The patient is likely suffering from hyperthyroidism secondary to toxic multinodular goiter. In this condition, there is increased production and release of thyroid hormones (T4 and T3) from thyroid epithelial cells. This results in a negative feedback on the anterior pituitary (as well as the hypothalamus) resulting in a subsequent decrease in serum TSH levels.
Many of the symptoms of hyperthyroidism are a result of the increased beta-adrenergic activity caused by elevated thyroid hormone levels. High thyroid levels lead to a heightened basal metabolic rate, resulting in increased heat production (heat intolerance, warm/hot skin, sweating) as well as unintentional weight loss. Furthermore, the beta-adrenergic hyperactivity of high thyroid levels can lead to tachycardia, palpitations, or arrhythmias.
Reid et al. review the diagnosis and treatment of hyperthyroidism. They conclude that Graves' disease is the most common cause of hyperthyroidism, with other common causes including thyroiditis, toxic multinodular goiter, toxic adenomas, and medication side-effects. Initial diagnostic work-up typically begins with measurement of serum TSH levels. When serum blood tests are inconclusive as to the etiology, radionuclide uptake scans can be useful in distinguishing among the possible causes.
Lee et al. discuss the use of propranolol in the treatment of hyperthyroidism. They note that beta blockers are often effective at neutralizing many of the symptoms of autonomic hyperactivity in the setting of hyperthyroidism, most significantly controlling tachycardia. Franklyn et al. discuss another benefit of propanolol therapy, namely the inhibition of the peripheral deiodination of T4 hormone. Therefore, not only does propranolol alleviate some of the downstream effects of high serum thyroid hormone levels, it can also act to decrease serum thyroid hormone levels directly.
Illustration A summarizes the hypothalamic-pituitary-thyroid axis. Illustration B depicts the mechanism of thyroid hormone synthesis and transport into the blood stream.
Answers 1, 3, and 4: High TSH would be more consistent with primary hypothyroidism since there would be decreased negative feedback on the anterior pituitary from the low thyroid hormone levels.
Answer 5: Although one would expect low TSH in hyperthyroidism (via increased negative feedback), one would not expect to find low serum T4.
Reid JR, Wheeler SF. Hyperthyroidism: diagnosis and treatment. Am Fam
PMID:16127951 (Link to Abstract)
Lee TC, Coffey RJ, Currier BM, Ma XP, Canary JJ. Propranolol and thyroidectomy in the treatment of thyrotoxicosis. Ann Surg. 1982 Jun;195(6):
PMID:1352677 (Link to Abstract)
Franklyn JA, Wilkins MR, Wilkinson R, Ramsden DB, Sheppard MC. The effect of propranolol on circulating thyroid hormone measurements in thyrotoxic and euthyroid subjects. Acta Endocrinol (Copenh). 1985 Mar;108(3)
PMID:3920853 (Link to Abstract)