Differential Diagnosis

The patient presents with depressed mood, fatigue, and a gravelly voice, along with a diffusely enlarged, firm, nodular, and non-tender thyroid gland. Given her symptoms and laboratory results, the following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • Hypothyroidism: The patient's symptoms of depressed mood, fatigue, and gravelly voice, combined with an elevated thyroid-stimulating hormone (TSH) level of 6.0 and a low free thyroxine (FT4) level of 0.8, strongly suggest 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. This condition can cause hypothyroidism, but the thyroid gland is typically tender, which is not the case here.
  • Lithium-Induced Hypothyroidism: The patient is on lithium carbonate for bipolar disorder, which can cause hypothyroidism as a side effect. However, the presence of thyroid peroxidase antibodies suggests an autoimmune component.

• Do Not Miss Diagnoses

  • Thyroid Cancer: Although less likely, the nodular thyroid gland could potentially harbor cancer. The patient's symptoms and laboratory results do not strongly suggest cancer, but it is essential to consider this possibility, especially given the firm and nodular nature of the thyroid gland.
  • Pituitary Disorder: A pituitary disorder, such as a tumor, could cause elevated TSH levels. However, the presence of thyroid peroxidase antibodies and the patient's symptoms suggest a primary thyroid issue rather than a pituitary problem.

• Rare Diagnoses

  • Riedel's Thyroiditis: This rare form of thyroiditis causes a hard, fibrous thyroid gland, which could potentially explain the patient's nodular thyroid gland. However, Riedel's thyroiditis is typically associated with more severe symptoms, such as difficulty swallowing and breathing, which are not present in this case.
  • Thyroid Lymphoma: This rare condition could cause a rapidly enlarging thyroid gland, but it is typically associated with more severe symptoms, such as neck pain and difficulty swallowing, which are not present in this case.