Differential Diagnosis

• Single most likely diagnosis
  • Hypothyroidism: The patient's symptoms of depressed mood, fatigue, and hoarse voice, along with the laboratory results showing an elevated TSH (6) and low free thyroxine (0.8), are consistent with hypothyroidism. The presence of thyroid peroxidase antibodies (48) also supports this diagnosis, indicating an autoimmune cause, likely Hashimoto's thyroiditis.
• Other Likely diagnoses
  • Postpartum thyroiditis: Although the patient's baby was weaned two months ago, postpartum thyroiditis can occur up to a year after childbirth. The patient's history of postpartum anemia and recent childbirth, along with the thyroid gland being firm and nodular, could suggest this diagnosis. However, the absence of a reported hyperthyroid phase and the presence of thyroid peroxidase antibodies make hypothyroidism more likely.
  • Lithium-induced hypothyroidism: The patient is being managed with lithium carbonate for bipolar disorder, which is a known cause of hypothyroidism. The patient's laboratory results and symptoms are consistent with this diagnosis, but it is less likely than primary hypothyroidism given the family history of hypothyroidism and the presence of thyroid peroxidase antibodies.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Thyroid storm: Although the patient's symptoms do not suggest a thyroid storm, it is a life-threatening condition that requires prompt recognition and treatment. The patient's heart rate and blood pressure are within normal limits, making this diagnosis unlikely.
  • Medication non-adherence: The patient is being managed with lithium carbonate, and non-adherence to this medication could lead to a relapse of bipolar disorder. Although not directly related to the patient's current symptoms, it is essential to consider medication adherence in patients with a history of psychiatric illness.
• Rare diagnoses
  • Pituitary disorder: A rare cause of hypothyroidism, pituitary disorders (e.g., pituitary adenoma) could lead to central hypothyroidism. However, the patient's laboratory results and symptoms do not suggest a pituitary disorder, making this diagnosis unlikely.
  • Thyroid cancer: Although the patient's thyroid gland is firm and nodular, there is no other evidence to suggest thyroid cancer. This diagnosis would require further evaluation, including imaging studies and fine-needle aspiration biopsy.