Differential Diagnosis

The combination of normal TSH, low T4, high T3, and low QRS voltage on an electrocardiogram (ECG) presents a unique set of findings that can be associated with several thyroid and cardiac conditions. Here's a differential diagnosis organized into the requested categories:

• Single Most Likely Diagnosis

  • T3 Toxicity: This condition, where there's an excess of T3, can occur due to various reasons including thyroiditis, exogenous thyroid hormone ingestion, or a T3-secreting thyroid adenoma. The high T3 level can suppress TSH and T4 production, explaining the lab findings. Low QRS voltage can be seen in conditions affecting the heart, and T3 toxicity can lead to cardiac effects.

• Other Likely Diagnoses

  • Thyroiditis: Specifically, subacute or silent thyroiditis can cause a phase of thyrotoxicosis (high T3) followed by a hypothyroid phase. Initially, T4 might be low due to the inflammation and damage to the thyroid gland, while T3 remains elevated.
  • Exogenous Thyroid Hormone Ingestion: Intake of thyroid hormone supplements can lead to elevated T3 levels, suppressed TSH, and potentially low T4 if the supplement primarily contains T3 or if the body rapidly converts T4 to T3.
  • Cardiac Amyloidosis: This condition can cause low QRS voltage on ECG and can be associated with thyroid dysfunction due to amyloid deposition in the thyroid gland, although this is less direct.

• Do Not Miss Diagnoses

  • Pituitary or Hypothalamic Disease: Although less likely, diseases affecting the pituitary or hypothalamus (like a TSH-secreting pituitary adenoma or hypothalamic dysfunction) can lead to abnormal thyroid hormone levels. Missing these diagnoses could lead to inappropriate treatment and significant morbidity.
  • Cardiac Sarcoidosis: This condition can cause low QRS voltage and can also affect the thyroid gland, either directly through sarcoid granulomas or indirectly through systemic inflammation.

• Rare Diagnoses

  • Familial Dysalbuminemic Hyperthyroxinemia (FDH): A rare condition characterized by abnormal binding of T4 to albumin, leading to low T4 levels with normal TSH and potentially elevated T3. However, this would not directly explain the low QRS voltage.
  • Resistance to Thyroid Hormone (RTH): A rare genetic disorder where there's reduced sensitivity to thyroid hormones. This can lead to elevated T3 and T4 levels, but with normal or slightly elevated TSH. The low T4 in this scenario doesn't perfectly fit, but variations in presentation can occur.