Hypertensive Encephalopathy: Given the patient's history of hypertension (HTN), a sudden tonic-clonic seizure could be indicative of hypertensive encephalopathy, a condition where severely elevated blood pressure leads to brain dysfunction, including seizures.

Stroke or Transient Ischemic Attack (TIA): Although the patient has no recent illness, a stroke or TIA could present with a seizure, especially in older adults with hypertension.
Metabolic Disorders: Electrolyte imbalances (e.g., hyponatremia, hypocalcemia) or other metabolic issues could precipitate a seizure, particularly in the context of underlying conditions like hypertension.
Medication Side Effects: Certain medications, including those used to treat hypertension or low testosterone, could potentially lower the seizure threshold.

Subarachnoid Hemorrhage: A sudden, severe headache accompanied by a seizure could indicate a subarachnoid hemorrhage, which is a medical emergency.
Meningitis or Encephalitis: Infections of the brain or meninges, although less common in this presentation, could lead to seizures and are critical to diagnose promptly due to their high morbidity and mortality.
Brain Tumor: Although rare, a new-onset seizure in an older adult could be the first presentation of a brain tumor.

Vasculitis: Conditions like central nervous system vasculitis could present with seizures among other neurological symptoms, but are less common.
Mitochondrial Disorders: Certain mitochondrial myopathies can present with seizures, among a wide range of other symptoms, but are rare and typically have other systemic manifestations.
Paraneoplastic Syndromes: These are rare disorders that are triggered by an altered immune system response to a neoplasm, and can sometimes present with seizures as an initial symptom.