Subclinical hypothyroidism (SCH) is a mild form of thyroid underactivity that often flies under the radar but affects a notable portion of Australians. Defined by elevated thyroid-stimulating hormone (TSH) levels while maintaining normal thyroid hormone (free T4) levels, SCH presents a diagnostic and management challenge for both patients and healthcare providers. This blog explores what subclinical hypothyroidism means for Australians, its prevalence, symptoms, and current approaches to treatment.
What Is Subclinical Hypothyroidism?
Subclinical hypothyroidism occurs when blood tests show a higher-than-normal TSH level, but thyroid hormones like free T4 remain within the normal range. Unlike overt hypothyroidism, where hormone levels are low and symptoms are pronounced, SCH may cause mild or no symptoms at all. It can, however, progress to full hypothyroidism in some individuals.
How Common Is Subclinical Hypothyroidism in Australia?
Research indicates that approximately 4-5% of Australians have subclinical hypothyroidism, with higher rates observed in women and older adults. The condition is especially prevalent in females over 60 years of age. Many people with SCH remain asymptomatic, which means they may only discover the condition during routine blood tests or investigations for vague symptoms like fatigue or weight gain.
Causes of Subclinical Hypothyroidism
In Australia, the most common cause of SCH is Hashimoto’s thyroiditis, an autoimmune condition where the immune system attacks the thyroid gland, leading to gradual dysfunction. Other causes include:
- Previous treatment for hyperthyroidism (e.g., radioactive iodine therapy)
- Thyroid surgery
- Iodine deficiency (rare in Australia due to iodised salt programs)
- Certain medications or radiation exposure
Symptoms and Impact on Quality of Life
Many individuals with SCH experience few or no symptoms. However, some report mild hypothyroid symptoms such as tiredness, weight gain, cold intolerance, or mood changes. A recent Australian study suggests that while thyroid hormone therapy does not generally improve quality of life for most with SCH, there may be a subgroup who experience significant impairment and could benefit from treatment. Exercise therapy is also being explored as a potential way to improve wellbeing in these individuals.
Diagnosing Subclinical Hypothyroidism
Diagnosis is made through blood tests showing:
- Elevated TSH above the normal reference range (typically >4.0 or 5.0 mIU/L)
- Normal free T4 levels
Because TSH can fluctuate, guidelines recommend repeating tests after 6-8 weeks to confirm persistent elevation. Testing for thyroid antibodies (especially thyroid peroxidase antibodies) is important to identify autoimmune thyroiditis, which predicts a higher risk of progression to overt hypothyroidism.
Should Subclinical Hypothyroidism Be Treated?
The decision to start thyroid hormone replacement in SCH remains controversial in Australia and worldwide. Current evidence suggests:
- Treatment is generally recommended if TSH exceeds 10 mIU/L or if symptoms are significant.
- For TSH between 5 and 10 mIU/L, treatment decisions are individualized, considering symptoms, antibody status, cardiovascular risk, and patient preference.
- Many people with mildly elevated TSH normalize without treatment.
- Routine treatment has not consistently shown benefits in improving overall survival, cardiovascular health, or quality of life in asymptomatic individuals.
Monitoring and Follow-Up
For those not immediately treated, regular monitoring of thyroid function every 6-12 months is advised to detect any progression. Lifestyle factors such as maintaining a healthy weight and regular exercise may support thyroid health and wellbeing.
Subclinical hypothyroidism affects a meaningful segment of the Australian population, especially women and older adults. While it often causes minimal symptoms and may resolve without intervention, some individuals experience reduced quality of life and may benefit from treatment. If you have elevated TSH but normal thyroid hormones, working closely with your healthcare provider to monitor your condition and discuss treatment options is essential.
If you notice symptoms like fatigue, unexplained weight changes, or mood shifts, or if you have a family history of thyroid disease, talk to your GP about thyroid function testing to ensure early detection and appropriate care.