Interpreting Thyroid Function Tests

The question presents a specific pattern of thyroid function test results:

• TSH (Thyroid Stimulating Hormone) is mildly elevated: $5-20 \text{ mU/L}$.
• T3 (Triiodothyronine) levels are normal.
• T4 (Thyroxine) levels are normal.

This combination is key to diagnosing the condition.

Evaluating Diagnostic Options

Let's analyze the given options based on the test results:

• Primary Hypothyroidism: Characterized by high TSH levels due to the pituitary gland trying to stimulate a failing thyroid gland. However, in primary hypothyroidism, T4 and T3 levels are typically low, not normal. This does not match the findings.
• Subclinical Hypothyroidism: Defined by elevated TSH levels (often mild, as in this case) but with normal circulating levels of thyroid hormones (T4 and T3). The body's tissues are not yet showing signs of hormone deficiency because the thyroid is still compensating. This diagnosis aligns perfectly with the described lab results.
• Artefact: While lab errors can occur, the specific pattern of mildly elevated TSH with normal T3/T4 is a recognized clinical condition, making artefact less likely as the primary explanation without further evidence.
• Thyroid Hormone Resistance: This is a rare genetic disorder where the body's cells don't respond properly to thyroid hormones. It can present with elevated TSH and normal or high T4/T3 levels, but it's not the most common interpretation of this specific lab pattern, especially the "mildly elevated" TSH description.

Conclusion on Diagnosis

The laboratory findings of a mildly elevated TSH ($5-20 \text{ mU/L}$) concurrent with normal T3 and T4 levels are the classic indicators of Subclinical Hypothyroidism.