Non-Thyroidal Illness (NTI): Also known as euthyroid sick syndrome, this condition is characterized by abnormal thyroid function tests in the absence of actual thyroid disease, often seen in patients with systemic illnesses, starvation, or severe stress. The low TSH with normal Free T3 and Free T4 levels can be a manifestation of NTI, especially in the context of acute or chronic illness.

Pituitary or Hypothalamic Disease: Lesions or disorders affecting the pituitary gland or hypothalamus can lead to decreased TSH secretion. Although Free T3 and Free T4 are normal, the low TSH suggests some level of pituitary or hypothalamic dysfunction.
Thyroid Hormone Resistance: A rare condition where the body's tissues are resistant to thyroid hormones, leading to elevated levels of thyroid hormones and a compensatory decrease in TSH. However, in some cases, Free T3 and Free T4 might be within the normal range, especially if the resistance is partial.
Recent Thyrotoxicosis Treatment: Patients who have recently been treated for hyperthyroidism (e.g., with radioactive iodine or antithyroid medications) may initially present with low TSH levels as their thyroid hormone levels normalize.

Pituitary Apoplexy: A medical emergency where a pituitary tumor undergoes sudden hemorrhage or infarction, leading to acute pituitary dysfunction. Although rare, it's crucial to consider this diagnosis due to its potential for severe and life-threatening consequences.
Traumatic Brain Injury: Severe head trauma can affect the hypothalamic-pituitary axis, leading to transient or permanent alterations in TSH secretion.

Familial Central Hypothyroidism: A rare genetic disorder characterized by decreased TSH production, which can present with normal thyroid hormone levels if the defect is partial.
Iatrogenic Causes: The use of certain medications (e.g., dopamine, glucocorticoids) can suppress TSH secretion, leading to low TSH levels with normal Free T3 and Free T4.