What is The neuroendocrine carcinoma arising from parafollicular 'C' cells of thyroid is: and explain in detail?
MEDULLARY CARCINOMA OF THE THYROID (MCT)
These tumours arise from parafollicular ‘C’ cells which are derived from ultimobranchial bodies and not from thyroid foillicle.

These tumours present in two different ways.


Sporadic is common, seen in about 80-90% of cases.
Familial variety present as a part of multiple endocrine neoplasia (MEN).

MEN Type I

Pituitary adenoma
Parathyroid adenoma
Pancreatic adenoma

MEN Type IIa

Parathyroid adenoma
Phaeochromocytoma
Medullary carcinoma of thyroid


When it is associated with mucocutaneous neuromas involving lips, tongue, eyelids, it is called Sipple syndrome, with an occasional marfanoid habitus (MEN type IIb)
It has got a characteristic amyloid stroma 
These tumours are not TSH-dependent and do not take up radioactive iodine 

Hormones produced by MCT

Calcitonin 
Prostaglandins
Serotonin (5-HT), ACTH

Spread

Both by lymphatics and blood, thus, worsening the prognosis.

Key Concept:
Medullary Carcinoma arises from parafollicular C cells which are derived from ultimobranchial bodies and not from thyroid follicle. The medullary carcinoma produces hormones such as Calcitonin, Prostaglandins and serotonin.
Reference:  Manipal Manual of Surgery 4th ed page no 348