A 19-year-old woman who has always been healthy and maintained an average weight

Question

A 19-year-old woman who has always been healthy and maintained an average weight has lost 20lbs in the last 3 months, despite a voracious appetite. She complains of nervousness, sleeplessness, heart palpitations and irregular menstrual periods. She notes that she is “always hot” and wants the thermostat set lower than her roommates. On physical examination, she was restless and had a noticeable tremor in her hands. At 5’8” tall, weighed only 110lbs. Her blood pressure was 160/85 (normal is less than 120/less than 80), and her heart rate was 110 beats/min (normal 60-100 beats/min). She had a wide-eyed stare, and her lower neck appeared full; these characteristics were not present in photographs taken 1 year earlier. Based on her symptoms, the physician suspected that she had thyrotoxicosis, or increased circulating levels of thyroid hormones. However, it was unclear from the available information why her thyroid hormone levels were elevated. Lab tests were performed to determine the etiology of her condition.

Based on an understanding of the hypothalamic-anterior pituitary-thyroid axis, regulation of thyroid hormone secretion and the lab results, include or exclude (and explain why) each of the following four potential causes of this patient’s thyrotoxicosis:

a. increased secretion of thyrotropin-releasing hormone (TRH) from the hypothalamus

b. increased secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary

c. primary hyperactivity of the thyroid gland (i.e., Grave’s disease)

d. ingestion of exogenous thyroid hormones (factitious hyperthyroidism)

Explanation / Answer

Based on an understanding of the hypothalamic-anterior pituitary-thyroid axis, among the following four potential causes of this patient's thyrotoxicosis:

1. Excluded- This is because thyrotoxicosis is caused due to higher levels of thyroid hormone in blood stream(T4 and T3) which increases production of pyroglutamyl peptidase II which degrades Thyrotropin-releasing Hormone(TRH) from the hypothalamus. Hence Secretion will be decreased.

2.Excluded- This cause is excluded because secretion of thyroid-stimulating hormone(TSH) from the anterior pituitary is increased when the anterior pituitary senses the diminishing levels of thyroid hormone in bloodstream whereas in thyrotoxicosis, thyroid hormone is extra enough to shut down the release of TSH.

3.Included- This is one of the main cause of hyperthyroidism which is leading to thyrotoxicosis .Grave's disease is an auto immunity disorder in which Thyrotropin receptor antibody(TRAb) acts like the regularly pituitary hormone leading to over production of thyroid hormone(Hyperthyroidism). Also, many of the symptoms mentioned in the case of this 19 year old lady like wide-eyed stare(Ophthalmopathy), heart palpitations etc. are major symptoms of Grave's Disease.

4.Included - This is a potential cause because ingestion of exogenous thyroid hormones in the form of medication pills, or consumption of meat containing thyroid gland tissue in it may lead to hyperthyroidism which leads to thyrotoxicosis.

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