Medicine Matters oncology

Features: Symptoms of hypothyroidism are vague and nonspecific, and patients most commonly present with fatigue, muscle weakness, cold intolerance and bradycardia. Primary hypothyroidism is typified by a high TSH and a low free T4 (FT4). Occasionally, thyroiditis with transient increases in FT4 accompanied by suppression of TSH, can occur over a period of a few weeks, and can be followed by an acute drop in free FT4 with rises in TSH as the thyroid gland dies, resulting in longstanding hypothyroidism. This conversion from hyper- to hypothyroidism can be acute, resulting in profound hypothyroidism.

Graph to model conversion from primary hyper- to hypothyroidism:

Of course, distinguishing primary thyroid disorders from secondary hypothyroidism is critical to enable appropriate management. Pituitary disorders such as hypophysitis classically result in a low TSH, low FT4 picture.

Investigations: TFTs, including free T3 (FT3) and FT4, should be obtained. Where TSH is suppressed and FT4 is also low, HCPs should consider assessing anterior pituitary function, including cortisol, follicle-stimulating hormone (FSH), luteinizing hormone (LH) and prolactin (as well as testosterone in men). Regular monitoring of TFTs both during and upon completion of ICI therapy is recommended, as incidence outside of clinical trials is likely to be much higher.

Management: Hypothyroidism is typically managed by replacement therapy, starting at 25–50 mcg daily with incremental dosing as biochemically indicated. Although it has been suggested that a course of steroids can help cases of acute thyroiditis, there is little evidence to support this.