Subclinical hypothyroidism or central hypothyroidism-The danger of thyroid function misinterpretation

Biochemical transition during triphasic postpartum thyroiditis: confusion with central hypothyroidism

Summary

Hypothyroidism occurring in the postpartum period can be due to pituitary or hypothalamic disease as in Sheehan’s syndrome and postpartum autoimmune hypophysitis or due to a primary thyroid disease as in postpartum thyroiditis. It is important that the correct diagnosis is ascertained because hypothalamic or pituitary disorders are often associated with other pituitary hormone deficiencies, especially life-threatening adrenal insufficiency or adrenal crisis. A combination of various symptoms and biochemical markers, especially serum thyroid-stimulating hormone levels dictate the initial diagnostic pathway. We present a case of a woman who presented with a 2-month history of tiredness and neck discomfort following delivery. A thyroid function test demonstrated results, which we initially interpreted as central hypothyroidism. Follow-up results indicated that this was in fact the transition period between the thyrotoxic phase and hypothyroid phases of postpartum thyroiditis. This case highlights the potential for diagnostic confusion between central hypothyroidism and postpartum thyroiditis.

Learning points

Lessons of the month: A challenging presentation of hypopituitarism secondary to an intracerebral aneurysm

A 65-year-old man presented to ambulatory care with a 10-month history of muscle weakness, weight loss, dysphagia and fatigue.Prior to presentation he had been managed in general practice for hypothyroidism with a low T4 level and normal thyroid stimulation hormone (TSH). He was commenced on levothyroxine yet, despite dose titrations, had ongoing symptoms. He had been extensively reviewed by gastroenterology and rheumatology teams.The thyroid function tests (TFTs) pattern prompted a pituitary hormone profile test, which revealed panhypopituitarism with a cortisol of 22 nmol/L. Therefore, hydrocortisone was commenced. A pituitary magnetic resonance imaging (MRI) detected a left internal carotid aneurysm that was confirmed on computed tomography angiography. He successfully underwent embolisation of the aneurysm in the local neurosurgical centre.This case highlights the importance of correctly interpreting TFTs as this patient's initial TFTs indicated central hypothyroidism which should have prompted urgent pituitary hormone screening. The risk of addisonian crisis with commencement of levothyroxine without steroid replacement in secondary hypothyroidism emphasises the importance of TFT interpretation. Internal carotid artery aneurysms are a rare, yet important, cause of hypopituitarism resulting in high mortality and morbidity associated with delayed diagnosis secondary to the pressure effects of the aneurysm or the effects of aneurysmal rupture.

Central hypothyroidism or subclinical hyperthyroidism: can they be confused with each other?

SUMMARY

Isolated, adult-onset central hypothyroidism is very rare, and its diagnosis can be challenging. A 42-year-old patient was referred for evaluation of a 2.8 cm thyroid nodule. She referred symptoms that could be attributed to hypothyroidism and thyroid tests showed low TSH and normal-low levels of free T4. However, evaluation of the remaining pituitary hormones and pituitary MRI were normal, yet a radionuclide scanning revealed that the thyroid nodule was 'hot' and the tracer uptake in the remaining thyroid tissue was suppressed. Interpretation of these studies led to a misdiagnosis of subclinical hyperthyroidism and the patient was treated with radioiodine. Soon after treatment, she developed a frank hypothyroidism without appropriate elevation of TSH and the diagnosis of central hypothyroidism was made a posteriori. Long term follow-up revealed a progressive pituitary failure, with subsequent deficiency of ACTH and GH. This case should alert to the possibility of overlooking central hypothyroidism in patients simultaneously bearing primary thyroid diseases able to cause subclinical hyperthyroidism.

LEARNING POINTS

Although rarely, acquired central hypothyroidism can occur in the absence of other pituitary hormone deficiencies. In these cases, diagnosis is challenging, as symptoms are unspecific and usually mild, and laboratory findings are variable, including low, normal or even slightly elevated TSH levels, along with low or low-normal concentrations of free T4. In cases with low TSH levels, the coexistence of otherwise common disorders able to cause primary thyroid hyperfunction, such as autonomous nodular disease, may lead to a misdiagnosis of subclinical hyperthyroidism.