones on diabetes management

Excess circulating thyroid hormones (T4 and T3) increases blood glucose levels by:

• Increasing absorption of glucose from the gut.
• Increasing the production of glucose in the liver.
• Increasing insulin resistance.

This implies that the diabetes regimen might need to be changed and insulin doses adjusted. It’s worth remembering to check for thyroid disease if a patient is reporting a sudden need to increase insulin doses for no apparent reason.

The medications used to treat diabetes can affect thyroid hormone levels. Metformin has been found to reduce TSH levels whereas pioglitazone increases TSH and decreases T4.

This means that the relevant thyroid medication may need to be adjusted to compensate for this. These medications for both hypothyroidism and hyperthyroidism (Levothyroxine and NeoMercazole respectively) tend to lower blood glucose, both fasting and after meals. This increases the risk for hypoglycaemia, particularly in a patient with relatively good diabetes control. It also implies that there may be an improvement in blood glucose levels such that there is a decrease in the risk of developing Type 2 diabetes in certain people.

Subclinical- hypothyroidism and hyperthyroidism

Both hyperthyroid and hypothyroidism can exist in a subclinical form. This is when the bloods are partially outside of the normal range but not in diagnostic range. Usually there are no symptoms (subclinical) at this point.

Subclinical hypothyroidism is associated with high blood pressure and high cholesterol and so may present increased risk of cardiovascular disease in the patient with diabetes.

Subclinical hyperthyroidism can cause cardiac arrhythmias and bone loss (osteoporosis), especially in older adults. As patients with diabetes already have an increased risk of heart disease and osteoporosis this condition may need to be treated.

Diabetes, thyroid disease and pregnancy

The health of both mother and baby are significantly impacted by diabetes and thyroid disease, either hypo- or hyperthyroidism. There is an increased prevalence of hypothyroidism in women diagnosed with gestational diabetes.

It’s important to screen and diagnosis this timeously to avoid pregnancy complications and a low-birth-weight baby. One quarter of women with Type 1 diabetes are at risk of developing hyperthyroidism in the weeks after giving birth. Again, healthcare providers need to be aware of this risk and look out for signs, symptoms, and blood level changes in these women.

In summary

Diabetes and thyroid disease can have similar symptoms and signs: weight changes, water retention, fatigue. Indeed, there is a concern that one condition can mask the other. This means the presence of one should alert a healthcare provide to look out for the other. There is no consistent advice with respect to screening, but general advice would be:

TYPE 1 DIABETES screen for thyroid disease:

• At diagnosis
• Annually
• Six weeks after giving birth

TYPE 2 DIABETES screen for thyroid disease:

• If an enlarged thyroid (goitre) is found on examination
• If high blood cholesterol levels and high blood pressure
• History of gestational diabetes
• Family history of thyroid disease
• Repeat screening every three to five years depending on risks

THYROID DISEASE screen for diabetes:

• At diagnosis
• Once thyroid function is normal
• In autoimmune thyroid disease consider annually

Overall, we observe that thyroid disease affects glucose metabolism and abnormal glucose metabolism affects the risk of thyroid disease. This is further complicated by overlap of symptoms and how the drugs for each condition interact with each other. It’s important to be aware that there is an increased connection between the two conditions and screen when appropriate. The influence each condition has on the other can make it challenging to manage both.

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