This is from a Natural Progesterone newsletter by Catherine Rollins:
Thyroid Function and natural progesterone
The thyroid gland, largest of the endocrine glands, is the body's metabolic thermostat. It regulates body temperature and the rate of energy production, which greatly influences the rate at which all body organs function.
This butterfly-shaped gland, located close to the windpipe, secretes two hormones: thyroxine (T4) and tri-iodothyronine (T3).
Approximately 93% of the thyroid gland production is T4 with 7% T3.
Secretion of thyroid hormones is regulated by thyroid-stimulating hormone, secreted by the pituitary gland in the brain. The thyroid gland also secretes a hormone called calcitonin, involved in calcium metabolism.
Although all the body's hormones interact, there is a closer than usual relationship of thyroid to ovarian hormones. Most commonly, as a woman approaches menopause, estrogen excess or progesterone deficiency tends to cause a hypothyroid state.
Progesterone deficiency, for example, can cause fatigue, weight gain, irritability, depression, memory and concentration problems, headaches, irregular and painful mentstrual cycles, fluid retention, loss of sex drive, and cold hands and feet. These same symptoms are also characteristic of hypothyroidism and thyroid hormone resistance.
Emerging evidence suggests the symptoms of hypothyroidism occurring in patients with progesterone deficiency become less so when progesterone is added and hormone balance is achieved. In other words, progesterone supplementation can play a major role in balancing our hormones which can very likely include improved thyroid hormone function.
Many women suffering estrogen dominance are taking thyroid supplements that, when progesterone is introduced and estrogen dominance is corrected, can often reduced or eliminate thyroid medication altogether. Therefore, if you have been diagnosed with a thyroid problem, are on thyroid medication and now want to incorporate natural progesterone into your regime, there's no reason why you can't providing you do so under the strict supervision of your treating physician.
Your thyroid medication, however, may require regular adjustment as progesterone exerts an influence upon the thyroid gland (progesterone may cause a potentiation of thyroxine's effects leading to hyperthyroidism).
"I am 41 and the idea of someone my age taking hormones had never occurred to me before. I had a number of estrogen dominant symptoms, as well as low thyroid issues. I began using natural progesterone cream in November of 2003. It is now August of 2004 and I can honestly say I have never had more energy in my adult life than I have had in the past 9 months."
Jennifer
"I'm aged 46, have been on hypo-thyroid medication since I was 21, (low thyroid levels). It’s been two years now, and I think the fertility endocrinologist is as impressed as I am with the result of the progesterone cream!"
Carolyn
In light & love,
Catherine
http://www.natural-progesterone-advi...ogesterone.php
Thyroid Function and Natural Progesterone 
